The beachflea Platorchestia platensis (Kroyer, 1845): a new addition to the Polish fauna (with a key to Baltic talitrid amphipods)

The present paper reports for the first time on the occurrence of Platorchestia platensis (Kroyer, 1845) (Crustacea, Amphipoda) in Puck Bay (southern Baltic, Poland) in May 2005. A key to the Baltic talitrids is given, which can be used to identify males and females of the four species occurring on Polish shores (Talitrus saltator, Talorchestia deshayesii, Orchestia cavimana, Platorchestia platensis) and additionally Orchestia gammarellus, which may yet be found in the Polish coastal zone. Only three species of talitrid amphipod (Crustacea) have been recorded from Polish shores. The sandhoppers Talitrus saltator (Montagu, 1808) and Talorchestia deshayesii (Audouin, 1826) are ubiquitous beneath flotsam and jetsam or wrack cast up at the high water mark on sandy beaches * This work was funded through European Community project BALTDER under the fifth FP, contract No EVK3-CT-2002-80005 and the Polish Ministry of Scientific Research and Information. The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ 288 J. I. Spicer, U. Janas (Urbanski 1948, Drzycimski & Nawodzinska 1965, Żmudzinski 1990, 1999, Kruk-Dowgiallo 2000, Weslawski et al. 2000 a, b, Koziel 2004). Both species bear quite distinctive dorsal markings, which easily separate them from each other, and from the beachfleas (Photo 1 & 2). Orchestia cavimana (Heller, 1865) is a ‘freshwater’ beachflea common in the Szczecin Lagoon, at the mouth of the River Oder (Odra), from where it has reached the Polish Island of Wolin (Urbanski 1948); it has also been observed in the Vistula Lagoon and in the Dead Vistula River (Żmudzinski 1990). It has also recently been recorded for the first time on the Estonian coast (Kotta 2000). O. cavimana is commonly found in damp habitats, beneath stones and decaying vegetation close to fresh or brackish water. There is a fourth talitrid that has been recorded from Poland, the euterrestrial Talitroides alluaudi (Urbanski 1948, Cerny & Straskraba 1958). However, where it occurs in Poland, and in the other Baltic countries (e.g. Finland, Sweden), it is restricted to greenhouses (Vader 1972) and so is not considered any further here. The common beachflea of NW European shores Orchestia gammarellus (Pallas, 1766) extends into the Baltic to Vastervik along the Swedish east coast, and allegedly as far as the Gulf of Riga on the Latvian coast (Dahl 1946) but, according to Urbanski (1948) and Jazdzewski & Konopacka (1993), it does not occur in Poland. Its absence from Poland led Den Hartog (1963) to conclude that the record from Riga required checking. Where it does occur, it occupies the same kind of habitats as O. cavimana, but with a much more marine distribution. Here we report the first occurrence on Polish shores of the semiterrestrial beachflea Platorchestia platensis (Kroyer, 1845) (Photo 3). Unlike the other Polish talitrids, this species has a cosmopolitan distribution, occurring in supralittoral marine environments on the Atlantic coast of Canada and the USA (Bousfield 1973), Uruguay and Brazil in South America (Stebbing 1906), Bermuda (Kunkel 1910), the West Indies (Bousfield 1973), the Canary Islands (Andersson 1962), Israel (Morino & Ortal 1995), India (Chilton 1921), Japan (Ruffo 1949, Morino 1978), Korea (Jo 1988), Hawaii and other Pacific Islands (Stephensen 1935) (although note that Stock & Biernbaum (1994) have proposed some differentiation, delineating a tropical variation that they designate P. platensis f. monodi). P. platensis also occurs on a number of shores in NW Europe (see below) but until now has been considered absent from Poland (Jazdzewski & Konopacka 1993, Żmudzinski 1999). It is believed that P. platensis arrived in NW Europe on Danish coasts in the 1860s (Dahl 1946) and from there had extended its range to the Swedish west coast by the 1940s (Backlund 1945, Dahl 1946, Karlbrink 1969), north to southern Norway by the 1980s (Teigsmark The beachflea Platorchestia platensis (Kroyer, 1845) . . . 289 Photo 1. Talitrus saltator from the sandy beach in Gdansk, June 2005 (length 13.5 mm). Normally greyish-brown in colour, larger individuals possess a distinctive black/brown streak that runs the length of the dorsal surface Photo 2. Talorchestia deshayesii from Puck Bay, May 2005 (♀ length 8.5 mm). Although they can be variable in colour (brown, green or even orange/red), individuals often possess three lines of darker spots running the length of the dorsal surface 290 J. I. Spicer, U. Janas Photo 3. Platorchestia platensis from Puck Bay, May 2005 (♀ length 9.3 mm). The body colouration of this species can be very similar to Orchestia gammarellus and Orchestia cavimana, ranging from dark blue through dark brown and greenish brown, sometimes orange 1981) and to the Baltic coast of Sweden and Germany by the 1990s (Zettler 1999, Persson 2001, although see the much earlier record of Urbanski (1948), confirmed by Kohn & Gosselck (1989), showing that P. platensis was present at numerous points along the German Baltic coast more than a half century ago). It has also been recorded from the Netherlands (Den Hartog 1963) (but not the deltaic region (Platvoet & Pinkster (1995)) and the Thames in the UK (Wildish & Lincoln 1979). The Polish population of P. platensis was found beneath stones and debris wedged within a stone coastal defence structure on the southern shore of the Hel Peninsula at Kuźnica (Puck Bay). In May 2005 24 specimens were collected and identified using two key characteristics: uropod 1 outer ramus with distal spines only and the posterior margin of epimeral plate 3 crenulate. Additionally, one specimen of O. cavimana was recorded at the same place. P. platensis has been seen to co-occur with, and in some cases has outcompeted O. gammarellus (Backlund 1945, Dahl 1946, Persson 2001). No O. gammarellus were identified in our beachflea collection. It is not known whether P. platensis has ousted O. gammarellus from Polish shores or invaded a vacant niche. The fact that some studies have noted the absence of O. gammarellus in the past tends to favour the latter explanation, but The beachflea Platorchestia platensis (Kroyer, 1845) . . . 291 a more extensive sampling regime is required before any definitive statement can be made. Talitrids are notoriously difficult to identify, and marked sexual dimorphism in some species further complicates this process (Lincoln 1979). Below we give a key to the Baltic talitrids that can be used for both males and females of each of the four species already occurring in Poland; we have added also O. gammarellus, which may yet be found in the Polish coastal zone. Key to Baltic talitrid amphipods 1. Gnathopod 2 with enlarged propodus and dactylus (Fig. 1) . . . . . . 2. Gnathopod 2 with small propodus and dactylus (Fig. 2) . . . . . . . . . 5. 2. Long curved process on propodus of gnathopod 2 (Fig. 1a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . male Talorchestia deshayesii. No such process present on propodus of gnathopod 2 (Fig. 1b) . . . 3. 3. Uropod 1 outer ramus with distal spines only (Fig. 3a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . male Platorchestia platensis. Uropod 1 outer ramus with a row of spines along the dorsal margin (Fig. 3b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. 4. Gnathopod 1 merus with posterior lobe (Fig. 4a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .male Orchestia cavimana. Gnathopod 1 merus without posterior lobe (Fig. 4b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .male Orchestia gammarellus. 5. Uropod 1 outer ramus with distal spines only (Fig. 3a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . female Platorchestia platensis. Uropod 1 outer ramus with a row of spines along the dorsal margin (Fig. 3b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. 6. Gnathopod 1 propodus without palm (Fig. 5a) . . . . . . . . . . . . . . . . . . . 7. Gnathopod 1 propodus with small palm (Fig. 5b) . . . . . . . . . . . . . . . . 8. 7. Uropod 3 ramus almost as long as peduncle with single large spine as long as the ramus (Fig. 6a) . . . . . .male or female Talitrus saltator. Uropod 3, ramus much shorter than peduncle (Fig. 6b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . female Talorchestia deshayesii. 8. Gnathopod 2 merus without posterior lobe (Fig. 2a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . female Orchestia gammarellus. Gnathopod 2 merus with posterior lobe (Fig. 2b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . female Orchestia cavimana. 292 J. I. Spicer, U. Janas

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