J . ZOO/., Land. (1982) 196,371-384

Feeding behaviour and feeding ecology of the Octocorallia (Coelenterata: Anthozoa)

JOHN B. LEWIS The Redpath Museum and The Institute of Oceanography, McGill University, Montreal,

Canada

(Accepted 9 June 1981)

(With 1 plate and 3 figures in the text)

The feeding behaviour of some 30 species of Octocorallia was examined in the laboratory and in the field. All of the species from the Orders-Alcyonacea, Gorgonacea, Stolonifera and Telestacea, appear to have a common, basic feeding strategy. Fine particulate matter and zooplankton were captured in a raptorial manner by the tentacles and pinnules. Upon capture of food particles, the tentacles were flexed rapidly inwards and closed or wiped across the mouth. At the same time the mouth opened and ingestion was accomplished by directional ciliary currents in the mouth and the pharynx. Food capture by means of mesenterial filaments, mucus strands or ciliary currents was not observed. The examination of gut contents of Alcyonacea showed that they feed upon zooplankton. Epibenthic copepods from the demersal coral reef zooplankton were an important element of the diet of alcyonaceans.

Contents

Introduction . . . . . . Materials and methods . . . . Feeding observations . . . .

The Alcyonacea . . . . The Gorgonacea . . . . The Stolonifera . . . . The Telestacea . . . .

Food sources and rate ofdigestion Discussion . . . . . . References . . . . . . . .

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Page . . 371 . . 312 . . 373 . . 313 . . 317 . . 319 . . 380 . . 381 . . 382 . . 383

Introduction

Feeding has been examined in a number of species of the Octocorallia but there has not been any general agreement on precisely how they feed nor upon their sources of food. The alcyonacean Alcyonium digitatum, for example, was observed by Pratt (1 906) to capture small zooplankton and other particulate matter with its tentacles, but by Roushdy & Hansen (1961) to filter phytoplankton from the water. Gohar (1940) was unable to observe any feeding in the alcyonacean family Xeniidae despite repeated attempts. In the Gorgonacea, Kinzie (1 970) found that a number of species could capture fine particulate matter but only one, Eunicea clavigera, was able to seize and ingest actively-swimming zooplankton. Leversee ( 1976) recorded that another gorgonid, Leptogorgia virgulata, readily captured larvae of the brine shrimp Artemia, while Mariscal & Bigger (1977) found that the same

371 0022-5460/82/030371+ 14 $02.00/0 0 1982 The Zoological Society of London

3 12 J . LI. L k W l b

species fed indifferently. Very little feeding activity was observed in the gorgonid Pseudoplexaura crassa by Chester (1 9 13) but Abel (1970) reported that Corallum rubrum readily captured food particles with its tentacles. Feeding in the Pennatulacea was described by Hoare & Wilson (1977) who showed that Virgularia mirabilis was capable of capturing small, actively-swimming organisms. They also considered that Virgularia was capable of filtration of suspended matter. Kastendiek (1 976) observed that another pennatulacean, the sea pansy Renilla kollikeri, had great difficulty in catching motile prey.

As to the food sources of the Octocorallia, a number of authors have suggested that the orientation of gorgonids, at right angles to currents, is an adaption for the capture of zooplankton (Theodor, 1963; Wainwright & Dillon, 1969; Grigg, 1972; Leversee, 1972; Kinzie, 1973). Direct evidence of zooplankton capture by gorgonids was obtained by Bayer (1956) and by Grigg (1970), both of whom found mollusc larvae in the gastric cavities of several species. In the Alcyonacea, however, Pratt (1906) found only a few instances of remains of minute crustacea in the coelentera of many genera.

There is obviously a need for more detailed observations on the feeding biology of the Octocorallia in order to evaluate the conflicting observations in the literature. The following account describes the feeding behaviour of freshly collected live colonies of a number of species from four Orders of octocorals. Feeding strategies are related to natural and artificial food sources, and an attempt is made to provide a general outline of feeding behaviour in the Octocorallia.

Materials and methods

All the colonies examined were collected from shallow water on coral reefs and transported to the laboratory without exposure to air. In the laboratory they were maintained in tanks of running sea water and their feeding behaviour examined within a few hours. Ground dried fish (Tetramin), finely ground fish muscle stained with alcian blue or methyl blue, freshly hatched Artemia larvae and freshly caught net zooplankton were used as food sources.

Alcyonacea were collected from Britomart and Davies Reefs of the Great Barrier Reef and at Magnetic Island, Townsville, Queensland. Gorgonacea were also collected from Britomart and Davies Reefs and in the Caribbean at the Bellairs Research Institute of McGill University in Barbados. Observations on the Stolonifera (Tubipora) were made at Heron Island, Queensland and on the Telestacea (Telesto) at Barbados. A list of the species studied is provided in Table I.

In addition to the colonies maintained alive, others were also preserved in 10% neutral formalin or 70% alcohol for later examination and identification. The gut contents of Alcyonacea were examined by first making longitudinal cuts through the colonies and exposing polyp sections, gastrodermal canals and soleniae. Prey organisms were dissected out with fine needles and retained for identification.

Scanning electron microscopy of soft tissue was undertaken at James Cook University, Townsville, Queensland. Fully expanded specimens were plunged into 10% formalin for 10 min, removed, and the polyps teased open with fine needles. Colonies were then carefully returned to formalin without disturbing the open posture and fixed for 48 h. Some contraction of the polyps took place under this procedure but no more than with various attempts to obtain expanded polyps by anaesthesis. After fixing, the colonies were washed briefly in distilled water and dehydrated in graded ethanols. This was followed by low temperature freeze-drying, using liquid nitrogen after a modification of the method of Darley & Lott (1973). Specimens were then mounted, coated with gold palladium and examined under a scanning electron microscope.

In order to determine rates of digestion, experimental feeding of Alcyonacea with Arternia larvae was undertaken with laboratory-maintained colonies. Colonies were fed larvae for one hour and then

FEEDING IN OCTOCORALS

TABLE I List of Octocorallia e.uaminedfirfiediizg behaviour

313

Species Locality

Alcyonacea Cladiella sphaerophora (Ehrenberg) Lohophytum cristagalli Von Marenzeller Sarcophyton trocheliophorum Von Marenzeller Sinularia densa (Whitelegge) Sinularia microspiculata Tixier-Durivault Sinularia capillosa Tixier-Durivault Sinularia rnicroclavata Tixier-Durivault Sinularia inelegans Tixier-Durivault Sinularia,firma Tixier-Durivault Sinularia n. sp. Capnella lacert iliensis Mac fad yen Dendronephthya sp. Lemnalia sp. Paralemnalia digitiformis Mac fad yen Eflatournaria sp. Xenia elongata Dana Hetero-xenia elizabethae Kolliker

Isis hippuris Linnaeus Rumphella aggregata (Nutting) Juncellafragilis Ridley Subergorgia reticulata (Ellis and Solander) Briareum asbestinum (Pallas) Eunicea tourneforti Milne Edwards and Haime Pseudopterogorgia americana (Gmelin) Pseudopterogorgia acwosa (Pal fas) Muriceopsis,flavida (Lamarck) Plexaurajlexuosu Lamouroux Gargonia ventalina Linnaeus

Telesto riisei (Duchassing and Michelotti)

Tubipora musica Linne

Gorgonacea

Telestacea

Stolonifera

Heron Island, Queensland Britomart Reef, Queensland Britomart Reef, Queensland Britomart Reef, Queensland Magnetic Island, Townsville Magnetic Island, Townsville Magnetic Island, Townsville Magnetic Island, Townsville Davies Reef, Queensland Magnetic Island, Townsville Britomart Reef, Queensland Davies Reef, Queensland Davies Reef, Queensland Britomart Reef, Queensland Britomart Reef, Queensland Davies Reef, Queensland Britomart Reef, Queensland

Britomart Reef, Queensland Britomart Reef, Queensland Davies Reef, Queensland Britomart Reef, Queensland Barbados Barb ado s Barbados Barbados Barbados Barbados Barbados

Barbados

Heron Island, Queensland

removed to isolation tanks. Fed colonies were then fixed in alcohol at 2-, 6-, 12- and 24-h intervals, sectioned and the larvae removed for examination.

Feeding observations The Alcyonacea

A common pattern of food capture and feeding was observed in the laboratory amongst a wide range of species examined, with the exception of the family Xeniidae. All polyps preparing to capture food were well expanded with highly reactive tentacles and pinnules. Food was captured in a raptorial manner by the tentacles and pinnules and was subsequently transferred to the mouth and ingested.

3 14 J . 8. LEWIS

i t

FIG. 1. Feeding behaviour in the alcyonacean, Savcophyton trocheliophorum: (a) Preparatory feeding posture of polyp. (b) Tentacles of polyp turned inwards to cover mouth. (c) Tentacles of polyp folded inwards to form basket- like structure. (d) Complete contraction and folding of tentacles of polyp.

The fully expanded alcyonacean polyp is characterized by a flattened or slightly concave oral disc encircled by fully extended tentacles which lie in the same plane as the disc but are curved slightly inward or curled backward. The pinnules when fully expanded are elongate, tapered to a point and lie in the same plane as the oral disc, in a single row on each side of the tentacle. This position may be regarded as the preparatory feeding posture (Fig. 1 (a)).

Fully expanded polyps were frequently observed in the field in those genera bearing large autozooids (Sarcophyton) and in these and genera bearing smaller polyps (Sinuluria, Paralemnalia) in the laboratory. Fully expanded polyps were characteristic of fresh, healthy colonies in the laboratory and expansion was observed during both day and night.

There was a good deal of spontaneous nervous activity in expanded polyps, both individual movement of a single tentacle and coordinated movement of all the tentacles.

FEEDING I N OCTOCORALS 375

Periodically, one or more of the tentacles gave a sharp oral flexion at the same time as the oral disc contracted. The pinnules also showed considerable nervous activity by constantly waving, flicking and contracting.

Coordinated spontaneous activity occurred throughout colonies of several taxa. In the Xeniidae, for example, rhythmic pulsation of all or several of the polyps in the colony was observed. This activity has also been reported by Gohar (1940). In Xenia elongata and Heteroxenia elizabethae the polyps opened and closed rhythmically while the tentacles waved about in the water and were intermittently wiped across the mouth. In several species of Sinidaria the tentacles of expanded polyps were flexed backwards or aborally at irregular intervals, then closed rapidly over the oral disc and opened again.

In the laboratory, the activity of the polyps was increased in the presence of prey and several modes of food capture were observed. A comparison of polyp sizes and feeding abilities is shown in Table 11. Small particles of food adhered to the pinnules which then contracted or folded inward. At the same time the tentacle flexed rapidly inward towards the mouth (Fig. l(b)). As the tentacle swept across the oral disc, the mouth opened and the particle was sucked into the pharynx. The tips of the tentacles were frequently stuffed into

TABLE 11 Polyp sizes (expanded) and feeding ahilities of Octocorallia (measurements in mm: cl-tentacles closing,

wr-ten tacks ulr it h ing)

Species

Feeds on 7 ___-_ ~ -* Polyp Tentacle

diameter length No.of Ground Brine Live Feeding (mm) (mm) pinnules fish shrimp plankton type

Lohophyium cristagalli Surcvphyton trocheliophorum Sinidaria densa Sin ular ia cap i l h a Capnella lacertilliensis Dendronephthyu sp. Lemnulia sp. Paralemnulia digitiformis Efflatournaria sp. dYmia ekmgata Heteroxenia elizahcthae Isis hippurus Rumphella aggregata Junc,ella.fiagilis Suhergorgia reticulala Briareum ushestinum Eunicra tournefiwi,uli Pseudopierogorgia umeric~ana Pseudopterogorgia acerosa hf uriceopsis fluvida Pl..xauru,fft,.xuo.Pa GorKonia ventulina Telesro rii.rci Tuhipora rnusica

2.5-4.0 2.5-4.0 4.0-5.0 1.0-1 ' 5 0'6-1 '0 3.0-4.0 2.0-3.0 1.0-2.0 5.5-8.0

70.0-30.0 12.0-14.0 2.5-3 '0 7.0-2.5 2.0-2.5 1.8-2.1

17.0-1 8.0 1 .0-1.5 1.5-2'5 1.8-2.5 2.5-3.5 2.0-3.0 1.5-1.7 5.5-8.0

12'0-1 4.0

1.0-1 ' 5 1 '0-1 ' 5 2.0-2.5 0.3-0.5 0 , 3 4 5 1 '0-1 ' 5 0.8-1.0 0 , 4 4 8 2.0-3.0

100- 1 5.0 6.0-7.0 1.0-1.2 0.8-1 '0 0.7-1.0 0.7-0.8

0 . 4 4 5

0 . 7 4 9 1.0-1.5

0 , 7 4 8

5.0-6.0

7.5-8.0

0.6-1 .O

1.0-1.5

2 '0-3 '0

10-12 6-8

10-12 3-4 3 4

10-1 2 6-8 4-6

10-12 30-36 50-60 6-8 6-8 6-8 6-8

20-30 5-6 8-10 8-10 4-5 8-10 3-5

10-12 17-20

J J J J J J J \i J

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cl, wr cl cl cl cl

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cl wr wr cl cl cl cl

cl, W f

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cl, W f

316 J. B. LEWIS

the open mouth. After removal of the food particle the tentacle flexed outward again and assumed the expanded position. This sequence of events usually involved a single tentacle and was repeated by it or other tentacles.

The capture of larger food objects such as brine shrimp larvae or small copepods was accomplished by coordinated movement of several or all of the tentacles. The prey adhered to both the pinnules and the tentacles. Upon capture, the tentacles folded inward rapidly to form a basket-like structure holding the prey within (Fig. l(c)). The tentacles were then pressed downwards on the oral disc, the mouth opened and the prey was sucked into the pharynx.

?&./A 7

FIG. 2. Polyps expanded for feeding in (a) Xenia elonguta and (b) Telesto rzisei

Larger actively-swimming prey (> 2-3 mm) were also captured by all the tentacles which folded inward and contracted. Complete coverage of the oral disc resulted as the tentacles formed a fist-like structure (Fig. l(d)). As ingestion began the polyp stalk contracted and the whole polyp was frequently withdrawn into the colony matrix.

Neither food capture nor ingestion of food particles was observed in the laboratory in the two species of the family Xeniidae examined. Similar results were obtained by Gohar (1940) who contended that the Xeniidae depend upon their zooxanthellae for nutrition. However, the continuous writhing and flexing movement of the tentacles of Xenia was similar to the feeding activity in other genera. Furthermore, the presence of identifiable food in the polyps and coelentera of Xenia elongata and in Heteroxenia elizabethae as well as field observations of food capture indicate that heterotrophic feeding does take place.

The polyps of Xenia elongata were observed in a fully expanded condition in the field and an expanded polyp is shown in Fig. 2(a). From this Figure it may be seen that the tentacles are very elongate and the slender, pointed pinnules lie in a single row along each tentacle. Because of the tentacle length and their passive response to water movement the polyps apparently do not assume a fixed preparatory feeding posture but are swept back and forth in the water. Judging from observations made on the feeding of polyps with long tentacles in the Gorgonacea (next section), food particles are captured by the tentacles and pinnules, the tentacles are wiped across the mouth and the prey is subsequently ingested.

FEEDING IN OCTOCORALS 377

In contrast to other Anthoza (Lewis & Price, 1975; Lewis, 1978), no mucus nets or strands were observed in feeding polyps of any of the alcyonacean species examined. However, mucus is produced, often in copious amounts, by a number of species. Colonies of Sarcophyton trocheliophorum when excised in the field will emit long strings of a very elastic mucus from cut surfaces. Several species of Sinularia produce mucus when handled and Gohar (1 940) has reported mucus production by the Xeniidae. Nevertheless, the routine use of mucus nets for the capture of food was not observed in the Alcyonacea.

Nor were any ciliary currents observed on the tentacles, pinnules or oral discs of any of the species examined. Ciliary currents were present around the mouth and in the pharynx and were functional in the process of food ingestion. Detailed examination of the surface morphology of the polyps in four species (Sarcophyton trocheliophorum, Lobophytum cristagalli, Sinularia capillosa and Sinularia densa) showed bands of cilia around the mouth but only scattered cilia and flagella on the disc and tentacles. Plates I(a) and (b) show the mouth region with the dense cilia in Sarcophyton trocheliophorum and Sinularia densa. Plates I(c) and (d) show a tentacle of Sarcophyton and a pinnule of Sinularia without cilia but with the surface covered with densely packed microvilli. The surface of the colony disc of Sarcophyton trocheliophorum around the siphonozooids is also densely ciliated. In Plate I(e) cilia and scattered flagella are shown around the edge ofa retracted siphonozooid.

Finally, the use of mesenterial filaments for feeding was never observed in any of the species examined. Although this feeding strategy is common in other Anthozoans (Goreau, 1956; Lewis & Price, 1975; Lewis, 1978), it is not reported in the literature (Hyman, 1940; Barnes, 1980) as occurring in Octocorallia.

The Corgonacea Feeding behaviour in the gorgonids examined was similar in most respects to feeding in the

Alcyonacea. However, distinctions could be made between responses in species with short and long tentacles. A comparison of polyp sizes and feeding abilities is contained in Table 11. In short tentacle forms, such as Gorgonia ventalina, a preparatory feeding posture was characterized by fully expanded tentacles and pinnules projecting from a slightly concave oral disc. Both tentacles and pinnules were highly reactive in the presence of particulate food. A gorgonid polyp in this feeding posture is shown in Fig. 3(a).

In long tentacle forms such as Briareum asbestinum (Fig. 3(c)), the expanded tentacles did not remain in a fixed position but were constantly waved about. This was due partly to coordinated spontaneous nervous activity and partly to water movement. Thus there was not a fixed preparatory feeding posture but rather a condition of continuous tentacle movement.

Upon the addition of fine particulate food the polyps of both forms responded by rapid inward flexion of one or more tentacles, contraction of the pinnules and the stroking or wiping of tentacles across the mouth and the oral disc. At the same time the oral disc contracted, the mouth opened and food particles adhering to the tentacles were sucked into the pharynx.

In the presence of larger food particles such as live zooplankton, coordinated response of all the tentacles occurred but two modes of action were observed. In species with short tentacles, food capture was achieved by a rapid flexion or folding inwards of all the tentacles to form a fist-like structure over the oral disc. Prey was held within this structure and ingested as the mouth opened. Figure 3(b) illustrates the prey capture process in Gorgonia ventalina.

378 J . 9. LEWIS

PLATE I. Surface features of polyps of Alcyonacea. Scanning electron micrographs (SEM) of: (a) Surcophyton trocheliophorum, mouth showing dense ring of cilia (x 600). (b) Sinufaria densa. mouth region showing dense ring of cilia (x 600). (c) Surcophytun trucheliuphurum, tentacle surface showing densely packed microvilli ( x 600). (d) Sinuluriu densa, pinnule showing densely packed microvilli (x 1800). (e) Sucrophyton trocheliophorum, rim of retracted siphonozooid showing cilia and scattered flagella (x 1200).

F E E D I N G IN OCTOCORALS

( b )

3 79

I Ornrn -

Re. 3 Feeding behaviour in the Gorgonaceae. (a) Gougonla ventalina. preparatory feeding posture of polyp (b) Gorgonia ventahna, food capture by polyp. (c) Briareum asbestinurn, expanded polyp. (d) Briareum arbatinurn, food capture by polyp.

In species with long tentacles, closure of the tentacles over entrapped prey also occurred but individual tentacles continued to writhe about over the oral disc. Prey capture thus appeared to be a continuous action rather than a single event as in short-tentacle species. The prey capture reaction in Briareurn asbestinurn is shown in Fig. 3(d).

As in the Alcyonacea, no ciliary currents were observed in the gorgonids examined except in the region of the mouth and in the pharynx. Capture of food by extruded mesenterial filaments was not observed, nor was food capture by mucus strands or nets seen. Mucus is produced by some gorgonids, for example in Pseudopterogorgia arnericana (Bayer, 196 I ) , but was not seen to be utilized for feeding.

The Stolonifira Feeding behaviour was examined in a single species of this Order, the organ pipe coral

Tuhipora rnusica. Food capture was achieved by raptorial movement of the tentacles and pinnules and was similar to the behaviour of species of the Gorgonacea with long tentacles. Expanded tentacles and pinnules were waved about continuously due to water currents and

380 J. B. LEWIS

nervous activity. Fine food particles adhering to the tentacles and pinnules were wiped across the mouth and oral disc and ingested in the open mouth. Larger food particles such as actively-swimming zooplankton were captured by coordinated movement of all the tentacles to form a basket-like structure over the oral disc.

The Telestacea Examination of live colonies of the Caribbean telestacean, Telesto riisei, showed that food

capture and ingestion were essentially similar to other Octocorallia with short tentacles. The polyp assumed a characteristic preparatory feeding posture (Fig. 2(b)) and in the presence of food the tentacles turned rapidly inward towards the mouth to form a fist-like structure covering the oral disc. At the same time the mouth opened and the prey was ingested. Individual tentacles holding prey were wiped across the oral disc.

TABLE I11 Summary ofprey organisms ofAlcyonacea

No. of No. of No. of No. of Prey organisms colonies colonies polyps prey/ ( )Yo total prey

Species examined with prey examined 10 polyps [ ] size range in mm

Xen ia 25 elongafa

Sacrophyton 25 lrocheliophorum

Lemnalia sp. 12

Lobophytum 10 cristagalli

Sin ularia 12 densa

Sinularia 15 capillosa

Sinularia 12 m icrocla vata

17 727 3 Harpactacoid copepods (71), [0.5-1.5]; calanoid copepods (8), [1.0-1.5]; polychaetes(l8), [0.5-1.0]; mollusc larvae (2); invert. eggs (I), [O. 14.21

[0.5-1.5]; calanoid copepods (lo), [1.0-1.5]; mollusc larvae (2 ) ; invert. eggs (12), [0 .142] ; Unident. (4)

[ < 1.01; calanoid copepods (1 0), [ 1.0-1.51; mollusc larvae (33); Unident. ( 1 5)

copepods (5); mollusc. larvae (65) , [0 .245] ; Unident. (1 5)

8 100 < 1 Mollusc larvae (5 I ), [0 .243]; nauplii (40), [0.34.5]; Unident. (9)

[ < 1.01; mollusc larvae (7); invert. eggs (3 I ) , [0.2-0.3]; nauplii (10); Unident. (43)

6 176 2 Harpactacoid copepods (10); mollusc larvae (52); invert. eggs (29); Unident. (9)

22 2 74 7 Harpactacoid copepods (72),

4 200 < 1 Harpactacoid copepods (42),

9 397 1 Harpactacoid copepods ( 1 5); calanoid

12 769 2 Harpactacoid copepods (9),

FEEDING IN OCTOCORALS 38 I

Food sources and rates of digestion

The results of the examination of gut contents o f seven species of Alcyonacea are shown in Table 111. There was a marked similarity of prey types and dietary composition amongst the species examined. Mollusc larvae, copepods and invertebrate eggs were the dominant prey types and all prey were small, less than 2 mm in length.

Mollusc larvae, both gastropods and pelecypods, were found in all species. Copepods were observed in all species but Sinularia densa, and occurred most abundantly in genera.with large polyps (Xenia and Sarcophyton). Although none of the copepods could be identified positively to species because o f decomposition, 75-9Oo/o could be identified as harpactacoid epibenthic forms. Each of the species of Alcyonaceans examined also contained unidentifi- able prey remains in various stages of decomposition.

TABLE 1V Rates of digestion ofArternia larvae by Sarcophyton trocheliophorurn

Digestion time (h) Digestion stage* No. of Artemia

2

6

12

24

25 0 0 0

1 17 15 3

0 21 35

8

0 2 4

36

*Description of stages; I . Appendages present, no apparent digestion. 2. No appendages, body pear-shaped, prominent eye spot. 3. Body rounded, no eye spot, orange colour. 4. Body broken into fragments, oil droplets.

Prey species were not found in all the colonies examined but the frequency of occurrence was higher than reported by Pratt (1 906). The abundance o f prey in individual colonies was generally low and varied between less than one prey/lO polyps in Sinularia densa and seven prey110 polyps in Sarcophyton trocheliophorum.

Estimates ofthe rate ofdigestion ofArtemia larvae consumed by Sarcophyton are shown in Table IV. Digestion was well advanced within 6 h of capture and 88% of the larvae consisted ofbroken fragments and oil droplets 24 h after feeding.

382 J . 8 . LEWIS

Discussion The clear definition of feeding behaviour in all of the species examined was dependent

upon the use of freshly collected colonies. Colonies held in tanks in the laboratory for periods longer than 24 h were seldom fully expanded and were slow or inactive feeders. Pratt (1 906) has also remarked on the loss of sensitivity of colonies of alcyonaceans kept for pro- longed periods in captivity. The lack of agreement of earlier authors on precisely how octocorals feed may be attributed to failure to make observations on freshly collected material.

All of the Octocorallia examined appear to have a single, common feeding strategy. Particulate matter and zooplankton were captured in a raptorial manner by the tentacles and pinnules. Fine particulate matter was captured by individual tentacles while larger particles were trapped by coordinated movement of all the tentacles. Upon contacting food particles, the tentacles were flexed rapidly inwards and wiped across or closed over the mouth. At the same time, the disc contracted and the mouth opened. Ingestion was accomplished by directional ciliary currents in the mouth and pharynx which detached food carried there by the tentacles. Food capture by means of mesenterial filaments, by mucus strands and by ciliary currents on the tentacles or disc was not observed.

Thus, Octocorals differ from the other Anthoza which have more than one feeding strategy. In the Actinaria, Scleractinia (Lewis & Price, 1975) and Antipatharia (Lewis, 1978), polyps capture food with their tentacles but can also entrap prey by means of ciliary currents and mucus nets. In the Zoantharia some species are primarily tentacle feeders while others are muco-ciliary feeders (Reimer, 197 1 ; Trench, 1974; Sebens, 1977). Reports of the ability of some octocorals to filter fine particulate matter from water (Roushdy & Hansen, 1961; Hoare &Wilson, 1977) are likely due to the transport of such material by the ciliary currents in the mouth and pharynx. The absence of any filtration structure in octocorals and the lack of ciliary-mucoid mechanisms make it unlikely that any significant amount of food could be accumulated in this way.

This view of octocorals as simple raptors was supported by examination of the surface morphological features of the polyps. A dense circle of cilia surrounded the mouth and drove currents into it. There were no ciliary tracts on the disc, or the tentacles as has been observed in the Scleractinia (Lewis & Price, 1976). Instead, the surface of the disc and the tentacles was covered with densely packed microvilli. Microvilli are usually thought to have an absorptive function, and Mariscal & Bigger (1 977) have suggested that the microvilli in octocorals were able to take up dissolved organic matter from the surrounding water. However, microvilli with associated cilia or flagellae have also been shown to have a sensory function in octocorals (Mariscal & Bigger, 1976). In the gorgonid Leptogorgia virgulata a sensory system of microvilli and ciliary cones is associated with nematocysts and appeared to be tactile in function. In phoronids, bryozoa and brachiopods, cilia with rings of microvilli around their bases may serve as particle detectors (Gilmour, 1978, 1979).

Although rings of cilia or flagella were not observed in the Alcyonacea examined, nevertheless the prominence of the dense cover of microvilli in this group and the high degree of nervous activity of tentacles and pinnules suggest that the microvilli have a sensory function, possibly acting as chemoreceptors. The presence of numerous sensory receptors on the pinnules would obviously be of advantage in raptorial feeding.

Feeding observations indicated that the Octocorallia were capable of capturing food over a wide size range, depending upon the size of the polyps. The dominance of epibenthic

FEEDING I N OCTOCORALS 383

harpactacoid copepods in several species suggests that they were acquiring a large proportion oftheir food from the demersal reef plankton (Alldredge & King, 1977; Hamner & Carleton, 1979). However, they fed also upon pelagic plankton for the molluscan larvae found in many species were presumably part of the pelagic meroplankton carried onto the reef.

The occurrence of small numbers of prey per polyp was consistent with observations on other Anthoza. Similar results were obtained by Sebens (1977) who found between less than one and three prey items per 10 polyps amongst four zoanthid species. Porter (1974) found between less than one and four prey organisms per 10 polyps in the reef coral Montastrea ca vernosa.

The rate of digestion appeared to be much slower in the Alcyonacea than in the Scleractinia. Porter (1 974) found that brine shrimp were rapidly digested by Muntastwa caveriiosa with only the black granular eye distinguishable within one to two hours. Yonge (1930) found digestion to be well advanced after only two or three hours in a number of Australian corals.

I a m grateful to Dr John S. Bunt and Dr David Barnes of the Australian Institute of Marine Sciences, to Dr A. J. Bruce at Heron Island and to Dr Finn Sander of the Bellairs Research Institute of McGill University, for the use of laboratory facilities. Dr P. Alderslade of the Roche Institute of Marine Pharmacology at Dee Why, N.S.W., Australia, very kindly provided identifications of Alcyonacea, Dr T. Ikeda of the Australian Institute of Marine Sciences identified the copepods. Dr F. M. Bayer of the U.S. National Museum identified the Gorgonacea. I am also grateful to Professor C. G. Alexander of the James Cook University, Townsville, Queensland, for assistance with the scanning electron microscope operation, and to Mrs Susan Gabe for the preparation of the drawings.

This study was supported in part by a grant in aid of research and a travel grant from the Natural Sciences and Engineering Research Council of Canada.

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