Bursatella leachii in prawn ponds
December 6, 1998
From: Doug Pearson
Dear Dr Rudman,
I am the Manager of a large Prawn Farm situated on the Logan River half way between Brisbane and the Gold Coast. One of the most important parts of growing prawns in ponds is the maintaining a stable algal bloom.
This season however I have encountered a problem with several of my ponds which I can not seem to overcome. I can not establish an algal bloom in these ponds, none of the 'tricks' I have learnt over my past 12 years in this field have achieved any substantial result. The one thing that does appear to be common among the problem ponds is a fairly large presence of Bursatella leachii. We regularly find large masses of eggs (spaghetti we call it, blue in colour) on the feed trays in the ponds. Am I right in assuming these are the eggs of Bursatella leachii?
My main questions are:
Do the larvae of this creature live in the water column and consume algae? If so, what do they look like?
Do you think that these creatures might be the cause of my problem?
If so, what are their weakness? How might I get rid of them.
Thanking you in advance for your time. Looking forward to your reply.
Regards,
Doug Pearson
Ph: 07 5546 1361
darah@ion.com.au
Pearson, D., 1998 (Dec 6) Bursatella leachii in prawn ponds. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/359Dear Doug,
Its possible the Bursatella are a byproduct of the cause of no algal bloom.
I am not an expert on algal blooms but in garden ponds a way to prevent algal blooms is to have other plants ..water weeds, water lilies etc which will use up the excess nutrients.
Perhaps in the ponds with algal bloom problems there is something else using up the nutrients. One possibility would be filamentous blue green algae which could form a mat over the bottom of the pond. Why I suggest that is because blue-green algae are the preferred food of Bursatella leachi and probably the only plant which will trigger the planktonic larvae of Bursatella to settle out of the plankton.
Your identity of the "spaghetti" is probably correct but I have never seen their egg strings coloured blue, although they can be coloured orange, brown, yellow or green. Their larvae are herbivores and millions are produced, but I wouldn't know if their feeding would outpace the ability of the phytoplankton to breed.
If you want to know what the larvae look like your best bet would be to keep a mass of eggs in a container with aerated seawater for a few days and see what they look like when they hatch out. In West Africa a subspecies has been reported to hatch 6 days after laying.
If the Bursatella are only in some ponds then we have to ask why?
There are two basic choices.
1. There are blue green algae in all the ponds and by chance some ponds got Bursatella larvae in their seawater when the ponds were filled.
2. There were Bursatella larvae in all or many of the ponds but they only survived in those ponds in which there was blue green algae.
If the second alternative proves correct then perhaps you need to look at some way to get rid of the blue green algae..or perhaps the Bursatella could be left to do the job. If, on the other hand, all the ponds have blue green algae then perhaps the Bursatella larvae are to blame. If so I guess you would have to remove as many of the Bursatella as you could and all the egg masses you can find. I'm afraid the egg masses will be all over the bottom of the ponds as well as the feed trays.
One suggestion I have is that you allow me to post your question on the Sea Slug Forum.
If there is anyone out there with information on the subject they will reply within a few days either directly to you or to the website.
Best wishes,
Bill Rudman
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