The marine gastropods of the genus Mitra get their genus name (as well as their common name of 'mitre shells') from the resemblance of many species, at certain angles, to the pointy hat of a bishop (and indeed, the species names Mitra episcopalis, M. pontificalis and M. papalis all appear to be floating around out there). They are fairly middling-sized shells - three or four centimetres long would seem to be a respectable Mitra size - and most of them are slender and pointed at one end (the technical term is 'fusiform', and the discription 'cigar-shaped' gets bandied about regularly). Members of the subgenus Strigatella, however, are shorter, more globular animals.
Mitra are members of the family Mitridae, which is in term a family of the Neogastropoda. Neogastropods have been featured at this site before (here and here), albeit without having been identified as such, and there is a fair probability that if you go looking for gastropods on a trip to the beach that the first one you find will be a neogastropod. This is not so much because neogastropods are that much more abundant than other marine gastropods (although they are a fairly speciose bunch) as because neogastropods tend to be a lot more active than other gastropods, and are much more likely to be visibly on the move while other gastropods are sitting clamped to rocks. And the reason for the greater mobility of neogastropods is a matter of diet.
The ancestral diet for gastropods was a reliable, if somewhat unexciting, scraped meal - algae rasped off rocks, or the fruits of scavenging. As a result, mobility is not at much of a premium for most gastropods - it doesn't take much speed to chase down a patch of algae - and the only reason to move is to get to the next patch of algae. Neogastropods, however, tired of this diet and went for something a little more exiting - they became active predators. Mobile neogastropods at the beach are on the hunt for prey (or, alternatively, pre-deceased animals to scavenge off). One of the most distinctive features of neogastropods to the casual observer is their elongate siphon, which in live animals can usually be seen extended from the front of the shell (which has a distinct notch or anterior extension for it to extend through), waving back and forth as the animal moves, sniffing for any appetising scents. The radula (the tongue-like structure covered with teeth in the mouth of a gastropod) has become adapted to the predatory life-style, with the number of teeth reduced but each individual tooth much larger and sharper. The fusiform body-shape as seen in most Mitra also appears well-suited to mobility, and is shared by a significant proportion of neogastropods.
Members of the family Mitridae possess a particularly elongate proboscis, often longer than the rest of the animal. Running along the inside of the proboscis is the radula and a muscular rod called the epiproboscis, which can be even further extended. Mitrids are specialist feeders on sipunculan worms, which live buried in sediment or burrowed into corals (Taylor, 1989), and the epiproboscis is used to capture their prey. Suggestions that it is used to inject digestive enzymes into the prey for external digestion are incorrect, as the prey is usually swallowed directly without allowing time for digestion (Taylor, 1989) The method used by Mitra idae to capture a sipunculan was described by West (1990), and as the morphology of the epiproboscis is fairly constant within the Mitridae other species probably use the same or a very similar method. After locating a sipunculan with its siphon, the gastropod would extend its proboscis until it contacted the worm, then the epiproboscis to grab onto the worm. The first move of the Mitra would then be to try and suck the worm directly out of its burrow. If this failed (which I suspect would be the norm), it would then use its radula to rasp a hole through the worm's skin before inserting the epiproboscis through the hole. The epiproboscis would entwine itself around the worm's viscera and grab directly onto its intestines. The viscera would then be hauled out through the hole in the sipunculan's skin and slurped down the waiting proboscis. Once the Mitra had pulled as much of the worm's guts out as it could, it would close its proboscis over the remaining husk and finish drawing the worm out from its hole. Insertion and retraction of the epiproboscis took a little under ten seconds. The whole process, from initial insertion to final withdrawal, could take up to twenty minutes.
REFERENCES
Taylor, J. D. 1989. The diet of coral-reef Mitridae (Gastropoda) from Guam; with a review of other species of the family. Journal of Natural History 23: 261-278.
West, T. L. 1990. Feeding behavior and functional morphology of the epiproboscis of Mitra idae (Mollusca: Gastropoda; Mitridae). Bulletin of Marine Science 46 93): 761-779.
Are these critters closely related to Conus? The shell-form and proboscis seem reminiscent, and the epiproboscis sounds as if it might be homologous-- or associated with homolgs of-- the conefish dart.
ReplyDeleteConus are indeed neogastropods, although they are not particularly close to mitrids within Neogastropoda, being more closely related to turrids (turret shells)and terebrids (auger shells). Mitrids on the other hand seem more related to muricids (murexes), volutids (volutes) and suchlike.
ReplyDeleteThe darts of Conus are modified radula teeth, not homologues of the epiproboscis.
What Adam said, though I'll add that the mitrid epiproboscis has been suggested to be a venom delivery system like that found in Conoidea* (conids, turrids and terebrids), though I suspect that this was always intended as an analogy rather than a suggestion of homology. At present, however, there's no evidence for venom in mitrids.
ReplyDelete*I'm rather disappointed by the wholescale adoption of the name Conoidea for this group, because Toxoglossa (poison-tongues) was such a cool and appropriate name.