Halgerda willeyi? from the Adriatic

February 6, 2001
From: Tom Turk

Dear Bill,
I am preparing a paper on the Opisthobranch mollusks of the Adriatic sea and a slide came to my attention showing a nudibranch that without much doubt belongs to the genus Halgerda. The specimen was photographed by my colleague France Velkovrh in July 1988. He collected the slug in the southernmost tip of the island Cres, near Baldarin Bay [Croatia, northern Adriatic sea, about 45 degrees North]. The specimen (4 cm in length) was found in mid summer (end of July) under a stone in 1 m deep water. The water temperature was about 25 degrees C. I discussed the finding with France who is reliable person, and a specialist on prosobranch molluscs, so any mismatch of the location is out of the question. He was also surprised by the finding, but did not pay particular attention since Nudibranchs have never been in the main focus of his research. I must emphasize that the area where the specimen had been found is very familiar to me. I have spent every summer there in the last 15 years. However, I must also say, that despite intensive diving in the area, including turning over many stones in the shallow water I never came across anything like this slug.

In my opinion the specimen is most probably Halgerda willeyi, Eliot 1903, but I would like to hear a second opinion. The specimen shows typical characteristics of the above species but also some differences. However, it is my understanding that different colour morphs of this species are quite common. As far as I know, this is the only record of any species of the tropical genus Halgerda in the Mediterranean sea. What is even more striking is the fact that the specimen was found far to the north of the Mediterranean basin.

There seems to me to be two reasonable explanations for its presence in the northern Mediterranean. There is an increasing number of Indo-Pacific species entering the Mediterranean through the Suez channel. A typical example within the mollusks is the large opisthobranch Bursatella leachi Blainville 1817 that has been found on several occasions even in the Gulf of Trieste and near Grado (De Min and Vio, 1998). There are many reports of this species from the coast of Turkey and Italy, some from up to 60 years ago. This clearly points to a slow but steady spreading of the species towards the north. In the case of a single finding of Halgerda willeyi such a distribution is less likely since this is the only record in the Mediterranean and was made 12 years ago. Since than, the warming of the Mediterranean sea is in progress but there are no new records of this species, not even from the southeastern Mediterranean basin where there are known to be larger numbers of lessepsian immigrants from the Red Sea. In the case of Halgerda willeyi I would think that distribution by means of ship ballast waters is more likely. In principle, its veliger larvae could survive in the northern Adriatic where during summer the surface water temperature easily matches that of tropical seas. One problem is the availability of an appropriate sponge, for metamorphosis and adult feeding. It is however possible that Halgerda willeyi is not a specialist and could survive by feeding upon locally available food. Marshall and Willan (1999) list Halgerda willeyi as a species that preys on different marine sponges and consider it as a non specialist

I would appreciate your comments and answers.

Best regards,
Tom Turk

tom.turk@uni-lj.si

Turk, T., 2001 (Feb 6) Halgerda willeyi? from the Adriatic. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3301

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