Okenia rhinorma feeding

January 18, 2010
From: John Chuk


Dear Bill,

When I found the egg-laying Okenia rhinorma specimen seen in my previous message [#23110] I noticed many ascidians buried in the sand nearby (only visible by their protruding siphons) and decided to collect some to see if the nudibranch would take an interest in them in the lab.

An ascidian was buried in sand in a lifelike position and the O. rhinorma specimen placed on the sand about 10cm from the ascidian. The O. rhinorma immediately partially buried itself and went into motion. It took about two minutes to track a curved course to the ascidian and there it stopped. The rapid closing of the ascidian siphons suggested that it had begun to feed. It proceeded to do so for the next two and a half hours.

The ascidian was then taken from the sand so as to see what the nudibranch was doing. The nudibranch had buried itself deep in the ascidian with only that portion of the animal above the pallial line exposed. It continued to feed and, as the ascidian had a translucent test, it was possible to observe it moving within the ascidian to reach almost all parts.

After a total of three hours feeding the specimen crawled out of the ascidian. The ascidian was then dissected and found to be an empty test. The ascidian would appear to be a Molgula sp. but I have yet to identify which of the several local Molgula species it is.

The first image shows the nudibranch and the track it took to the ascidian. The second image is of the nudibranch shortly after beginning to feed and the third image is of the nudibranch well buried in the ascidian.

Locality: Blairgowrie, 8m, Victoria, Australia, Port Phillip Bay, 28 December 2009, silty sand. Length: 17mm. Photographer: John Chuk.

Sorry to overload you with messages on this species but I thought the information worth passing on.

Best wishes,

John.

jchuk@optusnet.com.au

Chuk, J., 2010 (Jan 18) Okenia rhinorma feeding. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/23111

Dear John,

You don't need to apologise about overloading me when you have such interesting new observations to share. When I named this species (Rudman, 2007) I commented that its shape and radular morphology suggested that it would be one of the species of Okenia which feed on ascidians rather than bryozoans, and your observations certainly bear that out.

Another of these relatively 'high' species of Okenia, O. ascidicola is reported by Morse (1972), to be a specific feeder on the solitary ascidian Molgula manhattanensis. It has seldom been found, probably because of its association with its food. It eats through the tough outer skin of the ascidian and nestles in the cavity it makes, leaving only the gills protruding. It has been reported only from Massachusetts on the Atlantic coast of Nth America. This Nth American species may prove to be a synonym of one of the European species, but whatever it is, it has a very similar biology to O. rhinorma, more similar to that of a species of Goniodoris than of a bryozoan-eating Okenia.

  • Morse, M.P. (1972). Biology of Okenia ascidicola spec. nov. The Veliger, 15(2): 97-101.
  • Rudman, W.B. (2007) Two new species of Okenia (Gastropoda: Nudibranchia: Goniodorididae) from eastern Australia and Tanzania. Zootaxa, 1657: 57-67.

Best wishes,
Bill Rudman

Rudman, W.B., 2010 (Jan 18). Comment on Okenia rhinorma feeding by John Chuk. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/23111

Factsheet

Okenia rhinorma

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