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Guest Blog by Dr. Mike Vecchione on Heteropods
Heteropod snails by Mike Vecchione
I am out here on the R/V Point Sur primarily to study cephalopods, the squids, octopods and their relatives. However, I have decided to write about a different group of very interesting animals. The swimming snails called heteropods are a group about which most people know nothing and nobody knows much. They are not closely related to the other, and somewhat better known, group of snails that spend their entire lives in the pelagic environment, the pteropods.
Heteropods are active visual predators and, although most are small (planktonic), some can get quite large -- up to a half meter in length. I studied them many years ago and am now becoming reacquainted because we have been catching them regularly in our samples from the upper water layers. Three families exist, which seem to show a progression of evolution into pelagic life. Animals in the family with the smallest animals but most species have a coiled transparent shell into which they can withdraw their body and seal it off with a standard snail trap door, called an operculum. The shell has a keel on it, similar to what you might see on the bottom of a sailboat. The second family gets much larger but still has a keeled shell; for most species in this family the shell covers the guts like a hat. The third family loses the shell during transformation from the larval stage. They are streamlined and can swim quite fast. They should definitely be considered nekton rather than zooplankton.
All heteropods have well developed, but peculiar, eyes. They have a narrow retina that the can rotate up and down. They probably form an image much the same way that the image forms on a TV screen, aggregating one line after another rather than the whole picture forming at once. Another characteristic of all heteropods is a single fin on the belly. They normally swim by undulating this fin. Additionally, the larger species can swim in rapid bursts by eel-like undulation of the entire body, which is shaped somewhat like a fish including a flattened tail. The bodies, especially in the larger species, are remarkably transparent except for the eyes (which have to catch light), the guts, and the organs of the mouth. Often when we are sorting our catch, the only way you can tell that there is a large heteropod is by the eyes, guts, and mouth.
The mouth, located at the end of a trunk-like proboscis, has a structure characteristic of snails and most other molluscs, a tooth-covered tongue called a radula. This is used to grab and tear the heteropod's prey. The proboscis is why these animals are sometimes called "sea elephants". What they eat varies among the three families. The small species in the fully shelled family eat other snails, especially pteropods. They have a sucker at the base of the fin and use it to hold the shell of their prey. Not much is known about the natural prey of the other two families. It has been proposed that they eat a variety of prey, ranging from jellyfish to arrow worms, perhaps even including small fishes. Incidentally, the sucker is reduced in the other families and seems to be used to hold a sexual pair together during mating, so that the male can transfer a packet of sperm (spermatophore) to the female.
We are finding that these active predatory snails are quite common out here, It will be interesting to figure out their importance in the pelagic ecosystem of the open Gulf of Mexico.
Carinaria lamarcki is an example of heteropod from the family Carinariidae (top image).
A close-up of the shell of Carinaria lamarcki
An example from the another family of heteropod - the Pterotracheidae. This is Pterotrachea coronata.
An example of the heteropod family Atlantidae. This is Oxygyrus inflatus.
For more imformation, see http://tolweb.org/Pterotracheoidea/27801