Mollusca – Creatures of the Month: March 2019 DDAT

Creatures of the Month – March = Mollusca

Molluscs; although they are such are wide group of creatures they are often overlooked.

If you talk with any diver for too long, you will soon realize that we love to chat about the different critters we see on our dives. Many highlights include dives with sharks, manta rays and other big marine animals. But there are many other small, interesting animals we see too! So many, in fact, that this blog will only cover the Mollusca phylum; and with over 50,000 species, we will barely even scratch the surface!

A simple breakdown of molluscs

Mollusca are invertebrates, meaning they lack backbones (vertebrae). Mainly all molluscs are defined by their have three body regions consisting of a head, a visceral mass, and a “foot”. Although all share these features they vary widely in appearance. It might be surprising to know that this phylum contains the nudibranchs, the scallops, and the squids! 

In scientific terms, the phylum Mollusca can be broken down into 7 distinct classes. 

  1. Monoplacophora
  2. Scaphapoda 
  3. Aplacophora
  4. Gastropoda 
  5. Polyplacophora 
  6. Bivalvia
  7. Cephalopoda 

Of this list, the first three classes contain species that are rarely seen and/or found in extremely deep waters. The remaining four classes are made up of some creatures more familiar to the diving community and society in general. Below we go into more detail on these groups….



Firsly the Gastropods (slugs and snails): The literal meaning of this word is “stomach-foot”. Referring to the internal anatomy that has developed in such as way that the stomach lies above the “foot” area of the animal. Snails and slugs are familiar land-dwelling gastropods, but many other species are found in the ocean. Some gastropods including snails and limpets have a shell, while others; slugs and nudibranchs, do not. Certainly one of our personal favorites are nudibranchs and many macro divers will agree!

Nudibranchs, meaning “naked gills” are so named for the external gill-like structures prominently displayed on the outside of their body. These animals are exciting finds on any dive due to their flashy, colorful appearance. Nudibranchs come in all manner of bright and sometimes neon colors. They can range in size from 12 inches long to some which are barely distinguishable with the human eye. Timor-Leste is home to an exceptional array of nudibranch which are found all along the coastline. However, many species we can find just outside the main city at our local dive sites!


Similarly, the name of this class comes from an antomoical standpoint – it means “many-plated” and comprises of animals generally called “chitons”. Defined by the eight plates on their body. Chitons are slow creatures, crawling along rocks and other substrates scraping away at algae for food, sometimes moving only ten feet in an entire year!


Meaning “two-shells”, bivalves are one of the more widely known classes of mollusks. Many bivalves have been used throughout history and even today as a source of food. Including mussels, oysters, clams and scallops. Out of the approximately 15,000 known species of bivalve, around 80% are marine. Vast numbers can be spotted on the reefs of Timor-Leste if you know what to look for. Mentionably,  giant clams, oysters and colorful scallops. Timor-Leste boasts a host of different bivalves. A highlight for us is the electric clam (Ctenoides ales). Long thought to be creating its signature electric display through bioluminescence, researchers have found that this effect is actually caused by two different compositions found on the clam’s mantle or lips. One side of the “lips” absorbs light, while the other side is highly reflective, thus when the clam is feeding, the opening and closing of the lip flaps creates the electric-looking effect that we see. 

Need more interesting facts to convince you of how cool bivalves are? You might not see it, but many bivalves, like scallops, have hundreds of eyes all along the edge of their shell! 



Finally, also commonly knowm is our last class of molluscs. Cephalapods includes octopus, squid, cuttlefish and nautilus. Of these four groups, only the nautilus has a full outer shell. Of all molluscs, cephalopods have the most developed nervous systems and a well-developed eye. Hence giving them a high level of intelligence and great eyesight. Octopus, squid and cuttlefish also have an interesting characteristic, the presence of chromatophores found in the skin. Basically pigment sacs which can open and close as desired by the animal. By expanding or contracting the pigment within the sac, allowes the animal to change colors. In addition, the presence of papillae, sections of skin that can change texture, allow some species the ultimate camouflage. Interestingly, allowing them to mimic different substrate textures and colorations.

It truly is incredible how diverse life in the ocean is. We could go on and on about all the different creatures in the mollusk phylum, but don’t just take our word for it. Come for a dive and see them for yourself! 

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