Perhaps the most common is a tough shell, or carapace, to protect their vulnerable bodies, as seen in crabs, shrimps, urchins, clams, and snails. Finding these animals does not often present much of a problem, but any fish that wants to gain access to a juicy meal must first work out how to get past the hard shell. Plenty of fish, ranging from moray eels and groupers to wrasses and from triggerfish to puffer-fish, are equipped to a greater or lesser extent to tackle hard-shelled prey.
Crustaceans, such as hermit crabs , shrimps, and amphipods, are especially important prey for reef fishes. There are two main ways of getting at the tasty flesh of a shelled animal. One is to bite through the carapace, as pufferfish do. Fish that browse on small reef invertebrates often have deep bodies, making them highly maneuverable, for example.
As well as body shape, the mouthparts and teeth of fish within a guild are often quite similar, yet quite diverse between different guilds. For example, the rasping teeth of algae-eating surgeonfishes and damselfish are quite different from the amazing beaks of parrotfish or the fangs of a moray eel. The pressure of food competition on the reef has led to many species of fish becoming in their appearance: the beaks of coral-eating parrotfish, the long snouts of plankton-eating seahorses and pipefish, and the enormous mouths of predatory groupers are all examples of this.
The advantage of being a specialist is that the fish gain a competitive edge. They become expert at finding and catching their particular prey, so much so that they can easily out-compete a generalist. The downside is that their diet becomes increasingly inflexible. The algae that grow in your aquarium will provide a certain amount of this, but unless you have a lot of it, you will need to supplement their food supply. Prepared foods, such as many flake foods , are concentrated food and depending on the type can supply the fish with everything they need.
Feeding your fish a little bit of food several times per day is closer to the way they eat in the wild than feeding them a bunch of food every 2 or 3 days. Most fish even sharks will only eat what they need to survive. If you watch your fish when you feed them, you will see that they will actively eat for a few minutes, then ignore the remaining food for hours. The food that remains in the aquarium is wasted and will end up on the bottom, creating toxins as it decays.
If you only feed your herbivorous fish once every 2 or 3 days, it is not the way most fish eat in the wild, which is what we should try to duplicate as closely as possible. In my experience, feeding twice per day, only what is consumed in about minutes has been the best. This gives the fish what they need to remain healthy and at the same time doesn't create any maintenance problems in the future, such as more water changes to reduce nitrate created from uneaten food left on the bottom of the aquarium.
Carnivores, on the other hand, are a different thing. Eels are an excellent example. They will go for days without eating, then suddenly feed voraciously for a couple of minutes. A herbivore's digestive tract is quite a bit longer than a carnivore's because it takes more time for a herbivore to break down vegetable matter fiber that contains cellulose. The herbivore's digestive tract contains a huge amount billions of symbiotic bacteria, which break down the cellulose fiber.
This fiber contains no protein, but an herbivore gets protein by digesting the symbiotic bacteria themselves. The herbivore's system can digest starches, which the fish needs for energy. A carnivore's digestive tract is much shorter than an herbivore's. It produces digestive enzymes and acids which break down meat so it can be absorbed and utilized. Marine herbivores consume plant life such as macroalgae, microalgae , and true flowering marine plants such as the seagrasses. Many herbivores, such as the surgeonfish, primarily dine on algae but are not averse to eating something with a meat base when given the opportunity.
Good examples of this are the yellow tang and naso tang which will consume certain red algae and green algae by the bucketful but will gobble up mysid shrimp when given the chance. On the other hand, most fish which are carnivores, such as the dragon moray eel , volitans lionfish , or striped mandarinfish will not touch algae even if they are starving to death.
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