Friday, July 13, 2012

The Violet Coral Eating Snail, Coralliophila violacea

Coral fragment adorned with snails. Photo M. Heckman
In terms of a food source, corals would not seem to be the best of choices. Their skin is very thin (just two layers thick - not much food value there), plus they can sting. With the coral's zooxanthellae (symbiotic algae) and body fluids, there is some nutrition available, the question is just how to get it. Some coral-eating animals graze off the tissue (see the previous blog entry on a coral-eating seaslug), some pluck or pick (like the multiband butterflyfish), some do the occasional scraping (such as parrotfishes), and some just get down and digest (like the crown-of-thorns seastar as well as some bacterial diseases). 
Coral eating Phystilla seaslug, Multiband butterflyfish and Crown-of-thorns seastar - Photos by M. Heckman and K.Stender

Violet coral-eating snail, Coralliophila violacea. Photo K. Landers
Our animal for the week does none of the above - instead of biting, grazing or scraping, it just sips. It is the violet coral snail, Coralliophila violacea. The image above shows one on a fragment of finger coral (Porites compressa) and another to the side. The name of the snail is easy to understand, note the elegant purple edging on the inside of the shell.
These animals are part of the Murex snail group which includes our familiar drupe snails and muricid snails. The group in general are predators, often on animals with shells. For instance, the common granular drupe, Morula granulata or maka 'awa that you can find on rocks all along the Hawaiian intertidal, drills a hole into barnacles, limpets or other shelled animals that can't run away, then slurps out the insides. The drilling is undoubtedly a bit of work, but provides a good end result (for the drupe).
Granular drupe. Image K. Stender
The violet snail's prey, coral, does not require any drilling. In a coral (unlike a barnacle) the tissue comes first, then the skeleton. The violet snail can simply insert it's proboscis into the coral tissue and sip, siphon or suck -  depending on how you feel about these animals taking a meal from the coral.
C. violacea next to its feeding scar. Photo K. Landers
The result of this feeding is a predictable white feeding scar. There was a nice study done on the effect of this suction on a coral by Oren, Brickner and Loya. By labeling the coral's fluids with radioactive carbon 14 (this is very slightly radioactive), they were able to show that fluids from other parts of the coral would slowly be drawn down into the area that the snail was siphoning from.

So, theoretically, if a snail has moderate needs it can stay in one place and just gently feed without killing the host, a time honored parasitic lifestyle. However, this does not always work out; the Caribbean version of the violet coral snail, the abbreviated coral snail (Coralliophila abbreviata), creates larger and larger denuded areas as it feeds on elkhorn corals, leading to significant tissue loss. With Caribbean and Atlantic coral reefs in such poor shape, any population of abbreviated coral eating snails is not looked upon with favor. The host corals are already dying from a variety of causes and the more aggresive coral eating snail is not helping the situation.

Violet coral snail on P. compressa fragment. Photo K.Landers
The violet coral snail does not cause as much damage. It typically just stays in one spot and does not leave large swaths of dead coral tissue in its wake. According to Dr. Cindy Hunter, this actually makes them hard to find. There is no obvious dead tissue area around them to help make them easy to spot. Other Coralliophila snails around the world may be intermediate in their effect, certainly any tissue damage to a host can lead to secondary infections and other issues.  I removed the snails from our experimental tanks. It seemed unlikely that our coral fragments could survive while feeding a snail almost as large as themselves.

Other interesting facts about these snails:

Photo K.Landers
Violet coral snails are protandrous hermaphrodites, that is, they start out male, then turn into females. Just a short bit of thinking should reveal some advantages to this system. The animal can breed throughout its life, even when it is too puny to produce eggs. Eggs are costly to make (metabolically, with all those fats and nutrients and such).  So when a snail gets large enough to effectively be a female, it turns into a female; when it is smaller, it can function as a male and still be in the reproductive pool.

Secondly, rather than laying the eggs out in the open, the female broods her eggs under the edge of her shell until they hatch.  Most mollusks do not show such parental care, they just lay their eggs about in ribbons or patches and then crawl off to let the eggs hatch on their own.

Finally, while other snails within the violet coral snail family, Coralliophilidae, live on corals, some live in the  coral. One type bores into the skeleton of mushroom corals, making a burrow so tight it cannot even turn around.

If you see one of these snails in the wild, just admire it. Unless it is doing obvious damage, it is just part of the system.



References for this article and on these snails tend to be a bit scarce or technical, but if you have time are very interesting.

For info on the drupe snail feeding, John Hoover's Hawai'i's Sea Creatures always recommended.

For Hawaiian snails and shells in general, the slightly dated but still excellent, Hawaiian Marine Shells: Reef and Shore Fauna of Hawaii, Section 4: Mollusca.Alison Kay. 1979. Honolulu: B.P. Bishop Museum Press.  Look for a used copy.

Image of abbreviated coral snail damage (and another Caribbean Coralliophila that causes much less damage) can be found at this site: Field Guide to Western Atlantic Coral Diseases and Other Causes of Coral Mortality.

Image of Coralliophila on a Red Sea coral, see the Living Oceans Foundation's, " The hungry hungry corallivores" blog post.

Other selected references (accessed 07/12/12)

Ecology of a corallivorous gastropod, Coralliophila abbreviata, on two scleractinian hosts. II. Feeding, respiration and growth. I. B. Baums, M. W. Miller and A. M. Szmant. Marine Biology Volume 142, Number 6 (2003), 1093-1101, DOI: 10.1007/s00227-003-1053-4
Host effect on size structure and timing of sex change in the coral-inhabiting snail Coralliophila violacea. Ming-Hui Chen, Keryea Soong and Min-Li Tsai. Marine Biology Volume 144, Number 2 (2004), Pages 287-293

Prudent sessile feeding by the corallivore snail, Coralliophila violacea on coral energy sinks. U. Oren, I. Brickner, and Y. Loya. Proc Biol Sci. 1998 November 7; 265(1410): 2043–2050.

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