Those amazing mollusks (or do we spell it molluscs?)

Slug, Snail or Clam?
It seems simple, just count the number of shells, none for slug, one for gastropod/snail and two for bivalve. But not always that simple.

• Sea Hare. A gastropod/snail. The shell is entirely internal, and can take any of the shapes shown at right. One sea hare species can reach a weight of 14 kg, making it arguably the largest gastropod species.

• Cryptella canariensis. A semi-slug. “semi-slugs are land gastropods whose shells are too small for them to retract into, but not quite vestigial”. According to wiki, there are twice as many species of semi-slug as species of slug.

• Cavolinia tridentata. A sea butterfly, and therefore a free-swimming snail.
  

• Julia exquisita. A bivalved gastropod. Originally classivied as a bivalve/clam, this is now reclassified as a member of the gastropods/snails.

• Ephippodonta. A bivalve that walks around on its foot like a gastropod.
  

Black sea hare. They can grow quite a lot bigger than this.

Bubblecar said:

Black sea hare. They can grow quite a lot bigger than this.

Apparently the Black Sea Hare is the largest sea-slug, but they are all toxic to one degree of another:

Sea hares can also have a semi-toxic slime layer over their bodies and toxins infused through their tissues. The slime is a defence mechanism and, combined with their great camouflage capabilities, means they have almost no predators. Their toxicity is highly variable and depends on what the individual hare eats.

https://www.australiangeographic.com.au/topics/science-environment/2019/01/toxic-slime-camouflage-and-purple-dye-sea-hares-explained/

Having once collected sea shells, it was not uncommon to find the internal shells of various mollusc species, but alas information then was sketchy at best and most remained unidentified. With the Internet it is so easy these days to get information about most groups of organisms including plants. Even without the Internet there are so many illustrated books available, whereas not so long ago there was virtually nothing. I remember Mrs PF and myself buying each other the same marine biology book for Xmas, simply because it was a new publication and only the second general marine biology reference available at the time.

PermeateFree said:

Having once collected sea shells, it was not uncommon to find the internal shells of various mollusc species, but alas information then was sketchy at best and most remained unidentified. With the Internet it is so easy these days to get information about most groups of organisms including plants. Even without the Internet there are so many illustrated books available, whereas not so long ago there was virtually nothing. I remember Mrs PF and myself buying each other the same marine biology book for Xmas, simply because it was a new publication and only the second general marine biology reference available at the time.

It’s been a very long time since I collected seashells. I narrowly missed visiting the seashell museum in Lakes Entrance a couple of months ago, which sparked my interest again.

• Teredo navalis, the shipworm, is another weird one. A bivalve, but you’d hardly know it because it’s more worm-like.

• And then there’s Brechites giganteus, the giant watering pot. A bivalve with a single tubular shell. It begins life as a bivalve, the two original shells are that the bottom of the following image, but then it builds a calcarious tube from that and cements the original shells in. The tube will pick up and cement in whatever other shells it passes.

We often find sea hares around the rock pools.

This is a rose petal bubble snail. They are carnivorous and eat bristle worms.

fsm said:

We often find sea hares around the rock pools.

This is a rose petal bubble snail. They are carnivorous and eat bristle worms.

Bleedin beautiful. Like.

A piece of QI style trivia for you all. You know how lizards will deliberately lose a tail to lose a predator.
Some gastropods do too. Including the Harp shell.