Text by Chetana Purushotham

Images by  Scott “Gutsy” Tuason

Evolution is often driven by the basic elements of survival. In this case, the need to eat.

From the massive to the microscopic, the number and kinds of Technicolor beings we encounter on a coral reef is simply breath-taking. Divers and snorkellers the world over will agree that few terrestrial places rival the astounding diversity of life harboured by our reefs. Attempting to observe a rich and colourful assembly of reef fish, for instance, is at once an absorbing and distracting experience, leaving us wondering where to look, whom to follow, or what to photograph.

A staggering 4,000 species of reef fish have been recorded worldwide. With a chance of seeing at least 20 species on a single reef, one question often arises – how do so many species occur in such close proximity to each other? Scientists worldwide are still debating the answer to this question. Yet, on any given dive, by choosing the appropriate lens with which to observe the reef (metaphorically speaking, of course), anyone can observe some interesting patterns.

THE FOOD FACTOR

One of the more intriguing patterns is the close relationship between what fish look like and what they eat – associations which start to make sense of the wide variation we observe in the structure and shape of different species of reef fish.

Coral reefs, being highly dynamic and crowded metropolises, provide an assortment of potential food for fish to choose from. There are hairy and leafy algae, microscopic drifting zooplankton, larger invertebrates like corals, jellyfish, crabs, squid, worms and, of course, so many other fish. Each food source offers different benefits and challenges to its consumers. Over evolutionary time, competition for these rich resources has led to an incredible array of unique adaptations and feeding mechanisms both in fish body structure and behaviour.

Perhaps now, looking around the bustling reef, it is not as hard to differentiate the herbivores, like parrotfish with their beak-like teeth, from the large-mouthed ambush predators like the grouper. But what determines what they eat? Which part of the reef do they prefer? Do they have dinner buddies too? Understanding fish through their morphology (structure and form) offers new perspectives.

BIOMECHANICS OF A GOOD BITE

If having a meal were to be broken down into steps, it is evident that there are a number of challenges along the way. The predator first needs to encounter its prey. This is followed by an attack, which ultimately may or may not be successful. Among ambush predators, it is unlikely that a small lizardfish will feed on prey that are significantly bigger than its own mouth, while it is quite likely that big groupers will give very small fish a miss and wait instead for the larger ones to come along. A lizardfish, despite its camouflage and razor-sharp teeth, is still restricted in the size of its meal by how much its mouth can accommodate. On the other hand, a grouper may prefer fish on the larger side, gaining more energy from each onslaught despite the greater effort it would require. The design of the feeding apparatus, general body plan and energy requirements are therefore crucial in determining what a fish can and should eat.

At the core of their anatomy, the feeding structure of all fishes is made up of the same basic elements: a mouth opening, teeth, jaw bones and associated muscles. These are then modified in different species, to suit particular types of prey and modes of capture. For instance, the jaws may be modified to deliver a strong, forceful bite, like in the case of a shell-cracking porcupinefish, or to provide the advantage of speed for predators such as the barracuda, whose prey can be quick and elusive.

 

Read the rest of this article in SD Issue 5, AA No.83 of Scuba Diver magazine by subscribing here or check out all of our publications here.

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