In the 'Encyclopaedia' section, we explore the flora and fauna of the regions where we go on our sailing adventures. Last time we told you about the unique White Sea, in this article — Mauritius full of secrets.

The tropical underwater world is like another universe. It is astonishing in its colourfulness and the hundreds of different, sometimes bizarre creatures that live in a complex relationship with each other. In this brief guide to reef fauna, we take a look at the who’s who of the creatures you can see when snorkelling in the Indian Ocean.

Corals, like actinia and jellyfish, are coelenterates — primitive animals with a single hole in their body and a gut instead of a tube. But unlike the same actinia, corals are colonial organisms: their entire arrangement is made up of many tiny polyps connected to each other.

Corals do not build their skeletons by themselves, but with the help of symbiotic microalgae: they live inside the polyps, photosynthesise and, thanks to this chemical process, corals can capture carbonates from the water and deposit them in the form of insoluble lime. The result is a reef — perennial and even centuries-old growths of calcareous skeletons.

Sea Corals

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The structure of the reef makes it a habitat for a wide variety of invertebrates and fish that use the coral as shelter, food source or breeding ground.

INTERESTING FACT

Corals also build a calcareous skeleton, but not externally, but internally: the growing coral is a calcareous structure covered with a thin living layer of colonial polyps — these can be seen if you look closely at living coral on reefs.

The reef is usually made up of many species of coral — sometimes more than 200 — which is why it is so colourful (the symbionts are responsible for the colour) and varied in shape.

Here are the main corals found off the coast of Mauritius

Вот основные кораллы, которые можно встретить у берегов Маврикия:

But storms often nail the "Portuguese ships" to the shore, and then you have to be careful: its tentacles contain millions of poisonous stinging cells that can cause very serious burns.

Siphonophores are similar to jellyfish in that they are jelly-like, swim in the water column and have tentacles, but they are also colonial organisms from the class of hydroid coelenterates.

Siphonophores

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If you see these bluish bubbles on the beach or in the water, don’t touch them with your hands.

The most common siphonophoran is the "Portuguese ship" or physalia, Physalia physalis. Physalia usually drifts in the open sea: it has a gas bubble to keep it at the surface and long tentacles to catch prey — plankton and small fish.

Polychaetes live in a variety of conditions — there are species that burrow in the sand, there are those that crawl on the surface of the ground, there are those that settle in the crevices of dead coral and build house tubes on the reef (they are called sedentary) — then on the surface you can see a pair of modified tentacles with a crown of fluff.

If you think you see two Christmas trees a few centimetres high on the surface of the reef, these are the tentacles of sedentary polychaetes filtering their food — small organic particles — from the water.

Polychaetes

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Polychaetes get their name from the fact that each part of their segmented body is covered with tufts of bristles, making them look like furry caterpillars.

One of the most colourful polychaetes found on the reefs of Mauritius is Sabellastarte pectoralis, also known as the feathered corolla. This sedentary worm actually has tentacles that resemble pinnate opahalae, and its many filtering tufts make it efficient at capturing food particles.

Here and there on the reefs you’ll find various molluscs — bivalves, snails, cephalopods.

Some come out of hiding only at night, but shells are often found on the beach.

Molluscs

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Bivalves

The shell of the Tridacna can be completely hidden by the surrounding vegetation, and only the bright blue mantle can be seen in the open valves. The rich colour of the mantle is produced by symbiotic algae that feed the mollusc through photosynthesis (yes, just like corals).

Bivalve molluscs are those whose shells consist of two symmetrical flaps. Most lead a sedentary life, sometimes clinging to reefs like the Tridacna (Tridacna rosewateri). This mollusc is endemic to Mauritius, meaning that it is a species found only in this region.

Here and there on the reefs you’ll find various molluscs — bivalves, snails, cephalopods.
Some come out of hiding only at night, but shells are often found on the beach.

Bivalves

Gastropod molluscs

Gastropods typically have a spirally twisted shell. They are a variety of marine snails, ranging from very tiny symbionts that live on coral to large predators that feed on starfish and fish. Most gastropods are nocturnal, but some can be seen during the day on the reef or sandy bottom.

Gastropod molluscs

Some of them are endemic to Mauritius. For example, Ophioglossolambis violacea of the family Strombidae is a mollusc with a massive shell with a horned mouth. It uses the growths on its shell to defend itself against predators — you can’t get close to such a mollusc! Carrying a heavy shell is not easy either, which is why all Strombids have powerful legs.

Another pair of endemic species, Conus julii and Conus primus, are predatory clams that hunt their prey with venom. The molluscs have special glands for this purpose and their teeth are transformed into harpoon arrows with which the cones sting.

Predatory Cones

They should therefore never be handled: the cone’s venom is also dangerous to humans.

Хищные конусы

Cephalopods

In the reef-dwelling Mauritius nudibranch Glossodoris cincta, the gills are clustered in two small bundles on the back, and a brightly coloured skirt runs along the edge of the body.

There are exceptions to the rule of the shell in snails — in the representatives of the holodyna, the shell is reduced, just like in terrestrial snails. Instead of a shell, they have a series of projections on their backs — the very naked gills through which the molluscs breathe.

Cephalopods feed on a variety of sessile organisms: some may eat sponges, others small hydroid polyps.

Cephalopods

Cephalopod mollusc

Mauritian waters are home to the endemic Sepia saya — a relatively large cuttlefish that, like most cephalopods, can change colour and even texture. And the small sepioid Euprymna stenodactyla can glow thanks to symbiotic luminescent bacteria — the glowing dots blur the mollusc’s silhouette, camouflaging it.

Cephalopods, which include octopus, cuttlefish and squid, are another common reef inhabitant. While the latter live in the open ocean, several species of true cuttlefish — sepiids (or the closely related sepiolids — known in English as "squid dumplings" or "short squid") — like to inhabit the reefs.

Cephalopod mollusc

Sea stars and ophiurs

Most starfish are predators, feeding on molluscs, polychaetes and other invertebrates, but Acanthaster planci (known as the "crown of thorns" starfish) feeds on coral polyps and can cause serious damage to reefs.

Sea stars are also radially symmetric and have five or more rays, such as one of the most colourful species on Mauritius, Protoreaster linckii. On the underside of the rays are grooves — in these protrude numerous ambulacral legs with suckers, thanks to which the star and crawl.

On the starfish are similar to Ophiura — they also have five rays, but thin and wriggling, emanating from the central disc. This disc is buried in the sand when feeding, the tentacles are exposed upwards and they deposit food particles on them. They can be quite large, with rays 10−15 centimetres long, such as Ophiocoma scolopendrina, or quite small — ophiurs a few centimetres in diameter often live as symbionts in sponges or on sea urchins.

Sea stars and ophiurs

Fish are the most conspicuous inhabitants of a coral reef. They are numerous, mobile and colourful, and several hundred different species can coexist on a reef — all given enough space and ecological niches.

It is not uncommon for fish to develop special, often bizarre, adaptations to life on the reef.

Fishes

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Here are the highlights of the fish fauna of Mauritius

Вот яркие представители фауны рыб на Маврикии:

We think of rainforests as the lungs of the planet and a major centre of biodiversity, but it is actually coral reefs that are the honourable holders of both titles.

Coral symbionts produce more oxygen than terrestrial ecosystems, and scientists are discovering new species of reef dwellers every year. Every dive is like a journey in itself: time after time you can see something unique, the underwater world never repeats itself.

When you come to any sea, you should see it not only from the surface, but also from the inside — and sailing gives you that opportunity, as long as you have a good mask.

Discover Mauritius with SV Agent

Swim with whales and sperm whales in the open ocean, sample the local street food and find your favourite snorkelling reef. Or if you’re already a captain, take a catamaran with a cook and explore Mauritius on your own terms. It’s an adventure you’ll remember forever.