Trophic Biology of Reef Fishes

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Trophic Biology of Reef Fishes

• Planktivores
• Herbivores & Detritivores
• Benthic Carnivores
• Piscivores

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Planktivores

• Tropical Reef waters have abundant phytoplankton and zooplankton.
• These tiny animals and plants form the main food source for many of the fishes and other invertebrates on the reef.
• Their consumption also forms an important energy link between the open water communities and the reef community
• They are an abundant and nutritious food source, but present two problems to a consumer:
• the individual prey items are very small and hard to see
• consumers need to feed in the water column away from shelter, thus risking being eaten themselves.

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Phytoplankton

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Gelatinous Zooplankton

Planktonic jellyfish

Ctenophore

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Crustacean Zooplankton

Shrimp larva

Amphipod

Planktivores - Feeding Strategies

• To minimise the risk of predation, planktivorous fishes have both behavioural and physical adaptations.
• They form feeding schools in the water column, which increases target confusion in predators.
• These schools are often located above branching corals which provide shelter.
• They have large forked tails to facilitate both continuous swimming for feeding, and rapid escape from attacks.
• Planktivores aggregate in areas of high current flow, such as reef fronts, points, and passes.
• Most families of reef fishes have planktivorous representatives.

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Planktivore feeding schools - Damselfishes

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Planktivore feeding schools - Fusiliers

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Planktivores - forked tails

Pomacentrid

Chromis viridis

Pomacentrid

Abudefduf sexfasciatus

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Planktivores - forked tails

Pomacanthid

Genicanthus lamarck

Anthiid

Pseudanthias squamipinnis

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Planktivores - small and large

Blennid Ecsenius midas

Planktivores - Feeding Strategies

• Multi-species schools of planktivores show spatial niche-separation.
• Taxa with larger tails and streamlined bodies feed furthest from shelter.
• The new recruits and juveniles of many species feed on plankton, even if the adult diet will be different.
• There are 2 main groups of nocturnal planktivores - the Holocentrids (Squirrelfishes)and Apogonids (Cardinalfishes).
• These taxa have big eyes, and they feed on the large crustacean plankters that swarm above the reef at night.

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Spatial Niche separation in Planktivores

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Spatial Niche separation in Planktivores

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Nocturnal Planktivores

Holocentrid Myripristis vittata

Herbivores

• Algae on coral reefs is in the form of short (3-10mm) “turfs”, which cover most exposed surfaces.
• These turfs have low standing stock but very high production, equal to tropical rainforests.
• For herbivorous fishes, algal turfs are easy to obtain, but not very nutritious (opposite to plankton).
• Algae have cellulose fibres in the cell-walls, and often unpalatable chemical compounds too.
• Herbivores have had to evolve specialised methods of digestion to extract nutrition from algae; these vary among the 4 main families: Pomacentridae, Acanthuridae, Siganidae, and Scaridae.

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Algal Turfs

Macroalgae

Turf algae

Herbivores

• Pomacentrids: small-medium fishes which crop turf algae from within a small territory.
• They defend the territory against any other fishes (esp. other herbivores) which may steal the algae.
• They are very common, and their territories occupy most of the space on the reef not taken by corals, etc.
• Some of the larger species alter the algal community in the territory, creating conspicuous “farms”.
• Pomacentrids have long guts, and use acid lysis to digest the algal cells. (They must avoid eating CaCO₃ particles!).
• Recent research indicates that small crustaceans and detritus in the algal turf can also be nutritionally important to these fishes.

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Pomacentridae - Small Dark Herbivores

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Pomacentridae - Small Dark Herbivores

Pomacentrus chrysurus

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Pomacentridae - Large farming Herbivores

Plectroglyphidodon lacrymatus

“jewel damsel”

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Pomacentridae - Large farming Herbivores

Dischistodus melanotus →

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Pomacentridae - Large farming Herbivores

Dischistodus perspicillatus

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Pomacentridae – Large farming Herbivores

Dischistodus prosopotaenia

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Pomacentridae – Large farming Herbivores

Dischistodus pseudochrysopoecilus

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Pomacentridae - Large farming Herbivores

↓ Stegastes nigricans

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Pomacentridae - Large farming Herbivores

Stegastes lividus

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Pomacentridae - Large Herbivores

Roving Herbivores – cropping taxa

• Acanthurids: medium-large fishes which crop turf algae as they rove around the reef in schools (ie. roving herbivores).
• They also have long guts and use acid lysis to digest algae.
• Genus Naso are planktivores in the adult stage.
• All species have poison knives near the base of the tail.

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• Siganids: medium-large roving herbivores.
• They also have long guts and use acid lysis to digest algae.
• All species have poison spines in the dorsal and anal fins.

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• Both these families have a long tenure in the fossil record (at least 60My).

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Acanthuridae - Surgeonfishes - Ringtailed species

Acanthurus blochii →

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Acanthuridae - Surgeonfishes - Ringtailed species

Acanthurus blochii

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Siganidae - Rabbitfishes

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Siganidae - Rabbitfishes

Siganus doliatus – feeding school

Roving Herbivores – scraping taxa

• Scarids: Medium large roving herbivores, evolved from the Labridae in the last 10My.
• Parrot-like teeth used to scrape the reef surface, obtaining algae and also CaCO₃ particles. This causes massive bioerosion.
• The giant bumphead parrotfish feeds on live coral.
• Like the Labrids, the Scarids have a 2nd set of jaws in their throat, the “pharyngeal mill”.
• Algae & CaCO₃ particles are ground to a paste in the mill, and then pass rapidly through a short intestine.
• Scarids remove only a small fraction of the energy from the algae, relying on fast gut throughput to obtain adequate nutrition over the course of a day.

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Herbivorous Scarids – Feeding School

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Scaridae - Parrot-fishes - excavators

Giant Bump-head Parrot fish Bolbometapon muricatum

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Scaridae - Parrot-fishes – colour phases

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Scaridae - Parrot-fishes - excavators

Chlorurus bleekeri

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Scaridae - Parrot-fishes - excavators

Chlorurus microrhinos

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Scaridae - Parrot-fishes - scrapers

Scarus rivulatus

Detritivores

• Fishes which feed on organic detritus collected from benthic algal turfs.
• This is a highly abundant and relatively nutritious food source.
• This feeding mode has evolved in herbivorous taxa such as the surgeonfishes and some damselfishes.
• Detritivores have fine sieve-like teeth for slurping off the detritus, without removing the underlying algae or sand.
• Some damselfish such as Hemiglyphidodon plagiometapon grow luxurious algal farms specifically to trap detritus.

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Acanthuridae - Surgeonfishes - detritovores

Ctenochaetus striatus

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Pomacentridae – detrital farmers

Hemiglyphidodon plagiometapon

Benthic Carnivores

• Fishes which feed on animal material picked from the reef surface.
• Easily the largest trophic group in terms of # of species.
• Major families include the Labrids, Chaetodontids, Pomacanthids, Lethrinids, Haemulids, Mullids, Balistids, Nemipterids, Monocanthids, & Tetraodontids.
• There are a diverse range of feeding behaviours in these fishes, from delicate pickers to chisel-like crunchers to sand sifters.
• Some Chaetodontids and Labrids have specialised to feed on live coral polyps.

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Benthic Carnivores

Pomacanthid Pomacanthus imperator

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Benthic Carnivores

Labrid Thalassoma lunare

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Benthic Carnivores

Lethrinid

Lethrinus nebulosus

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Benthic Carnivores

Nemipterid

Scolopsis bilineatus

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Benthic Carnivores

Monocanthid

Oxymonocanthus longirostrus

Piscivores

• Fishes which prey on other fishes. Several categories:
• Diurnally active ambush predators – lie in wait for prey and then attack quickly over a short distance. Most active at dawn and dusk. Serranidae, Synodontidae, Cirrhitidae, Platycephalidae, Scorpeinidae.
• Diurnally active pursuit predators – swim in the open water near reefs, and come in close to take fishes at high speed. Can feed all day. Carangidae, Scombridae, Sphyraenidae.
• Nocturnal predators – moray eels(Muraenidae) which hunt sleeping fish by smell, and sharks (Hemigaelidae, Carcharinidae) which use smell and electroreception to detect prey.

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Piscivores - Diurnally active ambush predators

Cephalopholis cyanostigma

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Piscivores - Diurnally active ambush predators

Serranid

Plectropomus leopardus

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Piscivores - Diurnally active ambush predators

Platycephalid

Cymbacephalis beauforti

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Piscivores - Diurnally active ambush predators

Synodontid

Synodus jaculum

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Piscivores - Diurnally active pursuit predators

Scombrids

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Piscivores - Nocturnal predators - Moray eels

Muraenid Gymnothorax spp.

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Piscivores - Nocturnal predators - Reef Sharks

Hemigaleid Triaenodon obesus “White-tip” reef shark

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