What a fish's mouth reveals about its diet

At the Aquarium La Rochelle, we observe the eating habits of hundreds of species every day. After asking ourselves what each one should eat, with the help of a specialist nutritionist in particular, we turned our attention to the whys. Why does one species need to eat more regularly than another, or more consistently?

‘In marine animals, the shape of the mouth, the teeth, the size of the stomach and even the digestion time are all clues to the diet,’ explains Guillaume, a biologist at the Aquarium.

Mouth position: a valuable clue

The position of the mouth says a lot about how a fish feeds:

An upward-facing mouth indicates surface feeding.
A terminal mouth (centred) indicates an intermediate zone supply.
A mouth pointing downwards indicates a burrowing fish, which feeds on the bottom.

Carnivorous or vegetarian teeth?

Teeth also reveal many secrets. Each tooth shape is adapted to a specific diet:

The molars of gilthead bream and sars allow them to crush the shells of crustaceans and molluscs.
The sharp canine teeth of moray eels are ideal for grabbing and holding onto prey.
The flat, sharp incisors of saupes are indicative of their herbivorous diet, enabling them to cut plants.

The stomach: a variable-geometry organ

When we compare the digestive systems of teleost fish (marine bony fish), we see very different shapes:

Y- or U-shaped stomach: often associated with a macrophage-type diet (large foods, carnivorous or omnivorous diet).
I-shaped stomach or no stomach: microphagous species (herbivores, planktivores, omnivores) eat small, continuous meals.

Around 20% of marine teleosts have no stomach: this is the case, for example, of gobies and blennies.

Large stomach, slow digestion

Food transit time varies greatly:

Macrophages: slow digestion (12 to 20 hours).
Microphages: rapid digestion (from 1 minute to 10 hours).

This has a direct impact on… the way we feed them at the Aquarium!

Feeding adapted to each type of eater

For microphages: food is a round-the-clock process!

Some species need to eat… every 15 minutes! It’s impossible to do this by hand. Here are our solutions:

Automatic injection of Artemia Nauplii: perfect for plankton eaters such as atherines, mullets, woodcocks, anthias, Heteroconger…
Mysis dispenser: developed for our seahorses, it works by overflowing with a pump.
Seaweed-based pellets: Acanthuridae, herbivores, are automatically fed seaweed-based pellets.

Macrophages: tailor-made with tweezers

Predatory species such as Aulostomus and moray eels require more attention:

Targeted hand-feeding: each individual is identified, trained and fed by hand.
Monitoring food intake: we check what each fish eats.
Avoid competition for food: for example, selene are too large to access moray platforms. Less conflict, more efficiency.

Observe for better nutrition

"Observing a fish's morphology helps us to understand how it eats - and therefore how to feed it properly. This is essential to ensure the well-being of marine animals. It's this daily attention, almost invisible to visitors, that enables the teams at the Aquarium La Rochelle to best respect the needs of each species", concludes Guillaume.

This article is based on a presentation Guillaume gave to members of the Union des Conservateurs d'Aquariums (UCA) in March 2025.