Monday, April 11, 2011
My time with a heteropod
We got a new addition last week, a Carinaria japonica an unusual animal commonly called a sea elephant, after its long snout-like proboscis. It is generally around 13 cm (5 in) but can get to 50 cm (20 in) long. This one is about 30 cm (12 in). This animal is a type of snail, called a heteropod, that is modified to spend its entire life swimming in the water column.
Its shell is very reduced, to decrease the weight on the animal, and located on the bottom of the creature. Because of the shell's weight, small that it is, the animal spends its life swimming 'upside down'. The flap that it uses to swim with is a modified foot, and it has a large radula at the end of its proboscis for capturing prey.
It has very well developed eyes for a snail, complete with spherical lenses and ciliated retinas. The retinas themselves are interesting, because they are a thin strip, rather that a large patch. Because of that its vision field is only a few degrees high, and 80 to 180 degrees long . Some species solve that problem by constantly sweeping their eyes up and down.
They eat other things in the plankton, preferring things like arrow worms, salps, copepods, and krill . The prey is ingested whole, and digestion generally takes place without any mastication. There really is not much known on how they capture their prey, and I'd like to make a few comments based on observations made while feeding this little guy.
I have been feeding him by hand, which is certainly not natural, but I have noticed that when I drop food or put food near him, he cannot capture it without some sort of pressure behind it. The jaws of the radula seem to push out, then open, clamp the food then drag it back in. If there is no pressure behind the food, the radula just pushes it away. He generally solves the pressure problem by maneuvering his proboscis so that the food is trapped between it and the wall of the tank.
Obviously, that trick would not work in the wild, but I have noticed a second trick which would work in the wild. With floating food, he sometimes curls in to a circle and traps it between his proboscis and his large broad tail. This may be a behavior which helps them capture food in the wild... we won't know for certain until someone seriously takes a look at that.
In the meantime, I will continue to observe and marvel at my new companion...