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R EGU L A R PA P E R
A comparative morphological investigation of otoliths of sixparrotfish species (Scaridae) from the Solomon Islands
Laith A. Jawad
4 Tinturn Place, Flat Bush, Manukau, Auckland
2016, New Zealand
Correspondence
Laith A. Jawad, 4 Tinturn Place, Flat Bush,
Manukau, Auckland 2016, New Zealand.
Email: [email protected]
Funding information
No external funding was used for this research.
A comparative morphological investigation of otoliths from six parrotfish species and two
genera, Scarus and Hipposcarus, was conducted to identify taxonomically diagnostic species-
specific characters. Ontogenetic trends in otolith morphology were evident: Otoliths of small-
sized specimens were most like those of large-sized Scarus oviceps and were least similar to
large-sized Hipposcarus longiceps.
KEYWORDS
morphology, otoliths, parrotfish, Scaridae, SEM, Solomon Islands
1 | INTRODUCTION
Using the morphology of hard structures, such as otoliths, scales and
bones, to identify fish species has proven to be less expensive than
other approaches, such as molecular and genetic markers (Aguirre and
Lombarte 1999; Assis 2003; Jawad, 2008; L'Abée-Lund & Jensen,
1993; Ponton, 2006; Tuset et al., 2003a, 2003b, 2006), yet equally
valuable. Sagittal otoliths are generally the largest of the three otolith
pairs, while the lapillus and asteriscus are smallest (Paxton, 2000), at
least in most teleost families. Nevertheless, in siluriforms, characi-
forms and cypriniforms in general, the lapillus may be larger than the
sagittal otoliths (e.g., Ariidae; Schulz-Mirbach & Reichenbacher, 2006;
Teimori et al., 2012). Despite this variability in size, the sagittal otoliths
are widely used for taxonomic and biological (e.g., age and growth)
studies in non-ostariophysan teleosts for many reasons: their typically
large size, relative ease of access, opaque and translucent seasonal
markings, morphological variation and resistance to digestion in the
alimentary canal of predators (L'Abée-Lund, 1988; Nolf, 1985; Platt &
Popper, 1981; Tuset et al., 2003a). They are located on either side of
the basioccipital bone and are separated by a thin septum arising from
the midventral ridge of the occipital (Jawad, 2008; Jawad et al., 2007;
Ruck, 1976).
For parrotfishes (family Scaridae), which are important within
coral-reef ecosystems as herbivorous bio-eroders, there exists no
comprehensive literature on otolith morphology and any comprehen-
sive treatment of this clade is challenged by its nearly 100 species
(Nelson et al., 2016). Data on parrotfish otolith morphology is cur-
rently distributed across a wide range of sources. Nolf (1985) provided
a line-drawing of the sagittal otolith of Scarus iseri (Bloch 1789).
Weisler (1993) highlighted the importance of otoliths of Scarus perspi-
cillatus (Steindachner 1879) for archaeological studies, while Smale
et al. (1995) provided descriptions for six species of the genus Scarus
Forsskål 1775 as well as for Calotomus spinidens (Quoy & Gaimard
1824), Leptoscarus vaigiensis (Quoy & Gaimard 1824) and Hipposcarus
harid (Forsskål 1775). Rivaton and Bourret (1999) provided images of
otoliths of nine species of the genus Scarus and two species of the
genus Cetoscarus Smith 1956 from the Indo-Pacific region. Tuset et al.
(2008) briefly described the otoliths of Sparisoma cretense (Linnaeus
1758) and Sparisoma rubripinne (Valenciennes 1840). Baremore and
Bethea (2010) provided an image for Nicholsina usta (Valenciennes
1840) from the Gulf of Mexico. Lin and Chang (2012) described the
otoliths of seven species of the genus Scarus and one species each of
the genera Calotomus Gilbert 1890, Chlorurus Swainson 1839, Hippos-
carus Smith 1956 and Leptoscarus Swainson 1839 from Taiwan.
Sadighzadeh et al. (2012) gave short descriptions of two species of
Scarus and Nolf (2013) provided line-drawings for the otoliths of
N. usta and S. iseri. Recently, Jawad et al. (2017) described the mor-
phology of two scarid species, Chlorurus sordidus (Forsskål 1775) and
Hipposcarus harid (Forsskål 1775), from the Egypian coast of the Red
Sea. The present paper provides a morphological analysis of parrotfish
otoliths from the Solomon Islands.
The aim of this study is to find distinguishing features, based on
an extensive description of the sagittal otoliths to provide for the
identification of each species and characterize ontogenic stages
between the initial phase and terminal phase (IP and TP) colouration,
which closely follows the change of sex in this family. This study con-
tributes to methods in parrotfish taxonomy, paleoichthyology and the
taxonomy of coral-reef fish.
Received: 21 March 2018 Accepted: 22 August 2018
DOI: 10.1111/jfb.13787
FISH
1046 © 2018 The Fisheries Society of the British Isles wileyonlinelibrary.com/journal/jfb J Fish Biol. 2018;93:1046–1058.
2 | MATERIAL AND METHODS
Fifty-four otoliths were collected from Hipposcarus longiceps
(Valenciennes 1840), Scarus dimidiatus Bleeker 1859, Scarus ghobban
Forsskål 1775, Scarus niger Forsskål 1775, Scarus oviceps Valenciennes
1840 and Scarus rivulatus Valenciennes 1840 of various sizes between
2014 and 2015. Fishes were speared at night around the Island of
Ghizo (western Solomon Islands, 8.0833� S, 156.8333� E; Figure 1)
and subsequently measured. The top of the cranium was sliced, the
brain was removed and the otic capsules were exposed and separated
with a sharp scalpel. Sagittal otoliths were gently removed with a pair
of fine tweezers, cleaned with 70% ethanol and stored dry.
Fishes were aggregated in length groups, which were determined
by species (Tables 1–6). It is important to note that the species stud-
ied can attain larger lengths than the specimens speared (Randall,
2005), but no fishes of the maximum reported size were caught during
this study. The data gathered, however, are sufficient to draw first
conclusions and waiting for the ideal situation, in which specimens of
all sizes become available for study, is not justified. Otoliths were
placed in their respective length groups by species so that species-
specific ontogenetic changes in otolith shape could be followed. Spec-
imens of all species except S. rivulatus were divided into three group-
ings (H. longiceps: group 1,100–200 mm, group 2,201–300 mm,
group 3,301–400 mm; S. dimidiatus: group 1,100–150 mm,
group 2:151–200 mm, group 3:201–300 mm; S. ghobban: group
1:100–200 mm, group 2:201–300 mm, group 3:301–400 mm;
S. niger: group 1:100–150 mm, group 2:151–200 mm, group
3:201–250 mm; S. oviceps: group 1:100–150 mm, group
2:151–200 mm, group 3:201–250 mm). Specimens of S. rivulatus
were placed in four groups (group 1:100–150 mm, group
2:151–200 mm, group 3:201–250 mm, group 4:251–300 mm).
Otoliths from each size class of all six species were examined with
scanning electron microscopy (SEM), which allowed observation of
morphological characteristics on the mesial face of the otoliths. In
preparation for SEM, otoliths were air cleaned and mounted on an alu-
minium stub using double-sided carbon tape. Stubs were sputter-
coated with gold in a vacuum of about 4,061,023 Torr. Otoliths were
viewed in a Hitachi SU-70, EM (www.hitachi.com), at 5.0 kV.
Terminology of otolith morphology follows Smale et al. (1995)
(Figure 2(a), (b)).
3 | RESULTS
The following characters are shared by small and large parrotfishes:
otolith width, otolith depth, shape of mesial and lateral surface, shape
of sulcus acusticus, ostio-caudal differentiation and rostrum thickness.
The otoliths of small (group 1) H. longiceps (Figure 3) differed from
those of larger (group 3) specimens in 14 out of the 22 characteristics
studied. Detailed descriptions of the otoliths from three size classes
are shown in Table 1. The otolith shape is oval–oblong in small and
oblong–ovate in larger H. longiceps (Figure 3). The dorsal margin is cre-
nate, sinuate and dentate in smaller fish (groups 1 and 2), becoming
irregular in larger ones (group 3). The shape of the dorsal margin
shows an increasing trend in irregularity from group 1 to group
3, including the presence of lobes and ridges. The ventral margin can
be flattened, gently irregular, indented, crenate and lobed in group
3. The posterior margin is usually not uniform in group 1 and group
3 and can be rounded, slightly or deeply indented, lobed and elon-
gated. In group 2, it is lobed with a shallow indentation or elongated
with a large indentation. The ostium in otoliths of different length
groups is flared and lumps of different sizes cover their floor. In smal-
ler specimens (group 1), it is straight in its anterior part, but curved in
group 2 and straight again in larger specimens (group 3). The ostium is
deep in group 1 and group 2, but shallow in group 3. The cauda is
flared in otoliths of all length groups and circular or oval in group
1 and group 2, becoming oblong in group 3. The floor is either smooth
or covered with lumps of different sizes in all three length groups. A
deep groove is present in otoliths of large fish (group 3). The crista
superior is usually ridge-like or well developed in smaller fishes (group
1 and group 2) and may be absent or have a ridge-like shape in larger
fish (group 3). The crista inferior is well developed in smaller fish, but
poorly developed in some large ones (group 3). The dorsal depression
is elongated in all length groups, but shallow in group 1 and group
3. The rostrum is blunt and oblique, short or long in group 1 and group
2, but irregular and long in group 3. The antirostrum in large fish
(group 3) is long and lobed. The collum is either low or absent in all
length groups. The latter two characteristics could not be used to
identify the relative size of H. longiceps. The antirostrum, if present, is
either small, moderate, large, broad, or pointed. In general, the larger
specimens appear to have a well-developed antirostrum (Figure 3).
Solomon Islands(a)
(b) SolomonIslands
Kolombangara
New
GeorgiaGhiza
Randova Island
Vanguru
Vella laVella
N
N
FIGURE 1 Map showing (a) the geographical position of the Solomon
Islands and (b) the collection localities
JAWAD 1047FISH
The otoliths of small (group 1) S. dimidiatus (Figure 4) differ from
larger ones (group 3) in 11 out of the 22 characteristics studied.
Detailed descriptions of the otoliths from the three size classes are
shown in Table 2. The following characters did not vary between indi-
viduals of group 1 and group 3: the shape of both crista superior and
crista inferior, the rostrum thickness, the dorsal depression and the
shape of vexcisura. In group 1, the shape varies between oval and
elliptic, becoming oval and oblong in group 2 and oval in group 3. The
dorsal margin gradually changes from lobed to irregular in group 1 and
group 2, to entirely irregular in group 3. The ventral margin is crenate,
serrated or lobed in group 1 and group 2, but lobed at the middle in
group 3. The posterior margin is broad in all three length groups, with
a pointed, rounded or straight rim, but irregular in larger fish (group 3).
The ostium is flared and shallow in all specimens examined with lumps
on the floor. The lumps change to a longitudinal ridge in medium and
larger sized specimens (group 2 and group 3). The anti-rostrum in
group 3 is very poorly developed. In group 1 and group 2, the cauda is
flared, shallow or deep and the floor covered with lumps, ridges or
smooth. In adult fish of group 3, it is flared, shallow, or a longitudinal
ride and the floor is smooth. The rostrum is usually long in the smaller
specimens, but very short and blunt in the larger specimens. The
antirostrum is short in all specimens examined, but pointed in group
1, poorly developed in group 2 and rounded and poorly developed in
group 3. The collum is a solid ridge in group 1 and group 2, but absent
in group 3.
The otoliths of small (group 1) S. ghobban (Figure 5) differ from
the larger (group 3) ones in 12 out of 22 characteristics studied. The
following characters do not vary between individuals of group 1 and
group 3: the ostio-caudal differentiation, the ventral depression, the
thickness of the rostrum and the excisura. Detailed descriptions of
otoliths of the three size classes are shown in Table 3. The shape of
the otoliths is either oblong or ovate in smaller fish (group 1 and group
2), but ovate in all larger (group 3) specimens. There is a tendency
toward irregularity in the dorsal margin, which is serrated, lobed and
emarginated in group 1 and group 2, but irregular in group 3. This also
is the case for the ventral margin. The shape of the posterior margin
varies from oblique, to sinuate, lobed and rounded with an extension
in group 1 and group 2, becoming irregular with an indentation in
group 3. The ostium is of a similar shape (flared and shallow) in small
and large individuals; its floor has a ridge in adults. The cauda is flared,
shallow and smooth or with a ridge in group 1 and group 2, but also
deep in group 3. The crista superior has a similar shape and is less
TABLE 1 Otolith characteristics of three size classes of Hipposcarus longiceps from the Solomon Islands
Characters Group 1 (100–200 LT mm) Group 2 (201–300 LT mm) Group 3 (301–400 LT mm)
Otolith shape Raised oval, oval Oval, oblong, raised oval Oblong, ovate
Otolith width Very thick Very thick Very thick
Otolith depth Deep Deep Deep
Mesial surface Convex Convex Convex
Lateral surface Concave Concave Concave
Dorsal margin Slightly crenate, sinuate Sinuate, irregular, dentate Irregular
Ventral margin Smooth with slight irregularity,coarse indentated
Coarsly indentated, dentate, crenate,irregular, lobed
Lobed
Posterior margin Rounded with slight indentation,irregular with deep indentation
Lobed, irregular with wide indentation,irregular, rounded, broadly elongated,lobed with medium indentation, coarselobed
Lobed with shallow indentation, elongatedwith wide indentation
Sulcus acusticus Ostial, homosulcoid Ostial, homosulcoid Ostial, homosulcoid
Ostium Circular, oval, shallow, flared, floorcovered with fine lumps
Flared, shallow, deep, upper and lowermargins straight, floor covered with fewlumps or smooth
Shallow, upper and lower margins staright,floor covered with coarse lumps
Cauda Upper and lower margins notcurved, shallow, floor coveredwith coarse lumps
Circular, oval, oblong, flared or not, floorcovered with coarse and fine lumps orsmooth, shallow
Oblong, flared, lobes at posterior end, floorcovered with coarse lumps or smooth,presence of deep fissure
Ostio-caudaldifferentiation
With dorsal and ventral ridges,poorly developed
With dorsal and ventral ridges, poorlydeveloped
With dorsal and ventral ridges, poorlydeveloped
Crista superior Ridge-like, well deveoped Ridge-like Ridge-like or absent
Crista inferior Well developed Well, poorly developed or absent Well or poorly developed
Dorsal depression Elongated, shallow Elongated, deep Elongated, shallow, wide
Ventral depression Absent Absent Absent
Rostrum shape Rounded Rounded with lobes Irregular
Rostrum size Short Short or long Long
Rostrum thickness Broad Broad Broad
Antirostrum Short, with fine tip or absent Short with broad or pointed tip, absent,with lobes
Long, broad with lobes
Collum Low Low or absent Low or absent
Excisura Absent Shallow, narrow notch, narrow angle orabsent
Shallow, narrow notch, narrow angle orabsent
Note. LT: Total length.
1048 JAWADFISH
pronounced in younger specimens. This is also the case for the crista
inferior in those otoliths of group 3 where it is developed, although it
is poorly developed in some specimens of group 1. There is a ten-
dency toward a poorly developed dorsal depression, which is shallow
and narrow in smaller specimens, but poorly developed in otoliths of
larger fish. The rostrum is usually short and blunt in all otoliths exam-
ined, it is irregular in some otoliths of group 2 and in all otoliths of
group 3. The antirostrum is mostly short and poorly developed in
group 1 and in some specimens of group 2, but long and pointed in
group 3. The collum becomes more developed as the otoliths grow
larger. It is either absent or poorly developed in specimens of group
1 and group 2 but has a ridge-like shape in group 3.
The otoliths of small (group 1) S. niger (Figure 6) differ from those
of group 3 in eight of 22 characteristics studied. The following charac-
ters do not vary between individuals of group 1 and group 3: the
ostio-caudal differentiation, the shape of the crista superior and crista
inferior, the shape of the rostrum, the size and thickness, the collum
and the excisura. Detailed descriptions of otoliths from the three
length groups are shown in Table 4. In specimens of group 1, the oto-
lith is oval, becoming elliptic in group 2 and either ovate or oval in
group 3. There is a tendency toward a lobed dorsal margin, slightly in
individuals of group 1 and group 2, but raised in group 3. The ventral
margin varies from lobed, crenated and irregular in group 1 and group
2 to irregular in group 3. The posterior margin is broad in all otoliths
examined, pointed in group 1 and group 2, but mainly oblique in group
3. In all otoliths examined the ostium and cauda are flared and deep,
the floor is covered with lumps in group 1 and group 2 but covered
with a ridge in group 3. The dorsal depression is elongated and narrow
in all length groups, deep in individuals of group 1, but gradually shal-
lowing as the fish grows larger (group 3). The rostrum is short, blunt
and oblique. The antirostrum appears to be short and pointed or
absent.
The otoliths of small (group 1) S. oviceps (Figure 7 and Table 5) dif-
fer from larger (group 3) ones in 6 out of 22 characteristics studied.
The following characters do not vary between individuals of group
1 and group 3: the ostio-caudal differentiation, the crista superior and
the crista inferior, the dorsal and ventral depressions, the shape of the
rostrum, the size and thickness, the collum, the shape of the antiros-
trum and the excisura. The shape of the otoliths varies between oval,
ovate and elliptic in the three length groups studied. The dorsal mar-
gin is emarginated, lobed or irregular in group 1 and group 2, but
mostly lobed or sinuate in group 3. Lobes and serrations are charac-
teristics of the ventral margin in fish of group 1 and group 2 but
diminishing in group 3. Lobes and occasional notches are found in oto-
liths of group 1 and group 2. The posterior margin is broad and has
two notches in group 3. The ostium and cauda are flared and shallow
and their floors are covered with lumps or ridges in all otoliths
examined.
The otoliths of small (group 1 and group 2) S. rivulatus (Figure 8)
differ from the larger (group 3 and group 4) ones in 10 out of 22 char-
acteristics studied. The following characters did not vary significantly
between small and large individuals: the shape of ostium, the
TABLE 2 Otolith characteristics of three size classes of Scarus dimidiatus from the Solomon Islands
Characters Group 1 (100–150 mm LT) Group 2 (151–200 mm LT) Group 3 (201–300 mm LT)
Otolith shape Oval. elleptic Elleptic, ovate, oblong Oval
Otolith width Very thick Very thick Very thick
Otolith depth Deep Deep Deep
Mesial surface Convex Convex Convex
Lateral surface Concave Concave Concave
Dorsal margin Lobed, straight with central lobe Irregular Entire
Ventral margin Crenate, coarse crenate Serrate, lobed Lobed at the middle
Posterior margin Rounded, blunt Broadly irregular, straight Broadly irregular
Sulcus acusticus Ostail homosulcoid Ostail homosulcoid Ostail homosulcoid
Ostium Flared, shallow or deep, floor covered withlumps
Flared, shallow withlongitudinal ridge
Flared, shallow with longitudinal ridge
Cauda Flared, shallow, floor covered with lumps,presence of horizontal ridge
Flared, deep, floor with alongitudinal ridge or smooth
Flared, shallow floor smooth
Ostio-caudal differentiation Well developed Well developed Well developed
Crista superior Ridge-like Ridge-like Ridge-like
Crista inferior Ridge-like Ridge-like Ridge-like
Dorsal depression Deep, elongated with edge ventrally Deep, elongated withshelve ventrally
Deep, elongated with shelve ventrally
Ventral depression Absent Absent Absent
Rostrum shape Lobed Lobed Lobed
Rostrum size Short Short Short
Rostrum thickness Broad Broad Broad
Antirostrum Short, pointed Short Short, rounded
Collum Solid-bridge Solid-bridge, developed Absent
Excisura Could be absent Could be absent Absent
Note. LT: Total length.
JAWAD 1049FISH
TABLE 3 Otolith characteristics of three size classes of Scarus ghobban from the Solomon Islands
Characters Group 1 (100–200 mm LT) Group 2 (201–300 mm LT) Group 3 (301–400 mm LT)
Otolith shape Obong Oblong, ovate Ovate
Otolith width Very thick Very thick Very thick
Otolith depth Deep Deep Deep
Mesial surface Convex Convex Convex
Lateral surface Concave Concave Concave
Dorsal margin Sinuate, lobed, irregular Irregular, emarginate, raised irregular Irregular
Ventral margin Sinuate Emarginate, crenate, serrate Irregular
Posterior margin Straight with indentation,pointed, irregular
Sinuate, irregular, lobed,rounded with extension
Irregular with indentation
Sulcus acusticus Ostial, heterosulcoid Ostial, homosulcoid Ostial, homosulcoid
Ostium Flared, shallow, floor coveredwith lumps or smooth
Flared, shallow, floor covered withlumps, ridges or smooth
Flared, shallow, floor coveredwith lumps, ridges or smooth
Cauda Flared, shallow, floor coveredwith ridges or smooth
Flared, shallow, floor covered withridges or smooth
Flared, deep, floor coveredwith ridges or smooth
Ostio-caudal differentiation Well or poorly developed Well or poorly developed Well or poorly developed
Crista superior Ridge-like, not developed, developed Ridge-like, developed Ridge-like
Crista inferior Ridge-like, poorly developed Ridge-like, poorly developed Well developed
Dorsal depression Narrow, deep or shallow, elongated Narrow, shallow, elongated, developed Developed
Ventral depression Absent Absent Absent
Rostrum shape Broadly pointed, absent With staright end or irregular Irregular
Rostrum size Short Short Short
Rostrum thickness Thick Thick Thick
Antirostrum Short, broad Short, broad, absent, pointed Long pointed
Collum Absent, poorly developed Absent, poorly developed, solid-bridge Solid-bridge
Excisura Absent Present Present
Note. LT: Total length.
TABLE 4 Otolith characteristics of three size classes of Scarus niger from the Solomon Islands
Characters Group 1 (100–150 mm LT) Group 2 (151–200 mm LT) Group 3 (201–300 mm LT)
Otolith shape Oval Elliptic Ovate, oval
Otolith width Very thick Very thick Very thick
Otolith depth Deep Deep Deep
Mesial surface Convex Convex Convex
Lateral surface Concave Concave Concave
Dorsal margin Slightly lobed Lobed, raised lobed Flat or raised lobed
Ventral margin Lobed Lobed, crenate, irregular Irregular, slightly irregular anteriorly,slightly crenate at the middle
Posterior margin Broadly pointed Broadly pointed, irregular Irregular, broadly pointed
Sulcus acusticus Ostial, homosulcoid Ostial, homosulcoid Ostial, homosulcoid
Ostium Flared, deep, floor covered with lumps Flared, deep, floor covered with ridges Flared, deep, floor covered with ridges
Cauda Flared, shallow, floorcovered with lumps
Flared, shallow, floor coveredwith lumps or ridges, smooth
Flared, shallow, floor coveredwith lumps or ridges
Ostio-caudal differentiation Well developed Well developed Well developed
Crista superior Ridge-like Ridge-like Ridge-like
Crista inferior Ridge-like Ridge-like Ridge-like
Dorsal depression Elongated, narrow, deep Elongated, narrow, shallow Elongated, narrow, shallow
Ventral depression Absent Absent Absent
Rostrum shape Broadly pointed Broadly pointed Broadly pointed
Rostrum size Short Long Long
Rostrum thickness Thick Thick Thick
Antirostrum Short, broad Short or long, broad Long, broad
Collum Solid-ridge Solid-ridge Solid-ridge
Excisura Absent Absent Absent
Note. LT, Total length.
1050 JAWADFISH
ostio-caudal differentiation, the shape of dorsal margin, the shape of
the rostrum shape and the size and thickness. The shape of the oto-
liths is either elliptic or ovate in small individuals, but elliptic in larger
fish (group 4). The dorsal margin is either emarginate, irregular or
lobed in group 1 to group 3, it is lobed in group 4. The ventral margin
is emarginate, lobed or crenated in group 1 and group 2, but serrated
in group 3 and group 4. Small otoliths tend to develop a mid-ventral
angle on the ventral rim (see also S. dimidiatus). The posterior margin
is characterized by a notch, which varies between narrow and wide in
specimens of group 1 to group 3. In some individuals this margin is
pointed, but in group 4 it is pointed with a deep notch. In the four
length groups studied, the cauda is flared and shallow and the floor
either smooth or covered with lumps or ridges. In specimens of group
4, those lumps change to ridges. The crista superior has a ridge-like
structure in group 1 and group 2, becomes less marked in group 3 and
has a ridge-like structure again in group 4. The crista inferior is poorly
developed and not always along the full length in the four length
groups studied. The excisura is either absent or shallow in groups 1 to
3, but medium or medium-angled in group 4 (Table 6).
4 | DISCUSSION
There are several landmark features of the sagittal otolith that assist
in taxonomic studies (Figures 1 and 2). Such features have been docu-
mented in the works of several authors since the early 20th century
(Chaine & Duvergier, 1934; Frizell & Dante, 1965; Nolf, 1985, 2013).
The teleost sagittal otolith is the most widely used anatomical feature
in comparative taxonomic studies, because of its large size and degree
of inter-specific variation (Jawad, 2008; Jawad et al., 2007). In this
study, we examined a wide range of otolith characteristics, of which
only a few turned out to be taxonomically important. It was possible
to recognize two groups of characteristics that can be used to distin-
guish small and large specimens: characteristics that are consistent in
the otoliths of fish across different length groups, which can be used
to identify individual species; characteristics that vary due to ontoge-
netic changes, but may be useful to define certain developmental
stages.
Many parrotfish species are recognized based on morphology and
phenotypic characteristics (e.g., coloration) and these traits are con-
gruent with the phylogenetic evolution of the Scaridae (Bellwood,
1994; Parenti & Randall, 2011). The otoliths of parrotfishes are gener-
ally small, most likely to avoid the background noise produced by
rough seas in shallower regions of coral reefs (Cruz & Lombarte, 2004;
Paxton, 2000; Volpedo & Echeverría, 2003). The small size of otoliths
was clearly noticed in the six parrotfish species studied here, illustrat-
ing the relationship of ecology and otolith size. Relationships between
otolith size, habitat and behavior are also seen in other groups, such
as some notothenioids (Klingenberg & Ekau, 1996; Lombarte et al.,
2003). Sadighzadeh et al. (2014) found a relationship between varia-
tion in the anterodorsal area of the sagittal otoliths and certain behav-
iors in snappers. The anterodorsal area is extended in Lutjanus
argentimaculatus (Forsskål 1775), a species that is active at night
(Martinez-Andrade, 2003); however, results of that study cannot be
TABLE 5 Otolith characteristics of three size classes of Scarus oviceps from the Solomon Islands
Characters GI (100–150 mm LT) GII (151–200 mm LT) GIII (201–300 mm LT)
Otolith shape Oval, ovate Elliptic, oval Elliptic, ovate, oval
Otolith width Very thick Very thick Very thick
Otolith depth Deep Deep Deep
Mesial surface Convex Convex Convex
Lateral surface Concave Concave Concave
Dorsal margin Emarginate, lobed Lobed, irregular Lobed, sinuate
Ventral margin Lobed, coarse serrate Lobed, sinuate Slightly serrate, slightly lobed
Posterior margin Lobed, broadly straight Lobed, broadly pointed Broad with notchs
Sulcus acusticus Ostial, homosulcoid Ostial, homosulcoid Ostial, homosulcoid
Ostium Flared, shallow, floorcovered with lumps
Flared, shallow, floorcovered with lumps
Flared, shallow, floorcovered with ridges
Cauda Flared, shallow, floor coveredwith lumps and ridges
Flared, shallow, floorcovered with ridges
Flared, shallow, floor coveredwith lumps and ridges
Ostio-caudal differentiation Well developed Well developed Well developed
Crista superior Ridge-like Ridge-like Ridge-like
Crista inferior Ridge-like Ridge-like Ridge-like
Dorsal depression Elongated, shallow Elongated, shallow Elongated, shallow
Ventral depression Absent Absent Absent
Rostrum shape Lobed Lobed Broadly pointed
Rostrum size Long Long Long
Rostrum thickness Thick Thick Thick
Antirostrum Long or short, broad Long or short, broad Long or short, broad
Collum Solid-ridge Solid-ridge Solid-ridge
Excisura Absent Absent Absent
Note. LT: Total length.
JAWAD 1051FISH
applied to the six parrotfish fishes studied, because they all have a
lobed anteriodorsal area and they all are day feeders. This study also
shows that the overall morphology of otoliths varies from small to
larger individuals. Out of 22 characters studied, H. longiceps has
14 that differ between small and larger invididuals. The fewest onto-
genetically variable characters were noticed in S. oviceps with only six.
In general, the present study has shown the presence of ontogenetic
changes such as the lengthening of the otolith, the disappearance of a
mid-ventral angle, the ventral rim becoming straighter and generally
less lobed or crenulated, the development of pronounced ridges and
lumps in the colliculi, the more pronounced cristae.
The six species studied share consistent characters from smaller
to larger individuals (Tables 1–6), but these characters cannot be used
as a tool to separate individual species within one genus or between
genera of the Scaridae. The ostial shape of the sulcus acusticus in
Scarus is shared by members of Hipposcarus (this study; Lin & Chang,
2012) and of Nicholsina (Baremore & Bethea, 2010). It cannot be used
either as a characteristic feature to separate the five species of the
genus Scarus studied here. Moreover, it is similar in nine species of
the genus from Taiwan (Lin & Chang, 2012) and one species from
Pacific islands (Weisler, 1993). Furthermore, Smale et al. (1995)
described the sulcus acusticus of 10 parrotfish species, including
seven species belonging to the genus Scarus, as ostio-caudal. Among
those seven species S. ghobban and S. niger were highlighted. Finally,
Sadighzadeh et al. (2012) described the sulcus acusticus of S. ghobban
as homosulcoid. This shows a great geographical variance between
these studies and my findings. Scarus ghobban is one of the most
widespread parrotfish species circumtropically and is generally
believed to be a species complex. Therefore, the characters described
cannot be reliably used to identify any of the six scarid species from
the Solomon Islands elsewhere, as there appears to be too much intra-
specific geographic variability in parrotfish otoliths.
In each of the six species studied, there are certain characters that
are conservative in the different length groups (Tables 1–6): three in
H. longiceps, five in S. dimidiatus, three in S. ghobban, eight in S. niger,
eight in S. oviceps and four in S. rivulatus. The consistency of these
characters can perhaps, with a degree of doubt, be used as a tool to
separate these species from other members of the family Scaridae in
the Solomon Islands, which is a task for future studies. Otolith shape
varies ontogenetically in all species studied. From small to large,
S. dimidiatus shows the highest degree of variability with four shapes,
H. longiceps and S. niger with three shapes and the least variable were
S. ghobban and S. rivulatus with only two shapes. Among the five spe-
cies of Scarus studied here, it was not possible to describe a uniform
shape for the genus. The general trend is oval–ovate–oblong–elliptic.
Such a variation through ontogeny is also reported by Smale et al.
TABLE 6 Otolith characteristics of three size classes of Scarus rivulatus from the Solomon Islands
Characters Group 1 (100–150 mm LT) Group 2 (151–200 mm LT) Group 3 (201–250 mm LT) Group 4 (251–300 mm LT)
Otolith shape Elliptic Ovate Ovate Elliptic
Otolith width Very thick Very thick Very thick Very thick
Otolith depth Deep Deep Deep Deep
Mesial surface Convex Convex Convex Convex
Lateral surface Concave Concave Concave Concave
Dorsal margin Slightly emarginate Raised emarginate, lobed Raised irregular Lobed
Ventral margin Emarginate Emarginate, lobed, crenate Sinuate Sinuate
Posterior margin Broadly pointed,with notch
Lobed with notch Broadly pointed with notch Irregular, with notch
Sulcus acusticus Ostial, homosulcoid Ostial, homosulcoid Ostial, homosulcoid Ostial, homosulcoid
Ostium Flared, shallow, floorcovered with lumps
Flared, shallow, floorcovered with lumps
Flared, shallow, floorcovered with lumps
Flared, shallow, floorcovered with lumps
Cauda Flared, shallow,floor smooth
Flared, shallow, floor coveredwith lumps or smooth
Flared, shallow, floorcovered with lumpsor ridges
Flared, shallow, floorcovered with ridges
Ostio-caudaldifferentiation
Well developed Well developed Well developed Well developed
Crista superior Ridge-like Ridge-like Ridge-like Ridge-like
Crista inferior Absent Absent Absent Absent
Dorsal depression Elongated, shallow Elongated, shallow Elongated, shallow Elongated, shallow
Ventral depression Absent Absent Absent Absent
Rostrum shape Lobed Lobed Lobed Lobed
Rostrum size Short Long Long Long
Rostrum thickness Thick Thick Thick Thick
Antirostrum Short, broad Long or short, broad Long or short, broad Long, broad
Collum Bridge-like Bridge-like, poorly developed Bridge-like, poorlydeveloped
Bridge-like
Excisura Absent Absent or narrow, shallownotch, narrow angle
Absent Medium, shallow notch,medium angle
Note. LT: Total length.
1052 JAWADFISH
(1995), which suggests that it is impossible to provide a key for the
genera based on the shape of the otoliths for the Scaridae. The ante-
rior margin has an anterodorsal angle, which is high in the scarids. The
anterior margin cannot be described as one unit compared with the
other margins; its different components, rostrum, excisura and anti-
rostrum, were described separately. The three other margins of the
otolith, dorsal, ventral and posterior, show a low range in variation.
There were grades of lobation and irregularities across the different
length groups. Similar findings were reported for triplefin species by
Jawad et al. (2007) and for the greater lizardfish Saurida tumbil (Bloch
1795) by Jawad (2008).
The ostium and cauda in all six species studied are flared shape,
mostly shallow and their floor is covered with either lumps of differ-
ent sizes or ridges. The shape of the sulcus acusticus, the crista supe-
rior and crista inferior is also very variable. Shape and nucleus location
result from the release of soluble Ca2+ on the proximal side (Ibsch
et al., 2004), which in turn precipitates as CaCO3 crystals due to an
increasing alkaline gradient, from the sulcal area towards the otolith
edge (Gauldie & Nelson, 1990). As a result, the growth of the crista
superior and crista inferior is preferred and there is a more important
development on the mesial side. The macula is elongated and narrow
in teleosts and the crista superior and inferior are proportionally more
important than the colliculum (Ladich & Popper, 2001; Lombarte &
Fortuno, 1992; Popper & Hoxter, 1981). The macula faces the collum
and prevents otolith growth at this level (Lombarte & Fortuno, 1992;
Pannella, 1980; Popper & Hoxter, 1981). This is clear in some species
where the collum is either poorly developed or absent. Lombarte et al.
(2003) showed that the variability in the shape of the sagittal otolith is
related to genetic, ontogenetic and environmental factors in Merluc-
cius Rafinesque 1810. Previous studies on fossil and extant otoliths
Crista superior(a)
(b)
Posterior
An�rostrum
Anterrior
1·0 mm
Excisura
5.0kV 15.7 mm x30 (SE(M) 1.00 mm
Sulcus
Ventral
Os�um
Dorsal
Cauda
Crista superior
FIGURE 2 (a) The mesial surface of the left otolith of Hipposcarus
longiceps, 187 mm total length (LT), illustrating various features foundon the otolith and described in the text. (b) the mesial surface of theleft otolith of Scarus oviceps, 230 mm LT, illustrating excisura
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k) 1 mm
FIGURE 3 Otoliths of Hipposcarus longiceps taken from fish of
different total lengths: (a) 187 mm (right-side otolith), (b) 196 mm(right), (c) 209 mm (left), (d) 220 mm (right), (e) 230 mm (left), (f )245 mm (right), (g) 255 mm (right), (h) 266 mm (left), (i) 274 mm (left),(j) 328 mm (right), (k) 360 mm (right)
(a) (b) (c)
(d) (e) (f)
(g) (h) 1 mm
FIGURE 4 Otoliths of Scarus dimidiatus taken from fish of different
total lengths: (a) 114 mm (left-side otolith), (b) 115 mm (left),(c) 150 mm (left), (d) 160 mm (left), (e) 178 mm (left), (f ) 187 mm(right), (g) 195 mm (left), (h) 211 mm (right)
JAWAD 1053FISH
have demonstrated that the sulcus morphology usually is consistent
among the species of a single genus (Nolf, 1985, 2013), which sug-
gests that this feature is probably genetically controlled
(Gauldie, 1988).
Interspecific variation in sulcus morphology has previously been
recorded for only a few other genera. For example, the Euro-African
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k)
(l)1 mm
FIGURE 5 Otoliths of Scarus ghobban taken from fish of different
total lengths: (a) 163 mm (right-side otolith), (b) 185 mm (left),(c) 194 mm (left), (d) 205 mm (right), (e) 215 mm (right), (f ) 227 mm(right), (g) 231 mm (right), (h) 245 mm (right), (i) 271 mm (right),(j) 283 mm (right left), (k) 283 mm (left), (l) 319 mm (right)
(a) (b) (c)
(d) (e) (f)
(g) (h) 1 mm
FIGURE 7 Otoliths of Scarus oviceps taken from fish of different total
lengths: (a) 115 mm (right-side otolith), (b) 133 mm (left), (c) 149 mm(right), (d) 156 mm (right), (e) 208 mm (right), (f ) 214 mm (right),
(g) 215 mm (left), (h) 230 mm (right)
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
(j) (k) 1 mm
1 mm
FIGURE 8 Otoliths of Scarus rivulatus taken from fish of different
total lengths: (a) 126 mm (right-side otolith), (b) 138 mm (right),(c) 144 mm (right), (d) 155 mm (right), (e) 168 mm (right), (f ) 175 mm(left), (g) 182 mm (left), (h) 191 mm (left), (i) 204 mm (right),(j) 217 mm (left left), (k) 261 mm (right)
(a) (b) (c)
(d) (e) (f)
(g) (h) (i) 1 mm
FIGURE 6 Otoliths of Scarus niger taken from fish of different total
lengths: (a) 140 mm (right-side otolith), (b) 151 mm (left), (c) 161 mm(left), (d) 172 mm (right), (e) 183 mm (right), (f ) 197 mm (left right)(g) 202 mm (left), (h) 211 mm (right), (i) 215 mm (right)
1054 JAWADFISH
TABLE
7Geo
grap
hicalvariationin
themorpho
logy
oftheotolithofHippo
scarus
longiceps,Scarus
ghob
ban,
Scarus
niger,an
dScarus
rivulatus
SolomonIsland
sArabian
–Persian
gulf
SouthAfrica
New
Caled
onia
Taiwan
Present
stud
ySa
digh
zade
h&
Tuset,2
012
Smaleet
al.,1995
Rivatonan
dBourret,1
999
Linan
dChan
g,2012
Hippo
scarus
longiceps
201mm
L T–
––
–203mm
L T
Oval
––
––
Shap
e:Ovate
Coarsely
lobe
d–
––
–Posteriormargin:S
traigh
t
Short
––
––
Rostrum:L
ong
Smooth,p
resenc
eofinde
ntation
––
––
Ostium:F
loorwithco
arse
lumps,noinden
tation
anteriorly
Scarus
ghob
ban
194mm
L T215mm
L T245mm
L T319mm
L T191mm
L T210mm
L T241mm
L T375mm
L T
Oblong
Oblong
Oval
Pointed
Shap
e;cu
neiform
-elliptic
Shap
e:Circu
lar
Shap
e:Oblong
Rostrum:R
ounded
Long
Lobe
dLo
bed
–Rostrum:S
hort
Posteriormargin:
With
notch,
nearly
smooth
Dorsalmargin:E
marginate
–
Absen
tEmargina
teSlightly
emargina
te–
Excisura:
Nono
tch
Ven
tralmargin:
Crena
teVen
tralmargin:L
obed
–
Welld
evelope
dLo
ng,p
ointed
Lobe
d–
Cristasupe
rior
andinferior:
Poorlyde
velope
d
Rostrum:S
hort,b
road
Rostrum:E
marginate
–
–Bridg
e-like
Lobe
d–
–Collu
m:A
bsen
tPosteriormargin:P
ointed
–
–W
elld
evelope
dOstial
––
Cristasupe
rior:Poorlyde
velope
dSu
lcusacusticus:Ostio-cau
dal
–
–Dee
pDev
elope
d–
–Dorsalde
pression:
Shallow
Collu
m:A
bsent
–
–245mm
L TW
elld
evelope
d–
–245mm
L TCristasuperior:Absent
–
–Slightly
emargina
teW
ithridg
es–
–Ven
tralmargin:
Crena
teOstium:S
mooth
–
–Pointed
––
–Rostrum:B
road
––
–Bridg
e-like
––
–Collu
m:A
bsen
t–
–
–W
elld
evelope
d–
––
Cristasupe
rioran
dinferior:
Poorlyde
velope
d–
–
–Dee
p,floorwithridg
es–
––
Cau
da:S
hallo
w,smooth
floor
––
–Dee
p,floorwithridg
es–
––
Ostium:S
hallo
w,floorwithlumps
––
–319mm
––
–430mm
––
–Oblong
––
–Sh
ape:
Circu
lar
––
–Coarse
lobe
d–
––
Dorsalan
dve
ntralm
argins;
fine
cren
ate
––
–Lo
bed,
long
––
–Rostrum:B
road
,veryshort
––
–W
elld
evelope
d–
––
Cristainferior;po
orlyde
velope
d–
–
–Bridg
e-like
––
–Collu
m:A
bsen
t–
–
JAWAD 1055FISH
TABLE
7(Continue
d)
SolomonIsland
sArabian
–Persian
gulf
SouthAfrica
New
Caled
onia
Taiwan
–Dee
p,floorwithridg
es–
––
Cau
da:S
hallo
w,floorsm
ooth
––
–Dee
p,floorwithridg
es–
––
Ostium:S
hallo
w,floor
Withfine
lumps
––
–W
elld
evelope
d–
––
-–
–
Scarus
niger
161mm
L T161mm
L T202mm
L T157mm
L T158mm
L T206mm
L T
Oval
Welld
evelope
dW
ithseve
rallobe
sSh
ape:
Circu
lar
Collu
m:A
bsent
Posteriormargin:
Withonelobe
Lobe
dBridg
e-like
Broad
lypo
inted
Dorsalmargin:
Withinde
ntation
above
caud
aCristainferior:Poorly
dev
eloped
Antirostrum;finepointed
Coarse
lobe
dDee
p,floorwithridg
esW
ithco
arse
ridge
Ven
tralmargin;
fine
cren
ate
Ostium
andcauda:
Shallow
withsm
ooth
floor
Ostium
andcauda:
Smooth
withoneridge
Coarse
lobe
dAbsen
tW
elld
evelope
dPosteriormargin:
Curve
dVen
traldep
ression:S
hallow
Collu
m:P
oorlydev
eloped
Long
,lobe
d215mm
–Rostrum;sho
rt,roun
ded
220mm
–
Dee
p,floorwithridg
esSlightly
cren
ate
–Cau
da:S
hallo
w,floorsm
ooth
Ven
tralmargin:C
oarsely
lobed
–
Floorwithlumps
and
ridg
esat
bottom
Broad
lypo
inted
–Ostium:F
loorwithcentralridge
Posteriormargin;straigh
t–
215mm
L TBroad
lypo
inted
–286mm
L TRostrum:W
ithtw
osm
all
projections
–
Withbroad
fissure
Poorlyde
velope
d–
Posteriormargin:
Pointed
Antirostrum:S
hort,p
ointed
–
Long
,sha
llow
Floorwithlumps
–Dorsalde
pression:
Absen
tCau
da:
Floorwithridge
s–
Floorwithco
arse
lobe
san
dridg
es–
–Cau
daan
dostium:F
loorsm
ooth
––
Welld
evelope
d–
–Cristasupe
rior:Poorlyde
velope
d–
–
Scarus
rivulatus
217mm
L T261mm
L T291mm
L T225mm
L T
Shap
e:Domed
withlobe
sElliptic
Shap
e;oblong
Dorsalmargin:E
marginate
Dorsalmargin:
Withde
epfissure
Finecren
ate
Dorsalmargin:C
oarse
lobed
Posteriormargin:
Withshallow
fissure
Rostrum:N
arrow,
fine
pointed
Long
,broad
lylobe
d,lower
edge
coarsely
lobe
dRostrum:S
hort,b
oard,lower
edge
emarginate
Rostrum:B
road
lypointed
Ecisura:W
ith
deve
lope
dprojection
Widewithwideno
tch
Ecisura:N
arrow
with
narrow
notch
Antirostrum:S
hort,b
road
Floorwithlumps
and
ridg
esCau
daan
dostium:
Floorsm
ooth
1056 JAWADFISH
species of Merluccius (Merlucciidae) can be separated from the
American ones, based on the variation of the sulcus; therefore, the
variation in sulcus runs parallel to zoogeography and phylogeny
(Torres et al., 2000). However, the variation in sulcus is also the result
of an acoustic specialization and thus interspecific sulcus variation
may also result from ecomorphological adaptations (Ramcharitar et al.,
2004; Popper et al., 2005). The shape of the sulcus is important in
sciaenid otoliths (Nolf 1985). Apart from sulcus morphology, the cor-
relation between particular otolith features (e.g., the proportions of
rostrum and antirostrum) and biological functions (e.g., swimming abil-
ity, feeding, or other activities) has not yet been established (Popper
et al., 2005). Considering the variety of teleosts, there may be some
correlation between the length of the rostrum and the swimming abil-
ity (Nolf, 1985; Volpedo & Echeverría, 2003), but this correlation is
not significant in the discrimination of closely related species
(Reichenbacher et al., 2007). In this study, the general morphology of
the rostrum in the six species examined is broad, short and lobed and
such a consistency agrees with Reichenbacher et al. (2007).
Selective pressures can sometimes act on sagittae so that their
morphology meets specific auditory needs (Gauldie, 1988; Platt &
Popper, 1981; Popper & Coombs, 1982). Constraints in terms of phys-
ical packing of sagittae within the skull have also been cited in several
studies, especially those examining closely related species with large
sagittae (Arellano et al., 1995; Gaemers, 1984; Smith, 1992). Otolith
growth has long been shown to have a positive relationship to fish
age (Sabetian et al., 2015), but otoliths in very large fishes can be
much smaller than those in small fishes and vice versa (Campana,
2004). A limited comparison made by Friedland and Reddin (1994)
suggests that the shape of otolith is also influenced by genetics. Dif-
ferent morphologies in otoliths of a species obtained from different
localities have been observed. The otoliths of the six scarid species
were compared with those of similar size from other localities and this
comparison is given here (Table 7). Rivaton and Bourret (1999) pro-
vided an image of an otolith from a S. rivulatus of 470 mm length, no
comparable length group was available for the present study. The oto-
lith described by Rivaton and Bourret (1999) is oblong with dorsal
margins nearly smooth and ventral margins crenulated. The rostrum is
long and lobed, there is no collum, the cauda is deep with a smooth
floor and the crista superior extends over the whole length of the
ostium. The largest otolith of S. rivulatus obtained for the present
study is elliptic and there are more differences in several morphologi-
cal aspects. The description of the otolith by Rivaton and Bourret
(1999) complements our knowledge of the otoliths of S. niger.
The results of this study highlight a degree of intraspecific varia-
tion in the shape of the otolith for each species, such as the angle of
the rostrum (straight v. oblique), or the shape of the excisura
(Tables 1–6). This study was unable to highlight enough variability in
otolith shape to use as a key for individual species from the Solomon
Islands, but there is enough variability to distinguish small–juvenile
from large–adult individuals provided the species has been rigourously
identified upon capture. Further investigation is required, including a
comparative study of the shape and geometry of the sagittal otolith,
to add further taxonomic characters for the identification of these
species from the Solomon Islands.
ACKNOWLEDGEMENTS
My sincere thanks are due to those people who collected the fish
specimens and extracted their otolith. Also to the Auckland University
of Technology (AUT) for the SEM imaging of the otoliths and for edit-
ing them in Photoshop.
ORCID
Laith A. Jawad https://orcid.org/0000-0002-8294-2944
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How to cite this article: Jawad LA. A comparative morpho-
logical investigation of otoliths of six parrotfish species (Scari-
dae) from the Solomon Islands. J Fish Biol. 2018;93:
1046–1058. https://doi.org/10.1111/jfb.13787
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