Most people are familiar with slugs and snails, but many many more interesting molluscs live in the sea.
Marine Gastropods
There are around 30,000 species of named marine gastropods (although that number rises daily), and it represents about half of the entire family (another 30,000 or so live on land or in freshwater). Their only rivals – in terms of number of species described – are the insects (65 thousand gastropods and a million insects).
This huge class includes all the snail-like animals, and a lot of unexpected 'weirdos', from beautiful sea slugs that entrance divers to obscure parasites. They all have one thing in common – their larvae have a twisted history!
Torsion in Gastropods
Marine gastropods usually have planktonic (float in the sea) larvae. Normally they develop shells which protect them a bit, but the ancient design left them with their head out when they pulled back into it.
Millions of years ago the gastropods solved this problem by turning their bodies round as they grew, putting the head in a safer place and leaving the tail to be nibbled.
Their is a lot of debate about exactly how this trick was achieved, and the old idea (that they simply mutated to rotate through 180 degrees) has been challenged. Some now think that the ancestors might have had two 'holes' to pop into (an ancestral 'double mantle cavity') and simply mutated to choose one over the other (Integrative and Comparative Biology).
However it came about this idea of twisting round so the head could be pulled into the shell first was successful. True there were inconveniences (faeces dropped on your head instead of behind you), but the advantages of bringing your gills and extra sense organs to the front (as well as head protection) outweighed them.
Many gastropods went on later to develop a tough 'lid' (operculum) on their tail to close the opening to the shell once the animal was safely.
Later (in evolutionary terms) some gastropods found this legacy of torsion inconvenient, and the nudibranchs (for example) cancelled the process in their development (secondary detorsion).
Sea Slugs and Nudibranchs
When a gastropod mollusc 'decides' to float around on the sea, or creep around on the sea bed it is convenient to undo the ancient trick of twisting round. Nudibranchs are a good example of gastropods that have done this.
These 'nudibranchs' are among the most beautiful creatures in the sea – they have a great following, and many amateur diver/photographers are entranced by them.
These enthusiasts collect information that is scientifically invaluable.
Sources:
- 'gastropod', Encyclopaedia Brittaniaca.
- 'Modern insights on gastropod development', Page, Integrative and Comparative Biology, 2006.
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