The SWIMMING ANEMONE Boloceroides mcmurrichi

Swimming anemone Bulloceroides mcmurrichi on mangrove seed. Image M. Heckman

Most sea anemones when harassed just hunker down, shoot off some stinging cells and wait it out. If the harassment continues over time (such as repeatedly getting stung by a larger neighboring sea anemone), they will slowly inch away to a better location.

Not so for swimming anemones in the genus Boloceroides - when the neighborhood goes down, they just get up, start pulsing the tentacles - and swim away.

A study here at the "Hawaii Marine Laboratory (HIMB)" in 1965 by Josephson and March, noted that these anemones actually have a couple of options. When stimulated (i.e. by tapping the tank or prodding them in a scientific fashion) they first tended to "glide away" from contact by releasing their grip and pulling themselves along the bottom with their tentacles. Swimming was the second response.

Swimming sessions were typically short, about 15 seconds long at a velocity of about 1.9 cm/sec (= a blazing 3/4 of an inch per second or .04 mph). As you can imagine, these are not wildly streamlined animals. Each stroke back by the tentacles is countered by the return stroke. "About 80% of the forward impulse produced during the effective portion of the stroke cycle is negated by rearward impulse during the recovery portion of the cycle."

Why swim? Several reasons - in 1982, researchers Lawn and Ross noted that Boloceroides are eaten (predated upon) by a the sea slug Berghia major.  The sea slug gets food and steals some stinging cells for its own defense from the sea anemone. The sea anemone - well, it just gets eaten. So being able to swim away is a good thing for Boloceroides. Even better, Boloceroides can drop its tentacles, so when the sea slug lunges onto it, Boloceroides releases its tentacles and swims off, shedding the predator as it goes.

Next, just getting around is always useful. As mentioned before, these anemones can move to avoid predation, but they can also possibly move to better feeding areas. Or, since they harbor the same group of symbiotic algae that corals do and can act as greenhouses for an internal bank of photosynthetic food producers, they may be able to move to areas better suited to their symbionts if needed (I see a project for one of our interns here).

Swimming anemone. Photo R. Shinsato

These are elegant looking anemones. Our local type, Boloceroides mcmurrichi has long banded tentacles and can reach sizes of about 6" across. They occur fairly frequently in our research tables, coming in with the raw seawater or on collected fragments, then grow to a size where they are noticed.

At that point, our researchers sometimes helpfully take them out of their own tanks and put them in my touch table - where the Boloceroides swim about, landing on other animals and stinging  the heck out of them. These are not good community members for our tank. They need their own space.

And consider the issues of having a stinging anemone in a "touch tank." Although their sting appears to be very mild, they have the disturbing habit of autotomizing (casting off) their tentacles. The tentacles are sticky (actually they are micro-harpooning whatever touches them with their stinging cells) and continue to writhe for some time after detaching. Although the tentacles are able to seal off and even have the ability to regenerate into a new sea anemone, the whole process is a bit traumatic to a visitor - and probably not that great for the anemone.

Check out this video of tentacles released during the removal of one of these anemones from our tank - pretty amazing.

So enjoy viewing these anemones if you see them, but don't disturb them - unless they need to be moved to a more appropriate neighborhood. They are a wonderful animal for the reef, but maybe less so for a community tank.

Aloha,

Mark


References (all accessed on 09-21-12):

A very nice site with great pictures (I like the red highlights on one of the anemones) and more video:
http://www.wildsingapore.com/wildfacts/cnidaria/actiniaria/boloceroididae.htm

We had an article on a similar sea anemone eating sea slug recently. See: "This Week, the Anemone Eating Sea Slug, Berghia."

Keoki and Yuko's site: http://www.marinelifephotography.com/marine/cnidaria/boloceroides-mcmurrichi.htm

As always, John Hoover's book is a great reference, see: http://www.hawaiisfishes.com/books/hawaiis_sea_creatures/creaturebook.htm

Technical articles: 

The swimming performance of the sea-anemone Boloceroides. RK Josephson, SC March. 1966. Journal of Experimental Biology 44, 493-506. Full article: http://jeb.biologists.org/content/44/3/493.full.pdf+html

The behavioural physiology of the swimming sea anemone Boloceroides mcmurrichi.  ID Lawn, DM Ross. 1982. Proceedings of the Royal Society of London B: Biological Sciences, 216 (1204), 315-334.