BIO, DIVES

During our dive at Île d’Or, we were able to observe the surprising diversity of marine life.
Impressive schools of white seabream swam through seascapes teeming with opisthobranch mollusks, all kinds of fish, tubeworms, and the large blue gorgonians, which appear red when illuminated…

The dive was organized around three rocky pinnacles.
On the side of one of them, we encountered blue gorgonians at a depth of 25 meters. These gorgonians are usually found at greater depths.
We also saw three octopuses, two of which were huddled side by side, protecting themselves from rocks that they had attached to their suckers.

During our exploration, we encountered many nudibranchs. This gives us the opportunity to tell you about three of the most common species in the Mediterranean:

• First of all, what is a nudibranch?

  Nudibranchs are small animals belonging to the opisthobranchs, which are part of the class Gastropoda. These gastropods are part of the Mollusca, the second largest phylum in the animal kingdom, after the Arthropoda (insects, crustaceans, etc.)! And yet, these opisthobranchs are relatively unknown! They are referred to as opisthobranch mollusks. “Opistho” means “behind,” so literally “gills behind the heart,” as opposed to prosobranchs such as the periwinkle: “gills in front of the heart.” Opisthobranchs comprise two main groups: nudibranchs and tectibranchs.
  Tectibranchs all possess a shell, more or less developed, while nudibranchs never have one (“nudi” = “naked,” hence “naked gills”). There are also heterobranchs, which may or may not have a shell. They can thus resemble tectibranchs or nudibranchs. Among nudibranchs, a distinction is made between Dorids (e.g., Doris) and Aeolids (e.g., Flabellina).

• The mauve flabellina, or Flabellina affinis, was found on the hydroids they feed on: Eudendrium ramosum. These flabellina are among the most common: we encountered at least five during the dive. Their size varies between 10 and 40 mm, and their bodies are a beautiful lilac color. They are common in Mediterranean coralligenous reefs and on hydroids, from July to September. The egg mass is meandering, the same color as the integument. Former name: Flabellina flabellina, Vérany, 1846.

  

• Flabellina hervia has many names: processionary hervia, or Cratena peregrina.They are among the most common species in coralligenous reefs and, like Flabellina affinis, can be found on hydroids from March to September. We observed two during our dive, feeding on hydroids alongside a larger Flabellina affinis. From a distance, they can be confused with mauve flabellina. But if you want to correctly identify nudibranchs, you need to pay attention to every detail of the animal. Indeed, hervia costai (named by Haefelfinger in 1961) is easily recognizable thanks to the two superficial orange spots visible in front of the rhinophores (erect tentacles at the front of the body that serve an olfactory function). Their size varies between 10 and 30 mm. The tactile tentacles are very long and tapered. The eggs are meander-shaped and rather white in color.

  

• Dalmatian doris, or Discodoris atromaculata, formerly known as Peltodoris, is a Mediterranean endemic species easily recognizable by its brown spots on a white background, resembling Dalmatian dogs. They range in size from 20 to 100 mm. They feed on the sponge Petrosia ficiformis, more commonly known as the stone sponge. Unlike the two previous species, the Dalmatian doris is not a flabellina, but a doris: its gills are rosette-shaped around the anus, whereas aeolids have numerous simple or branched papillae (cerata) on the sides of their bodies. The Dalmatian doris is the most common dorid in Mediterranean coralligenous reefs. The egg mass is white and spiraled into a ribbon.

  

  

In addition to the nudibranchs, this dive at Île d’Or was full of surprises. We encountered a large Mediterranean fanworm, and, right in the middle of the dive, a large dentex emerged from the blue to observe us. But the highlight of the show was sheltering in a small crevice in one of the rocky pinnacles. We first spotted two spider crabs taking refuge there. Looking more closely, I saw a small red scorpionfish. Then two. Then three! And at the very bottom of the crevice, a beautiful, larger brown scorpionfish was resting. Six of the most sought-after animals in a single shelter! Incredible what you can observe in a 45-minute dive, isn’t it?

William Zylberman

Bibliography : 1000 & une limaces de mer, Nardo Vicente, éditions gap.