February 16, 2009
From: Valda Fraser
Why are these "critters" crawling on Thorunna horologia? I see it often.
Locality: Park Rynie, 25 m, South Africa, Indian, 6 Feb 2009, Rocky reef. Length: 12 mm. Photographer: Valda Fraser.
Fraser, V.J., 2009 (Feb 16) Thorunna horologia and friends?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/22254
These 'critters' are caprellid amphipods - sometimes called 'skeleton shrimps' because of their shape. I have discussed a little bit about caprellids and their association with some nudibranchs in an earlier message [#19058] and you'll find a few more examples on the Forum if you search for caprellid. I have enlarged a couple of sections of your photo and one of them shows that the substrate and growths, on which Thorunna is crawling, are over-run with caprellids of various sizes. I think this is just a coincidence. If you have seen it often then perhaps the sponges that Thorunna feed on live in a place that caprellids find attractive.
On a seperate topic, your photo gives us a nice view of the purple oral tentacles. I noted in the original description of this species that specimens I had seen had the oral tentacles 'tipped with a washy purple' so it is good to get this character confirmed, as photos usually don't show it.
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