Re: Spatial Distribution of Bursatella leachii

February 4, 1999
From: Clare Kennedy

Dear Dr.Rudman,
I came to this web site looking for info on the Sea Slug Bursatella leachii because it is an essay question for the 1997 A.P. Biology exam. I saw messages from students that said they were "studying" this organism. Hmm... you mean trying to answer this A.P. Bio exam question? Anyway - here is the entire question.

A scientist working with Bursatella leachii, a sea slug that lives in an intertiday habitat in the coastal waters of Puerto Rico, gathered the following information about tthe distribution of the sea slugs within a ten-meter square plot over a 10 day period.

DISTRIBUTION OF SLUGS

Midnight 8.0cm
4 AM 8.9 cm
8 AM 44.8 cm
noon 174 cm
4 pm 350.5 cm
8 pm 60.5 cm
Midnight 8.0 cm

For the above data, provide information on each of the following
* summarize the pattern
* identify three physiological or environmental variables that could cause the slugs to vary their distance from each other.
* explain how each variable could bring about the observed pattern of distribution.

My thoughts: This might be a circadian rhythm. I cannot see any pattern within the day.

The mechanisms may have something to do with salinity and the rise and fall of the tide. Do the slugs have a structure analogous to a pineal gland that might be sensitive to light (secretion of melatonin).

claken@bergen.org

Kennedy, C., 1999 (Feb 4) Re: Spatial Distribution of Bursatella leachii. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/530

Thanks Clare,
I guessed there wasn't a gang of Bursatella research scientists all running down to Puerto Rico to repeat the same experiment. I still find the observations difficult to conceptualise. If you imagine a slug at the middle of the 10m square. At midnight he is surrounded by slugs 8cms away and each of them has slugs 8cm outside them etc etc. If they are arranged evenly thats about 110x110 slugs. By 8am the slugs outside the central slug have had to move a further 36cm away from the centre, and the slugs outside them have to move at least twice that, and those outside three times etc etc to maintain their spacing. By 4pm there will be only 3x3 slugs left in the 10 square metre area. Presumably there were lots of slugs outside the 10 metre square as well. It suggests there was an awful lot of pushing and shoving and very fast crawling for them all to keep increasing their distance apart.

To answer your questions. It's unlikely to be a tidal rhythm because the tidal cycle is not based on the day-night cycle. Yes aplysiid Sea-Hares that have been looked at, do have a day-night circadian rhythm. It seems to be associated with the eye which seems to have its own clock with a circadian rhythm of neuronal activity. Like many animals, there appears to be an underlying circadian rhythm which is kept "tuned" by the light-dark cycle.

It looks like the regular change in spacing of the Bursatella population observed in Puerto Rico resulted from a cycle of action and inaction, though I don't see why it caused them to "crowd" together during inactivity and disperse during periods of activity.

Basically I don't think its a very good question.
Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Feb 4). Comment on Re: Spatial Distribution of Bursatella leachii by Clare Kennedy. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/530

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