Feeding ecology of the earliest vertebrates
~oological Journal of the Linnean SocieQ (1984), 82: 261-272. With 2 figures
Feeding ecology of the earliest vertebrates
EARLY FISH ECOLOGY 263
paper is based on the tenet that pre-vertebrates and ancestral vertebrates were
microphagous suspension feeders (Jmgensen, 1966). Like ammocoetes and most
extant protochordates, they would have employed their pharynx to trap
microscopic food particles (phytoplankton and fine detritus. under about
0.2 mm in diameter). This is the traditional view of ancestral vertebrates and
recent evidence supports it (Mallatt, 1981): there are similar patterns of cilia
tracts in the pharynges of cephalochordates, ascidian 1 unicates, and
ammocoetes, and corresponding tracts act to direct cords of food-trapping
mucus in the same relative directions (Fig. 1 ) . For exposition of an alternate
view, that vertebrates primitively were predaceous, see Jollie (1982) and Gans
& Northcutt (1983).
Assuming a microphagous suspension-feeding ancestry, clues to the poorly
documented earliest stage of vertebrate history may lie in a peculiarity of larval
lamprey feeding. The ammocoete has been found (Mallatt, 1982) to move water
across its feeding apparatus at the lowest rate ever documented for a suspension
feeding animal (Table 1). Inability to obtain food-carrying water rapidly
implies either that ammocoetes must feed on highly concentrated suspensions to
obtain enough nutrient, or else that their metabolic rates are so low that they
can survive on a comparatively low rate of food intake. The second possibility is
ruled out by the data of Table 2, which indicate ammocoetes expend more
metabolic energy per unit of food-carrying water pumped than do other
suspension feeders. It follows that ammocoetes require more concentrated f o o d
suspensions than do any other known suspension feeder. Available data, although
incomplete, are consistent with this view: while most other suspension feeders
live and grow in waters containing under 1 mg suspended organic solids per litre
(Mallatt, 1982) the lowest concentration at which I was able to obtain
ammocoete growth was 4 mg/l (yeast diet: Mallatt, 1983). Suspended food
concentrations in the ammocoete habitat have been measured at up to 40 mg/l
(J. Moore, in Mallatt, 1982).
Other fish and amphibians (Table 1 ) pump water at rates (under 200 ml/g/h)
nearer those of ammocoetes than of most suspension-feeding invertebrates (over
200 ml/g/h). The mechanical reason for this is not known, (and should be
investigated. If it can be assumed, however, that ancestral suspension-feeding
vertebrates shared the relatively low pumping rates of later fish, certain
conclusions about their ecology necessarily follow. They were not typical
suspension feeders: an inability to obtain suspension quickly would have
generally excluded them from the open waters of oceans and lakes, which are
oligotrophic (< 1 mg/l; Mallatt, 1982). Confined to habitats where suspensions
were concentrated, ancestral vertebrates would have been primarily benthic-
for organic particles concentrate on and just above substrate surfaces, as they
settle from the water column and are then resuspended by bottom currents. This
deduction differs from the commonly proposed view that the earliest vertebrates
were nektonic, open-water animals (Berrill, 1955; Halstead, 1'369; Jagersten,
Figure I . Pharyngeal ciliated tracts in larval lampreys and amphioxus, drawn in a composite
pharyngeal tube. Only the anterior three pairs of branchial bars are shown. Name.< of the tracts in
the lamprey are written above the corresponding names in amphioxus. Directions in which cilia
move food-laden mucus are indicated by arrows. The lamprey's gill seam tracts ( i x . 2a) are
situated laterally, but initially develop medially as in amphioxus (2b). Given this. the patterns in
the two animals correspond closely. Dorsal is above.
264 J. MALLATT
Table 1. Typical water flow rates across the
biological filters of suspension feeders, and
through the pharynges of fish*
Average flow rates (ml/g/h)
*This table summarizes Table 1 in Mallatt (1982).
T = 10-20Â°C; all flow rates (F) have been corrected for animal
weight (W) according to FzWO 1 5 .
?Calculated from Sheltonâs (1970) Table 1; non-hypoxic
conditions only: shark, Scylior/pus: 70 ml/g/h; shark, Squulus:
200; eel, Ansuillu: 20; trout, Sulmo: 45, 130; sucker, Cutostomus:
75; bullhead, Ictulurus: 60; carp, Cyprinus: 35.
fMuch higher rates than this characterize fish that employ
ram ventilation instead of muscular pharyngeal pumping. Flow
rates for anchovies are about 2000 ml/g/h (Leong & OâConnell,
1972), but it is consistent with the evidently benthic adaptations (Moy-Thomas
& Miles, 197 1) of most ostracoderms.
A requirement for highly concentrated suspensions would select for the ability
to find appropriately rich patches of suspended food, and I reconstruct ancestral
fishes as shifting about the surface of the ocean floor exploiting such
microhabitats. They are pictured as actively seeking food in a non-homogeneous
environment, resembling predators in this respect. This âbenthonecktonicâ
existence is consistent with the derivation of vertebrate sensory structures (eye,
ear, electroreceptive organs) and vertebrate activity metabolism (Ruben &
Bennett, 1980), and it can answer the often asked question fJollie, 1977: 79-80)
of how benthic suspension feeders could have evolved the features of active
existence. T o equate benthic suspension feeding with inactivity ignores the
ecology of amphioxus and larval lampreys: both are exceptionally active within
the sediments they inhabit and are highly sensitive to changing local conditions
(Webb & Hill, 1958; Webb, 1975; and pers. obs.).
The above line of reasoning depends on an assumption, which should be
stated. Some extant fish employ ram ventilation (Roberts, 1975): by swimming
constantly with mouth open, they can process water fast enough to live as
typical open-water suspension-feeders (e.g. anchovies: Leong & OâConnell,
1969). The above model assumes ancestral fish did not primarily utilize ram
ventilation (most living fish do not).
EARLY FISH ECOLOGY
Table 2. For various suspension
feeders: ratio of rate at which water is
moved (ml/g/h) per unit of oxygen
consumed (pl/g/h) *. Note this ratio is
smallest for larval lampreys
Lamelli branchs 4-80
Cladocerans (Duphniu) 1.3-2.6
8. Macraphnpus particulate feeders
/ /? â
6 Macmpha&s 7 Micmphagous 4 Fish
suspenvon particulate parasites
I Microphapus suspension feeders
2 Depasif &eders 2 b Select big
cans and osteostracans
3 lnmprey ancestors, anaspids, osteostrocans (
4 Adult lomprey
7 Many larval ostrocoderms
8 Primitive gnothostome fish
Figure 2 The initial radiation of vertebrate feeding types, from a proposed microphagous
suspension feeding ancestral condition. According to this model, ancestral vertebrates (1 ) were
entirely benthic and included no open water species Much of this radiation could have occurred in
estuaries Vertebrates would have occupied all feeding zones shown (except 4) by the beginning of
the Devonian period Each zone could have been occupied by several different fish groups through
convergence, so no rigid phylogenetic sequence is implied Complex life cycles are expected among
early fish, with larvae and adults feeding differently In the evolution of gnathostomes ( 8 ) , the
sequence 1 +7+8 is considered more likely than the other possibility, 1 +6+8
Myxinoids feed by foraging through the mud for infaunal organisms such as
worms and shrimps ((Strahan, 1963; Shelton, 1978). Gut analyses indicate
hagfish ingest small quantities of mud. I t should be emphasized that, although
hagfish can also feed epifaunally, teleost fish do not comprise a significant part of
their normal diet (Strahan & Honma, 1960).
Jawless vertebrates possess no hyoid or opercular apparatus so it is reasonable
to assume the primitive vertebrate pharynx could produce only weak suction
(i.e., via elastic recoil of branchial arches during ventilatory inspiration: Hughes
& Ballintijn, 1965; Alexander, 198 1 : 45). Thus mouth, rather than pharyngeal,
features are expected to have become specialized for the uptake of heavy
particles in early deposit feeders: a variety of oro-bucchal scooping, raking, and
suction devices should have existed. While such structures may be echoed in the
complex mouth parts of modern lampreys and hagfish (Reynolds, 1931;
Dawson, 1963), direct documentation has been limited to the oral plates of some
heterostracans (Moy-Thomas & Miles, 1971). It is therefore significant that two
recently discovered, well preserved Palaeozoic agnathans exhibit strikingly
elaborate mouth parts (Bardack & Richardson, 1977).
Deposit feeding is ecologically related to feeding upon algae that cover rocks
(Fig. 2, epilithic algal feeding). This continuum is illustrated by African cichlids
where some species facultatively alternate between collecting food particle films
from sediment surfaces and removing periphyton from rocks (Fryer & Iles,
1972: 72). Many cichlid species exclusively utilize the latter food source.
Epilithic grazing is a reasonable feeding mode to propose for jawless fish, as it is
widespread among aquatic animals (Hynes, 1970: ch. 4), biting mouthparts are
EARLY FISH ECOLOGY 267
not required (scraping and combing elements can suffice), and the algal food
source undoubtedly existed in the Palaeozoic.
Among extant epilithic algavores, some stream-dwelling teleosts and tadpoles
cling to rocks via suctorial lips, not only to scrape off the periphyton but also to
resist fast currents (Noble, 1931; Alexander, 1974; Lagler et ,d., 1977). This
could have been the condition in anaspid ostracoderms (Parrington, 1958) and
ancestral lampreys (Ritchie, 1968), groups which seem to have included
fluviatile forms (Denison, 1956; Hardisty, 1979). Extant suctorial teleosts
resemble adult lamDrew in mouth structure and both use tidal ventilation for
irrigating the gills when the mouth is engaged (Randall, 1972; Alexander,
A behaviour of teleost rock-scraping fish may show how parasitism evolved in
lampreys. Neither the loricariid Hypostomus plecostomus nor some carp minnows
(Cyprinidae) can be held with fish that produce significant quantities of
epidermal mucus, for they scrape off the mucous layer and open welts in the
victimâs sides (Lagler et al., 1977). The nutritional value of fish epidermal mucus
is discussed by Gorlick ( 1980).
After vascular plants became abundant in the Devonian period (Andrews,
1961; Taylor, 1981), periphyton scraped from plants would have been a
potential food source for fish.
For the earliest known vertebrates (Cambrian to Ordovician), fossil evidence
points to a fully marine, often near-shore, habitat (Darby, 1982). There was an
unexplained preference for sandy sediment of medium grain size.
For vertebrates that lived somewhat later (Ordovician to Silurian), estuaries
and other marine fringing zones have been considered potenl ially important
habitats (Northcutt & Gans, 1983), primarily because they could have served as
springboards for the subsequent invasion of fresh water (Denison, 1956; Heintz,
1963; Spjeldnaes, 1967). Nutrient considerations also suggest the importance of
such environments. If ancestral fish required exceptionally high concentrations
of suspended organic particles, this need would have been met by estuarine
waters, where land-derived nutrients stimulate phytoplankton growth, and tidal
action continually resuspends settled detritus (Barnes, 1974; Teal, 1980). For
deposit feeders (Fig. Z), the substrate of estuaries supports rich growths of algae
(possibly richer in early Palaeozoic times before large shading plants appeared),
bacteria, and small animals (meiofauna). There is fossil evidence that some
ostracoderm groups initially inhabited marine-fringing environments
(cephalaspidomorphs in the mid-Silurian: Denison, 1956).
Vertebrates entered fresh waters no later than late Silurian times (Denison,
1956). Initially, eutrophic lakes may have been favourable habitats (based on
the cichlid analogy). Palaeozoic rivers and streams, on the other hand, could
have provided only limited opportunity: only the largest rivers support plankton
(Hynes, 1970), and detritus content in rivers and streams woulcl have been low
prior to the widespread occurrence of terrestrial plants, whose fallen leaves
support most extant fluviatile food webs (Cummins & Klug, 1980; Pomeroy,
1980). Initially, rock scraping may have been the only mode of feeding widely
available to fish in fluviatile environments.
268 J. MALLATT
DERIVATION OF THE GNATHOSTOMES
An evolution of jawed predaceous fish from microphagous suspension-feeding
ancestors necessarily involved two changes: increasing the size of particles taken
(microphagy to macrophagy) , and attaining selectivity for individual particles
(suspension to particulate feeding). Depending on which change occurred first,
two models of gnathostome derivation are possible.
By one scheme (Fig. 2: Stages 1+6+8) a filtrate of originally microscopic
food particles came to include macroscopic zooplanktonic organisms of
progressively larger sizes. (Zooplankton would have been a valuable energy-rich
nutrient supplement if early fish were limited in the rate at which they could
obtain suspension, as proposed above.) Later, as discriminatory abilities
increased (better vision), capture of the most desirable individual zooplankton
organisms would have become possible. Extant anuran tadpoles can be
arranged in a continuum of feeding types from microphagous to macrophagous
suspension feeders to obligate carnivores (Wassersug & Hoff, 1979; Wassersug &
Rosenberg, 1979) and this reflects the pregnathostome radiation proposed here.
Estuarine waters support rich concentrations of zooplankton as well as
phytoplankton (Barnes, 1974). Estuaries thus could have been primary sites for
the evolution of zooplanktivory in pregnathostomes. Some modern estuaries
support populations of macrophagous suspension-feeding fish (Blaber, 1979;
Mann, 1980). Zooplanktonic organisms available as prey in Palaeozoic times
included ostracods, protozoans, and the pelagic larvae of many metazoans
Uagersten, 1972; Tasch, 1973), but not todayâs dominant crustacean groups
(cladocerans, copepods: also see Miiller, 1983).
There is some indication, however, that the pre-gnathostome line never
passed through a macrophagous suspension-feeding stage. Extant gnathostome
fish of this feeding type show rather extreme structural features that contrast
markedly with traits of primitive gnathostomes. Gill rakers are elongate and
abundant, the pharynx tends to be highly expandable, and the teeth on the
upper and lower jaws are degenerate (Leong & OâConnell, 1969; Lagler et al.,
1977; Moss, 1977; Rosen & Hales, 1981). By contrast, primitive gnathostomes
seem to have had well developed jaws and teeth, and only a limited capacity for
expanding the pharynx (early osteichthyes: Lauder, 1982; placoderms: Miles,
1967; biting in primitive sharks: Moss, 1977). Acanthodians (Moy-Thomas &
Miles, 197 l ) , which initially possessed the typical features of gnathostome fish,
diverged markedly in morphology with the apparent adoption of macrophagous
suspension feeding in late forms.
Even if pre-gnathostomes were not macrophagous suspension feeders, other
early fish may have been (pteraspid heterostracans: Halstead, 1969).
The alternative to the above scheme is that microphagous suspension-feeding
ancestors abandoned suspension feeding early to take small individual particles
(microphagous particulate-feeding) , with an increase in the particle sizes taken
EARLY FISH ECOLOGY 269
following later in phylogeny (Fig. 2: Stages 1+7+8). One way this switch to
particulate feeding could have occurred is through neoteny. Most larval fish
necessarily take individual particles (Harden Jones, 1980), due 10 the tiny sizes
of their mouths. This holds true even when their adult stages are suspension
feeders (Ruelle & Hudson, 1972; Drenner et al., 1982), and it should have been
the case for many Palaeozoic jawless fish*-for even amphiotxus larvae are
particulate feeders (Webb, 1969; Gosselck & Kuehner, 1973). I t is proposed
that retention of larval proportions and feeding behaviours occurred in
pregnathostomes, resulting in a prolongation of particulate feeding throughout
the life cycle and the elimination of an originally suspension-feeding stage in the
adult; progressively larger particles would have been utilized a!; the fish grew.
The proposed neotenic step (Gould, 1977) could explain why comparative
anatomists have long considered the adult head of gnathostonne fish to be a
better reflector of the vertebrate embryonic plan than are the heads of known
aganthans. It should be stressed that the neotenic event advanced here is not
equivalent to that sometimes proposed (Berrill, 1955) to have produced
vertebrates from tunicates.
Once particulate feeding had evolved in the lineage leading to gnathostomes,
macrophagy would have directly followed. Observations on zooplanktivorous
predatory fish indicate preference for the larger prey items present (Hughes,
1980; Gardner, 198 1; Eggers, 1982), and this is expected since 1a.rger prey offer
higher yield per capture effort. Assuming no competition, the earliest
gnathostomes should have risen rapidly to the top of the food chain. Indeed, it is
noteworthy that most of the oldest known gnathostome groups (cladodont
sharks, arthrodires, large palaeoniscids, early dipnoans and crossopterygians)
seem to have fed on large prey (e.g., other fish, molluscs: Moy-Thomas & Miles,
197 1 ) . The initial gnathostome (or pre-gnathostome) feeding niche of predation
on zooplankton could have been retained by small acarithodians and
paleoniscids, and by the larvae of other jawed groups. However, today's great
range of zooplanktivorous teleosts (Lauder, 1982) seems to have been
conspicuously without analogue in the Palaeozoic record.
( 1 ) Early vertebrate evolution is discussed in an ecological context (Fig. 2) ,
using modern animals and environments to evaluate Palaeozoic (counterparts.
(2) Comparison of the pharynx in protochordates and larval lampreys
(Fig. 1 ) supports the idea that ancestral vertebrates were microphagous
( 3 ) Assuming that ancestral vertebrates pumped water like extant fish (and
did not employ ram ventilation in feeding), they must have obtained food
suspensions so slowly as to require eutrophic, mostly benthic habitats. Highly
* I assume that ancestral marine vertebrates, including pre-gnathostomes, had complex life cycles with
planktonic (and planktivorous) larvae and benthic adults. Benthic marine animals usually have pelagic larvae
(Mann, 1980), and such a life cycle seems to have been primitive for most marine metazom groups (Jagersten,
1972). Pelagic larvae are not typical for fluviatile or deep-sea animals, however, due to unidirectional water
currents instreams, and paucity of food in the deep sea water column (Mann, 1980). By this reasoning,
lamprey and hagfish life cycles are not primitive for vertebrates.
270 J. MALLATT
productive marine fringing environments exhibit the appropriate conditions.
Ancestral vertebrates are modelled as relatively active, epifaunal (not infaunal)
(4) From this benthic suspension-feeding ancestry, agnathan groups radiated
into the related niche of deposit feeding, and then to benthic foraging (hagfish)
or feeding on epilithic algae (lamprey ancestors). These feeding modes are all
interrelated, as illustrated by the recent radiation of cichlid teleosts in African
(5) The evolution of gnathostomes (macrophagous particulate feeders) from
microphagous suspension-feeding ancestors could have involved either an initial
increase in the particle sizes filtered (intermediate stage = macrophagous
suspension feeder), or an initial switch to individual particles (microphagous
particulate-feeding intermediates). The latter possibility seems more likely, as
extant macrophagous suspension-feeding fish show rather extreme speciali-
(6) Known fossil palaeozoic gnathostomes seem to have clustered at the top of
the food chain (large prey). Although the possibility of preservational bias in the
fossil record cannot be ruled out, zooplanktivorous fish seem underrepresented
in the Palaeozoic (cf. among modern fish).
Thanks are extended to Quentin Bone, Carl Gans, Philippe Janvier, Malcolm
Jollie, Richard Parker, Barbara Stahl, and Richard Wassersug for critical
discussion of the ideas in this paper, and to Barbara Comstock and Sue Rose for
typing the manuscript.
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