ANNALES • Ser. hist. nat. ■ 10 - 2000 • 2 (21)
original scientific paper	UDC 594.3(262.3)
received: 5. 12. 2000
THE OPISTOBRANCH MOLLUSKS (CEPHALASPIDEA, SACCOGLOSSA, NOTASPiDEA, ANASPIDEA AND NUDIBRANCHIA) OF THE ADRIATIC SEA WITH SPECIAL REFERENCE TO THE SLOVENIAN COAST
Tom TURK
Department o' Biology, Biotechnical Faculty, University of Ljubljana, SMOOO Ljubljana, VeCna pot 111
ABSTRACT
During the past 20 years, 49 species of Opistobranch mollusfa have been recorded with the aid of SCUBA diving and underwater photography in the Adriatic Sea. We found that certain localities have higher biodiversity than others. Some species are considered rare and only present at certain localities in a limited time of the year, while others can be found through the entire year at different localities and are therefore listed as frequent. Scuba diving and underwater photography are valuable tools in studying biodiversity of opistobranch molkmks. They provide us not only with records of a certain species but also with some other data, i.e type of their habitat, food, spawn, etc. In the paper we also report on the highly surprising find of the tropical species Halgerda sp. in the shallow waters of the island of Cres. To our best knowledge this is the only record of any Halgerda species in the Alediterranean Sea.
Key words: Opistobranchia, Nudibranchia, underwater photography, biodiversity, Halgerda sp.
GASTEROPODI OPISTOBRANCHI {CEPHALASPIDEA, SACCOCLOS5A, NOTASPiDEA, ANASP1DEA E NUDIBRANCMfA) NEL MARE ADRIATICO, CON PARTICOLARE RIUEVO
ALLE ACQUE COSTIERE Sl.OVENE
SINTESI
Con l'aiuto di equipaggiamento subaequeo autonorno e fotografía subaequea, gli autor i sono nusciti a determinare 49 specie di gasteropodi opistobranchi negti ultimi 20 snni. Han no inoltre constátalo che alcune localitá sono piú n'cche di altre in falto di specie, che alcune specie possono dirsi ra re in quanto si tiovano in poche localitá e solo in determinati periodi dell'anno, mentre altre possono definirsi comuni poiché sono stale tróvate in diverse localitá e durante fu fío l'anno. Le ¡inmersión; e la fotografía subaequea sono due mezzi preziosi nello studio della biodiversita dei gasteropodi opistobranchi. Con l'aiuto di tali teeníche, gli autorí hanno potuto raccogliere anche daii inerenti ¡'ambiente di vita, ¡I tipo di nutrimento e la forma delle uova che depongono queste specie. Nell'articolo viene segnalato anche ti primo ritrovamento della specie tropicale Halgerda sp. nella parle merídíonale deli'ísola di Cberso. Dalla revisione dei dati disponibilí, questa risulta essere la prima segnalazione del genere Halgerda nel mam Mediterráneo.
Parole chiave: Opisíobrancbifl, Nudibranchia, fotografía subaequea, biodiversita. Halgerda sp.
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Tom TURK:THF OPISTOBRANCH MOILUSKS (CtPHAlASf'lDtA. SACCOClOSSA. NOTASPtDEA. ANASPIDEA ANO NUOIBRANCH1A)..., 161-172
INTRODUCTION
Opistobranch moilusks are among the most interesting marine organisms. Although they are usually small, their shapes, striking colour patterns and living habits make them difficult to compare with anything eise living in the oceans. Therefore, it is no wonder that they became popular subjects of underwater photographers. The underwater pictures made opistobranch moilusks true stars among marine organisms and raised much interest in their life and habitats where they can be seen. However, despite their popularity, Jittle is known about biology of most of the species. Scuba diving and underwater photography, together with a careful observation of the circumstances in which a particular opistobranch has been caught on film, can provide us with valuable information about their biology, i.e. type of food, mating behaviour, life cycles and above all biodiversity of a particular area
In this paper, we shall focus mainly on Nudibranchia species that could be more or less frequently encountered in the Adriatic Sea by Scuba divers. There had been 230 species described for the Mediterranean and this number is probably close to the number of species that can also be found in the Adriatic, although far less have been described so far (Cattaneo-Vietti et a!., 1990). About 25% of these are endemic Mediterranean species, and about the same number belongs to the Atlantic-Mediterranean and boreal zoogeographical categories. There are only about 5% and 3% of the species that belong to the Tropical-Atlantic and fndo-Paclfic zoo-geographical regions, respectively. However, these numbers might change in the future due to the warming of the Mediterranean basin and increasing number of lessepsian species entering through the Suez Canal or being introduced by ships ballast waters.
During Scuba diving in the Adriatic in the past 20 years, 35 nudibranch species have been recorded and photographed. In addition, we also report on one species of the ordo Cephafaspidea, four species that are members of the order SaccogSossa, seven species that belong to the order Notaspidea, and two species from the order Anaspidea. Locality, depth and some additional data had been recorded for each photographed specimen.
MATERIAL AND METHODS
Pictures of opistobranch moilusks were taken by the author using underwater photo equipment during Scuba dives at different localities in the Adriatic Sea (Slovenia and Croatia), unless stated otherwise. Most dives were made in the Bay of Piran (Punta Madona), along the western coast of Istra, in the Cres-Losinj Archipelago and in the Velebit Channel. Data as to the locality, depth, time of the year, habitat, abundance and size are given for each specimen, unless stated otherwise. In
addition, some other notes have been provided for certain species. Single sightings or rare records that are mentioned for some species are based on personal observations and existing photographic records of several underwater photographers. Systematic nomenclature was adopted according to Sabelli et ai. (1990) and Le Renard (1997). The latest revision of the checklist was also considered on the CLEMAM web page www. mnhrrt. fr/base/malaco. html.
RESULTS
List of species with comments
Cephalaspidea Fisher P.. 1883
1.	Philinopsis depicta (Renier, 1804) [AgiajaJ = Doridium carnosum (Cuvier, 1810)
A single record of this probably rare species was made in mid-summer off Strunjan at a depth of 1 meter on the rocky bottom.
SaccogSossa Von Ihering, 1876
2.	Elysia timida (Risso, 1818)
Several specimens were found feeding on the green alga Acetabularia acetabularia. Size about 1.5 cm. Depth 1 m. May 1995. Water temperature 16°C. Another specimen (picture) was photographed in the Bay of ¿rnovnica near Sv. juraj at the beginning of June 1996. Water temperature 17°C, depth 3 m. Not common.
3.	ThuridWa hopei Bergh, 1872
This is a common species often found on the rocky substrates among bryozoans and cnidarians.
4.	BoseHia mimetica Trinchese, 1891
The specimen was found in front of the Marine Biological Station in Piran at the depth of 4 m in the month of September. It. was taken out of the water and photographed in a petri dish filled with sea water.
5.	Cafliopaea bellula D'Orbigny, 1837
Caught in plankton net close to the surface just off Piran during the month of August. Probably a juvenile, only few mm long.
Notaspidea Fisher P., 1883
6.	{Jmbraculum umbracu/um (lightfoot, 1786) = U. mediterraneum (Lamarck, 1812)
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Photographed several times in the Adriatic. The photo of the specimen was taken ai a depth of 36 m by 3 vertical wall. Location: northeastern coast of the island of Hvar, end of September 2000. The specimen measured about 15 cm in diameter. Considered rare in the northern Adriatic, but rather common in the southern Adriatic.
7.	Tylodina perversa (Gmelm, 1791)
This species is always associated with the sponge Veronals aerophoba on which it feeds. Usually a pair of these animals could be seen on a single sponge. The picture was taken at the isiand of Cres near Punta Križa at a depth of 8 m in 1999. Severa! specimens were observed from May to August in the same aquatory. Probably common.
8.	Pleurobranchus testudinariusCantraine, 1835
The photo of this specimen was taken in mid-sutn-mer in the southern Adriatic on the soft bottom close to the small island of Oglran near Mljet Depth about 60 m. Size about 15 cm. Probably rare.
9.	BertheUa aurantiaca {Risso, 1818)
Two specimens found under stone in 1 m deep water. Size 2-3 cm. Punta Križa, Cres Isiand. Juiy 1994. Not common.
10.	BertheUa ocellata (Delle Chiaje, 1830)
The only record comes from Piran. The specimen was taken out of the water and photographed. It was found under stone, 1 m deep, in the beginning of lune 2000.
11.	BertheUa stellata (Risso, 1826)
This only documented record also comes from Piran. The specimen was found under rock. Same as above.
12.	BertheUa cf. plumula
This particular tentatively identified pieurobranchid species was found under stone in a rock pool near Novigrad (Istra). Most probably BertheUa plumula (Montagu, 1803).
Anaspidea Fisher P., 1883
13.	Aplysia fascial a Poire!, [ 789
Probably a common species in the southern Adriatic. One of the largest opistobranch mcllusks. The specimen on the picture is about 25 cm long and was photographed while swimming at the southern tip or the is-
land of Mljet at a depth of 4 m (southern Adriatic).
14.	Aplysia punctata (Cuvier, 1803)
One of the most common opistobranchs. From March to May could be found in large numbers in upper Infralitoral zone among green and brown algae on which the species feeds.
Nudibranchia Biainville, 1814
15.	Trapania lineata Haefelfinger, 1960
Several records of this small species come from various localities in the Adriatic, The particular specimen was found crawling on the surface of the sponge Hali-dona mediternmea in Žmovnica (Stari mlin). Size about 1 cm. Endemic Mediterranean species.
16.	Trapania maculata Haefelfinger, 1960
Found several times. The picture is from Punta Kri2a, CreS Isiand. Size about 1 cm. it was taken 4 m deep at the beginning of May. Water temperature was around V5°C. The species is reported to feed on bryozoans, but in this case it was found on a sponge Ircinia sp.
17.	Diaphorodorispapillata Portmann & Sandmeier, 1960
Small species feeding on bryozoans. Specimen depicted here was found in a shady area of larger rocks in Zrnovnica (Stari mlin), it was photographed in the beginning of June at a depth of 7 m, with water temperature of 1 7°C. Not common.
18.	Crimora papillata Alder & Hancock, 1862
Same locality and conditions as above. One of several individuals found among brown algae Dictyoln di-chotoma and Cystoseira spp. This is a boreal species. In Mediterranean it is reported to feed on a. bryozoan Chattel!a tenella. Considered rare.
19.	Doris bertbeloti ? (D'Oi bigny, 1839)
flesa, at a depth of 15 rn, deiritic bottom in the month of May, water temperature 15"C. The species may be identified by a ridge of prominent tubercles on the top of the notum.
20.	Doris sticia (Iredale & O'Donoghue, 1923)
The picture of this rare species was taken in midsummer in the waters of BiSevo Isiand near cape Kobiia. on a soft bottom at a depth of 65 m. Size of the specimen about 3 cm.
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21.	Archidoris pseudoargus (Rapp, 1827)
Ftesa, at a depth of 15 m, detritic bottom in the month of May, water temperature 15°C. Probably rare species, at least in the northern Adriatic. Large species, the specimen on the picture found on detritic bottom and was about 10 cm long..
22.	Halgerda sp.
This tropical species was surprisingly found at the southern tip of the island of Cres near the Baidarin Bay ai the end of July 1988. The species was found while collecting other moilusks on a fine gravel bottom from which larger stones had been previously removed. Depth 1 m, water temperature 25"C. To our best knowledge, this is the first record of any Halgerda species in the Mediterranean. The species was taken out of the water and photographed alive in a petri dish filled with seawater. The specimen was tentatively identified as Halgerda willeyi Elliot, 1903.
23.	Chromodoris krohni (Verany, 1846)
Typical species of shady habitats and coralligenous biocenosis. Masimum reported length is about 3 cm, on average about 2 cm. The egg laying specimen was taken by surprise on a steep wall on the island of Vis. Not common.
24.	Chromodoris luieorosea (Rapp, 1827)
Medium sized species of up to 55 mm, but usually considerably smaller. It is a cryptic species found under stones down to 60 m in depths. I: feeds on a spongc-Aplysilla rosea. However, the portrayed specimen was found on a sponge Hamigera hamigera, together with several other specimens crawling on hard bottom in the shallow Punta Madona (Piran) waters.
25.	Chromodoris purpurea {\<\sso & Guerin, 1831)
Medium sized species of up to 5 cm, allegedly it lives in shady habitats where it feeds on sponges. This single specimen was found, however, in mid-summer almost washed up on the beach of Veli Melj Bay, ai the southern tip of Cres Island. The specimen was alive and was subsequently taken to a deeper water (of about 3 m) where it was photographed. Considered rare.
26.	Hypsehdoris foniandraui (Pruvot-fol, 1951) - H. messiniensis (Von ¡hering, 1880)
This is a common Mediterranean species, although ¡ess common in the Adriatic. It feeds on different horny sponges. The specimen was photographed at the en-
trance of an underwater cave on Ircmia sp. sponge that was shared with another much larger species Hyp$e~ lodoris picia. Mid-August, depth 25 m, 170C, Duboka Bay, Premuda Island.
27.	Hypsehdoris orsinii (V£rany, 1846)
This is the current valid name for the species, its previous name, H. tricolor, is now listed as a separate species (see bellow). This small specimen was photographed on Cacospongia spp. that is its main food. Common, could be found at various depths.
28.	Hypsehdoris picta {Schultz and Philippi, 183b) = Hypsehdoris valenciennesi {Cahtraine, 1841)
This recently renamed species is one of the largest dorids. It seems to be frequent at some localities in certain part of the year. The picture was taken at a depth of 25 m In August. During a typical dive (Premuda Island, Duboka Bay) one could find several specimens feeding on Ircinia sponges, mating and laving eggs.
29.	Hypselodoris tricolor {Cantraine, 1835)
This species could be found at various depths and localities, most frequently on different marine sponges. The photograph was made in July at a depth of 17 m (Susak Island).
30.	Hypsehdoris villafranca (Risso,1818)
Small, probably cryptic species living in shady habitats. Allegedly it feeds on the sponge Dysidea fragile. The picture of the two specimens was taken in shallow water, where they were found under stone (Novi-grad, Istra).
31.	Discodoris atromaculata (Bergh, 1830) = Pekodoris atromaculata (Bergh, 1880)
The most common dorid in the Adriatic. It could be found elsewhere in shady habitats feeding on the sponge Petrosia ficiformis. However, it has not been documented in the Gulf of Trieste as yet.
32.	Platydoris argo (Lmne, 1767)
This species lives on rocky bottoms, frequently under stones. The specimen portrayed here was found under a small rock 3 m deep in mid-summer in Baidarin Bay (Punta Križa, Cres). Another specimen was photographed in the shallow waters of Strunjan Bay while crawling among algae Considered rare.
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33.	Dendrodorisgrandiilora (Rapp, 182 7;
Common species,, seen and photographed a number of times. Frequent species in the 3ay o* Piran
34.	Dendrodoris limbata (Cuvier, 1804]
Another common species that could be found under stones or on soft bottom, Several colour morphs exist, but a common feature is a yellow noturn edge. This particular picture comes from the same locality as
above.
35.	Phyllidia flava Aradas, 1847
Characteristic species that can be usually found on sponges such as Acantheila acuta or Axinella spp. The specimen here is from the island of Hvar and was found in September at a depth of 46 m, temperature 16°C. Locally common.
36.	Tethvs fimbria linne, 1767
One of the largest opistobi anchs, up to 30 cm long Sometimes frequent, especially in the early spring. Sporadically occurring in large numbers close to shores. It crawls on the soft botorn, preferably on terrigenous mud. Specimen from Piran, 15 m deep.
37.	Janolus crista t us (Delle Chiaje, 1841)
Characteristic species, not common. Feeds on a bryozoans, mainly Bugula sp. The portrayed species is from Selca. The photo was taken in September at a depth of 12 m, 20°C. Size of the specimen 3 cm.
38.	Ae olid id la alderi (Cocks, 1852)
Usually found next to the sea anemones on which it feeds. The picture is from Punta Križa (Cres). It was taken at a depth of 3 m in May. Another specimen was detected on the same site while spawning.
39.	Berghia coerulescens (Deshayes, 1838)
Few confirmed records from the Adriatic. The specimen on the picture was collected in front of the Marine Biological Station in Piran, it was taken out of the water and photographed. Feeds on sea anemones.
40.	Berghia verrucicomis {Costa A., 1 867)
The only record is ¡Torn Strunjan, where the specimen was found under stone in tidal zone. Mid-winter. Feeds on actinians.
41.	Spurilla neapolitana (Delle Chiaje, 1841)
Quite common species of shallow waters where it-feeds on sea anemones. Several pictures exist from different Adriatic sites.
42.	Eubranchus (arrant (Aider & Hancock, 1844)
Boreal-Mediterranean species, so far photographed at two locations in the Adriatic. First specimen found at Zmovnica (Stari ml in) near Sv. |uraj. The picture was taken at the beginning of june, 12 m deep at the temperature of about 13°C near the underwater spring of fresh water. Few specimens observed among hydroid colonies. There are two additional pictures on web site www.medsiuRs.de from Medveja.
43.	Eubranchus tricolor Forbes, 1838
Same location as above. Boreal species with a single record from the described location. Found at a depth of 7 m.
44.	Cratena peregrina (Cmelin, 1791)
One of the most common nudibranchs. it could be seen frequently among hydrozoan colonies on which it feeds. Prefers Eudendrium sp.
45.	Dondice banyulensis Portrriann & Sandmeier, 1960
Photographed at 2rnovnica (Stari mJin) near Sv. Juraj. Probably rare, but in Zrnovnica very frequent or common during certain period of the year (beginning of June). Close to the underwater freshwater springs among hydroid colonies and on Cystoseira sp. By the end of June it disappears completely.
46.	unidentified species
This unknown small species was found at 2rnovnica in the beginning of june. It was found on a hydroid colony at a depth of 12 m. Most probably a juvenile Don-dice banyulensis.
47.	Flabellina affinis (Gmelin, 1791)
Common nudibranch feeding on hydrozoa. Could be found elsewhere in the Adriatic. Very common in certain areas. Picture of this specimen made off Piran at a depth, of 10 rn.
48.	Flabellina ischilana Hirano & Thomson, 1990
This recently described species is considered an endemic Mediterranean species and was known only from
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Fig. 4: Bosellia mimetica	Fig. 5: Calliopaea bcllula	Fig. 6: Umhraculum umbracu/um
. 4 . . 1
fc;, .¿k .
■ if- ..nt . J^*4 $ ■'
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Fig. 7: Tyfodina perversa
Fig. 13: Ap/ysia fasciata
Fig. 8: Pleurobranchus testudinarius
Fig. 9: Bert he I la aurantiaca
Fig. 12: Berlhella cf. p/umu/a
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Fig. 16: Trapania macu/ata
Fig. 19: Doris bertheloti ?
Fig. 17: Diaphorodoris papi/lata
Fig. 20: Doris sticta
Fig. 18: Crimora papH lata
Fig. 21: Archidoris pseudoargus
Fig. 23: Chromodoris krohni	Fig. 24: Chromodoris luteorosea	Fig. 25: Chromodoris purpurea
Fig. 29: Hypselodoris tricolor
Fig. 27: Hypselodoris orsinii
Fig. 30: Hypselodoris villafranca
Fig. 28: Hypselodoris picta
Fig. 31: Discodoris atromaculata
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Fig. 32: Platydoris argo
Fig. 35: PhyMdia flava
Fig. 36: Tethys fimbria
Fig. 33: Dendrodoris grandiflora
Fig. 39: Berghia coeru/escens
Fig. 42: Eubranchus farrani
Fig. 41: Spurilla neapolilana
Fig. 40: Berghia verrucicornis
Fig. 43: Eubranchus tricolor
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Fig. 47: Flabellina affinis	Fig. 48: Flabellina ischitana
Fig. 49: Flabellina pedata
the Tyrrhenian Sea. However, this species appeared to be very common in the Adriatic, too. Many pictures from various locations exist. Some specimens (particular those from Punta Madona of Piran) have bright red cerata that distinguish them from the less brightly coloured specimens inhabiting other locations.
49. Flabellina pedata (Montagu, 1815) = Coryphela pedata (Montagu, 1815)
Common, recently renamed species known from many localities. Feeds on hydrozoans. Certain individuals have brighter red colour of the cerata than others (Bay of Piran).
DISCUSSION
Forty-nine species of opistobranch mollusks were found and photographed by means of Scuba diving and underwater photography. Most of the photographs were made in situ; only few were taken of live specimens in petri dish filled with seawater. Of the 49 species, 45 were positively identified, one species tentatively (Fig. 19, marked with ?), while two species remain unidentified although a suggestion as to the possible species has been made (Figs. 12, 46). The number of recorded species is rather small compared to about 230 species described from the Mediterranean merely for the order Nudibranchia (Cattaneo-Vietti et ai, 1990). However, we are describing more than half of the species reported for the Adriatic Sea by Thompson (1986), a bit less than half reported by Jaklin (1989) who listed 125 opistobranch species and above twice as many as mentioned in the malacological survey for the shore waters near Cervar (Vio & De Min, 1999). The number of species in Thompson's paper was based on his personal collection from the vicinity of Rovinj and supplemented with previous observations and collections. A large portion of his data originate from half a century ago, which makes the records less reliable and of lesser significance for today's biodiversity. On the other hand, more than 90% of our records were made during the last five years and none of the data is more than 20 years old. Of the 49 species described in our paper, 23 (Figs. 1, 3, 4, 5, 10,
1 1, 14, 16, 19, 21, 24, 26, 32, 33, 34, 36, 39, 40, 41, 44, 47, 48, 49) were recorded in the Slovenian part of the Adriatic Sea in a short stretch of shallow coastal waters between Strunjan and the Marine Biological Station in Piran. Some of them (Figs. 1, 5, 10, 11) may well represent the only photographic record known to us in the eastern part of the Adriatic Sea.
Most of the opistobranch species are considered rare and are seldom seen even by Scuba divers, while some can be observed regularly. It appears that some species are more frequent at certain localities and absent or extremely rare in the others. This is probably true for the Adriatic, too. There are some locations where many species can be observed during particular time of the year, including the species that are absent or extremely rare at all other localities. One of such localities, as experienced by us, is a few kilometres long stretch of the sea in the Velebit Channel between Sv. Juraj and 2d ra I ova Bay with its many underwater fresh springs. The conditions of decreased salinity and lower water temperature in this particular locality could be the reason for the high biodiversity of opistobranchs, particularly those of boreal zoogeographical distribution. Another such locality is Punta Madona in the Bay of Piran, a place known for its high biodiversity index in which nudibranchs play an important part.
It is difficult, however, to claim that a certain species is rare or not known from a particular locality. We do not have sufficient data, and it is quite possible that a particular species is common but not frequently observed since it is cryptic or reaches its peaks during that particular part of the year when there is not much underwater human activity. As an example, before the times of Scuba diving, the today's very common species Discodoris (Peltodoris) atromaculata that one can see almost during every dive under dark overhangs, was considered extremely rare. By 1954, only a single specimen of this species had been recorded. Therefore, even today we should be cautious in claiming that a certain species is absent or rare at certain locality. In this paper, the comments considering the abundance of certain species are based on personal experience and observations and should be taken as such.
One should be careful in interpreting the abundance
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of certain species. As an example, the prominent opis-tobranch species Umbraculum umbraculum is considered very rare or even absent in the northern Adriatic, but quite common in the southern Adriatic, or may even occur in mass numbers in some other parts of the Mediterranean Sea (Di Mart ino & Stancanelli, 1999). Another example are nudibranchs that are listed as boreal species. In the Adriatic they could be sometimes present, according to our observations, even in substantial numbers at certain localities. These areas are characterised with lower water temperatures and decreased salinity due to the large inflow of fresh cold water from underwater springs. Boreal species, such as Crimora papillate, Eubrancbus farrani and E. tricolor, were found only at such localities, which is also true of the endemic Mediterranean species Dondice banyulen-sis. The latter is present in quite large numbers and could be found on hydrozoan colonies or Cystoseira algae in the vicinity of underwater springs. However, the presence of such species is short lived. At least during the warmer period of the year they reach their peaks in late spring, but completely disappear by the end of June. By then, the inflow of fresh water is almost gone and the water temperature rises to about 20 °C.
On the other hand, the species such as Cratena peregrina, Flabellina pcdata, Flabellina ischitana, Fla-bellina affinis and Discodoris atromaculata are more or less common all year round. The reason for their constant presence might be the yearlong availability of their prime diet. The first four species are specialists feeding on hydrozoan colonies, while Discodoris feeds on marine sponge Petrosia ficiformis.
The discovery of a nudibranch specimen of the genus Halgerda that has an Indo-Pacific tropical zoogeog-raphical distribution in the northern Adriatic came as a big surprise. The specimen from Cres Island show the visual characteristics of Halgerda willeyi Elliot, 1903, but may well represent a yet unidentified species. A closer and more detailed morphological inspection of the specimen would be necessary for positive identification. Halgerda willeyi is a well known species that could be found on coral reefs from the Red Sea to New Caledonia. Only one specimen was found at the southern tip of the island of Cres and to our best knowledge this is the only specimen of the tropical genus Halgerda that has ever been found in the Mediterranean Sea. We can only speculate how it made it so far to the north, but there are at least two reasonable explanations. There is an increasing number of Indo-Pacific species entering the Mediterranean through the Suez Canal. A typical example as far as mollusks are concerned is the large anaspidean opistobranch Bursatella leachi Blainville, 1817 that has been found on several occasions even in the Gulf of Trieste and near Crado (Jaklin & Vio, 1989; De Min & Vio, 1998). There are many reports on this species from the coast of Turkey and Italy, some of them
being as many as 60 years old. This clearly points to a slow but steady spreading of the species towards the north. In the case of the single find of Halgerda sp. such distribution is less likely since this is the only record in the Mediterranean and on top of it made 12 years ago. Since then, the warming of the Mediterranean Sea has been in progress (see Bethoux et a!., 1990; Bethoux & Gentile, 1995) but there are no new records of this species not even from the southeastern Mediterranean basin that is known for its larger numbers of lessepsian immigrants from the Red Sea. Therefore, in the case of Halgerda sp. a distribution by means of ship ballast waters is more likely. If the larva was transported in a week or so and then released into the northern Adriatic waters, a veliger could have been transformed into an adult animal in just a few weeks. For most of the tropical nudibranchs the entire developing process from veliger to an adult individual is rather short and could be completed within few weeks (lecithotrophyc larva). In principle, such a development could be completed even in the northern Adriatic, where the temperatures of surface water layers easily match, during the summer months, those of the tropical seas. However, for a development into an adult individual, we should also consider the availability of a suitable diet, which is especially problematic with highly specialised species that feed only on particular food. Food specialisation is widespread within nudibranchs. Marine sponges, bryozoans, tunicates and cnidarians are their most popular diet. Still, it is possible that Halgerda willeyi is not a specialist and could have survived by feeding on locally available food. The recent paper of Marshall and Willan (1999) lists Halgerda willeyi as a species that preys on different marine sponges, and the authors consider it a non-specialist.
Finally, we would like to point at some aspects of taxonomic confusion regarding the dorid nudibranchs belonging to the genus Hypselodoris. A taxonomic revision has been made recently and some new names introduced that are currently valid. Since at least three Mediterranean species are very similar in appearance, it is difficult to make a proper identification based only on a visually evident differences. We believe that Hypselodoris orsinii (formerly H. tricolor), H. (ontan-draui (formerly H. messinensis; H. coelestis) and H. tricolor (formerly Doris gracilis) were properly identified and a valid name was assigned to each of them according to the existing literature and pictures therein. There are two additional species that also belong to the group of blue, white and yellow coloured Mediterranean dorids: H. cantabrica and H. picta (formerly H. valen-ciennesi; H. elegans). In addition, a valid name for the very similar but formerly separate species H. webbi is now also H. picta. A great help in the attempts to clarify the problems in the systematics of bluecromatic dorids is the work of Ortea et at. (1996).
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Tom TURK: TME OPISTOBRANCH MOllUSKS (CEPHAL ASPIDEA, SACCOCIOSSA, NOTASPIDEA, ANASPIDEA AND NUDIBRANCHIA).... IGM 72
ACKNOWLEDGEMENT
I would like to thank the following persons who contributed photographs for the present paper: Marijan Richter (1, 5, 10, 1 1, 13,19, 21, 24, 27, 36, 41, 48, 49)
Borut Furlan (8, 20, 23) and GriSa Planinc (4, 39). Special thanks go to France Velkovrh who contributed the picture and the accompanying data of the Halgerda sp. specimen (22).
Fig. 22: Halgerda sp.
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ANNALES • Ser. hist. nat. ■ 10 - 2000 • 2 (21)
Ton TUB K: THi OPISTOBRAMCI i MOl.l.USf. > KtPHAlASPIDEA, SACCOGIOSSA. NOTASFIDEA, ANASTIDCA AND NUDIHRAMCHIAj .. 1i,1.172
POLŽI Z.AŠKRGAR)! (CEPHALASPIDEA, SACCOGLOSSA, NOTASPIDEA, ANASPIDEA IN NUDIBRANCHIA) V JADRANSKEM MORJU, S POSEBNIM POUDARKOM NA SLOVENSKEM OBALNEM MORJU
Tam TURK
Oddelek za biologijo, Biotehniška fakulteta. Univerza v Ljubljani, SI- '000 Ljubljana, Večna pot «11
POVZETEK
V zadnjih 20 letih nam je v Jadranu med potapljanjem z avtonomno potapljaško opremo s podvodno fotografijo uspelo določiti 49 vrst morskih polžev zaškrgarjev. Ugotovili smo, da se nekatere lokacije močno razlikujejo od drugih po Številu vrst. Za nekatere vrste lahko trdimo> da so redke in da se pojavljajo le na nekaterih lokacijah v določenem letnem obdobju. Druge vrste so razmeroma pogoste in jih lahko najdemo na različnih lokacijah skozi vse leto. Potapljanje in podvodna fotografija sta dragocena pripomočka pri ugotavljanju biodiverzitete polžev za-Škrgarjev. S pomočjo obeh tehnik lahko pridobimo tudi podatke o življenjskem okolju, kjer smo določenega polža našli, o tipu hrane, s katero se prehranjuje, 0 obliki mresta, ki ga odlaga, itd. V članku poročamo tudi o odkritju tropske vrste Halgerda sp. na skrajnem jugu otoka Cresa, kar je glede na razpoložljive podatke prva opisana najdba golega polža iz tropskega rodu Halgerda v Sredozemlju.
Ključne besede: Opistobranchia, Nudibranchia, podvodna fotografija, biodiverziteta, Halgerda sp.
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