Thursday, October 20, 2016

Hawaiian Swimming Clam (Limaria keohea)

File clam collected during our invasive algae lab. Image by HIMB.

The Hawaiian swimming clam Limaria keohea, is one of the most unique invertebrates found in Hawaiian waters.  It can swim, it can drop its sticky tentacles like a lizard loosing its tail, and it lives in a tidy hidden nest it has made out of byssal threads.   

During our invasive algae lab, students are asked to replicate some of the methods used by DNLR's Supersucker.  This includes sorting through the algae for any animals or native algae, and separating them from the invasive gorilla ogo seaweed Gracilaria salicornia.  One of the most spectacular species found in this lab is this Hawaiian swimming clam. A type of file clam, once pulled from the water it appears as an orange gooey blob, but when it is submerged back into the water, it gets lots of "oohs" and "ahhs" as it expands and starts swimming in rhythmic, mesmerizing motions.

The swimming is achieved by opening and forcefully closing the shell valves, ejecting jet-like streams of water from the hinge area to move forward. The tentacles may assist in the swimming and certainly assist the clam as it explores its world. Take a look at the video below.

L. keohea swimming.      Video  L.Tanabe/HIMB

File clam on byssal threads among gorilla ogo seaweed. Image by HIMB
The pink/orange tentacles are also used for defense. They are sticky and can be released to attach to a predator or other threat. A study of a related Limaria clam published by the Journal of Marine Biological Association of the UK found that the mucus responsible for the sticky tentacles is also distasteful to predatory fish. Nothing like having sticky distasteful tentacles stuck to your face and mouth. We don't know for sure if our Hawaiian clams tentacles are distastful to fish, but they are certainly sticky.

The prevalence of the Hawaiian swimming clam living in the alga around Coconut Island is very peculiar. These clams didn't start appearing in our alga labs on Coconut Island until around 2010. Apparently, it was a very good recruitment (settlement) of juvenile clams that year and they have been common ever since. Published literature suggests that this species are more commonly found in old coral blocks or under rocks, which makes their occurrence in an invasive alien alga all the more interesting. They have moved into a new habitat.

In the nest.  Photo by L.Tanabe/HIMB.

As students are pulling apart the clumps of alga, they rarely notice that they are pulling apart the clam's nests. The clams make the nests from loosely spun byssal threads. These are very similar to the threads that mussels use to attach themselves to the rocks, but in this case, the keohea clams use them to reinforce the sides of a burrow or nest in which they live. One study of a related Limaria clam, indicated that the clam creates thousands of nests in and around rocks and sediment on the bottom, suggesting the clams are actually creating biogenic clam reefs! The threads are a silky strong proteinaceous material.

Life in the alga - Limaria keohea embedded in the alien invasive alga Gracilaria salicornia. Video L. Tanabe/HIMB

Like other bivalves, file clams are filter feeders.  They use a siphon to intake water as they filter out food.  This water is also used to obtain oxygen for respiration. Perhaps the invasive alien alga, in a slightly murky food filled lagoon, is proving to be an excellent habitat for at least one Hawaiian endemic animal.

Written by Lyndsey Tanabe, M. Heckman and HIMB CEP Interns.

Donovan, Deborah A., John P. Elias, and John Baldwin. "Swimming Behavior and Morphometry of the File Shell Limaria Fragilis." Marine and Freshwater Behaviour and Physiology 37.1 (2004): 7-16. Web.
Eldridge, Jan. "Aquatic Invertebrate Important for Waterfowl Production." Invertebrate Natural History (1990). Web.
Gilmour,  T.H. J. "The defensive adaptations of Lima hians (Molluca, Bivalva). J. Mar. Biol. Assoc. U.K. 47, 209-221 (1967)
Kay, E. Alison. Hawaiian Marine Shells. Honolulu, HI: Bishop Museum, 1979. Print.
Morton, Brian. "Bivalve." Encyclopedia Britannica Online. Encyclopedia Britannica. Web. 31 Mar. 2016.

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