Keyhole Limpets (Jan 2007)

[zanzimog]

Keyhole Limpets: Hitchhiking Grazers

You will probably not see these guys for sale in your LFS, but there's a good chance one will ride into your tank on live rock at some point. They are interesting little buggers, and excellent algae grazers, but their broad tastes may make them unwelcome in a tank with soft corals, tunicates or sponges.



Description
All limpets have the same basic appearance, with a low, conical shell. Unsurprisingly, the keyhole limpets (see photo above), have a hole at the peak of their shells, while others do not. The fleshy limpets (photo below) are actually keyhole limpets, with the shell covered by an extension of the mantle. Both non-fleshy and fleshy species make their way into aquaria on live rock shipments.



Taxonomy and General Biology
All limpets are in the phylum Mollusca, class Gastropoda, along with snails and slugs. Although it is often kept hidden under the shell, limpets have a perfectly good, snail-like head, as can be seen in the grazing animal below. Despite all having a similar appearance, limpet species are from at least two divergent orders. The keyhole limpets are members of the primitive order Archeogastropoda, while other limpet species evolved independently and form the order Patellogastropoda.

Limpets are all adapted for life on rocks in strong current and surge. Unlike other snails, they cannot retract their bodies into their shells when disturbed. Instead, they clamp their conical shells tightly to the rock, which gives them low resistance to waves and currents, and makes them difficult to remove by predators. In several species, it has been documented that they have "home depressions," resting in the same place every day, and ranging to forage at night.

Most keyhole limpets are generalist grazers, using their strong, tongue-like radulla to scrape whatever edible stuff they can find from the substrate. In the wild, this can include algae, sponges, bryozoans and hydroids. Their diet in the aquarium is similar, and I have seen them taking the occasional divots from soft corals as well. The photo below shows one grazing on a sponge in an aquarium.



Care and Feeding
Because of their ruggedness and non-discriminating tastes, keeping keyhole limpets is not too challenging. Like any marine invertebrate, they require stable water conditions and will be killed by copper and other heavy metals. A small but steady supply of algae, which is not usually a problem in most tanks, will keep them well fed and happy for years. When moving them, keep in mind that they grip surfaces very tightly and can be damaged if pulled off rocks or glass. It is better to wait for them to move onto a small rock if you want to move them safely.

Reproduction
Sexes are separate in the limpets, and fertilization is external. Sperm and eggs are released through the excurrent hole at the top of the shell, making the limpet look like it is putting out a little puff of smoke. In the wild, there is presumably some way of coordinating the release of gametes by the sexes, so that eggs can meet up with viable sperm, but I have not found any literature on this. Embryos develop into free-swimming larvae called veligers, which settle after a period of time that varies between species. Surprisingly enough, despite the low odds of veligers surviving in reef tanks, there are reports of successful reproduction by limpets.

Reef Safe?
If you feel lucky, keyhole limpets can be added to a reef tank. Because they move slowly, they are pretty easy to remove if they cause trouble. My own experience has been mixed, with aggravation at seeing them eat pieces from cherished leather corals, but having them leave other species, like gorgonians, completely alone. On the other hand, they can be added without hesitation to FOWLR tanks. Their ability to protect themselves by hiding and clamping to surfaces makes them a good choice for predator tanks as well.

References/more info:
Snails and Limpets by Ron Shimek
Ruppert, EE and RD Barnes (1994) Invertebrate Zoology: A Functional Evolutionary Approach. 6th Ed. Harcourt Brace College Publishers.