First record of a fertilized female blue crab , Callinectes sapidus Rathbun , 1896 ( Crustacea : Decapoda : Brachyura ) , from the German Wadden Sea and subsequent secondary prevention measures

A single female specimen of the blue crab Callinectes sapidus Rathbun, 1896 was captured by a fisherman on 26 May 2008 in the East Frisian Wadden Sea, National Park of Lower Saxony, Germany. The specimen was transferred to a public marine aquarium in Dorumersiel. On 9 June 2008 the female extruded masses of fertilized eggs. Due to nature conservation efforts in this national park, management instructions are implemented by the appropriate authority for further dealing with the captured specimen and its potential offspring. This is the second record of a mature female and the first record of a fertilized female blue crab from German coastal waters of the North Sea.

A single fertilized female specimen of the American blue crab, Callinectes sapidus Rathbun, 1896, was caught by a shrimp fisherman using a shrimp beam trawl on 26 May 2008 while fishing in the East Frisian Wadden Sea at the Accumersieler Balje (53°43.7'Nand 7°27.1'E), the tidal inlet between the German islands Baltrum and Langeoog at a water depth of 8 m (Figure 1, Table 1).The carapace length was 8.3 cm, total carapace width was 16.5 cm and 12.5 cm width when excluding the longest lateral spines.It weighed 190 g.The specimen was directly transferred to the public marine aquarium of the National Park House in Dorumersiel.
After fourteen days in the aquarium at a salinity of 30 PSU and water temperature of 19°C, normal for Wadden Sea summers, fertilized eggs were extruded within a period of two hours to form a cohesive mass on the underside of the abdomen.Mating will have taken place some time before its capture because most females spawn for the first time two to nine months after mating (Hill et al. 1989).Moreover mating occurs when the female is ready to molt into the mature stage (Hill et al. 1989).The captured specimen had an extensive covering of specimens of the balanoid Balanus crenatus Bruguière, 1789 (Figure 1) which indicates a considerable period of time since its last molt took place.According to Hill et al. (1989) they copulate only once in their lives, the sperm from this mating is stored in seminal receptacles and may be used as often as a female spawns, generally two or more times during a one to two year period.
On 12 June 2008, immediately after the eggcarrying status of the captured female became known, the Lower Saxony Wadden Sea National Park Authority decreed that a release of this specimen into the wild would not be allowed.By producing this order the deliberate release of a potentially invasive alien species into the Wadden Sea, which is a multiple protected marine nature reserve (see Nehring and Hesse 2008), was prevented.This management instruction is in fully agreement with the 'Guiding Principles of the Convention of Biodiversity' (CBD 2002) and the 'European Strategy on Invasive Alien Species' (Council of Europe 2003) which oblige member states to prevent unwanted (re)introductions of alien species.Additionally the national park authority required that measures were taken to prevent the water of the aquarium, in which the female crab was held, to reach the Wadden Sea.Otherwise blue crab larvae may reach the environment.According to our current state of knowledge, this is the first time such secondary prevention measures for a captured aquatic alien species were ordered in Germany for the Wadden Sea National Park.
Initially the egg mass was orange-yellow in color, but it gradually darkened to black.After two weeks the eggs hatched and released an unknown number of blue crab larvae.Sandoz and Rogers (1944) reported that hatching of blue crab eggs must occur at salinities of 23 to 33 PSU and water temperatures of 19 to 29°C, conditions occurring in the aquarium.A release of crab larvae to the Wadden Sea was prevented by rigorous management of the closed water circulation system at the aquarium.Additionally all hatched larvae were extracted through continuous filtration and no juvenile crab was observed in the aquarium afterwards.On 2 July 2009 the captured blue crab died a natural death in the aquarium and was preserved.Live specimens of C. sapidus were recorded from the German North Sea coast in 1964, ca. 1965, ca. 1990and 1998(Nehring et al. 2008) (Figure 2, Table 1).These were males except for the second record where sex was not determined.The first mature female, an unfertilized specimen, was caught by a German shrimp fisherman in the outer Weser estuary in 2007 (Figure 2, Table 1) and died in a public marine aquarium 16 months later (Nehring et al. 2008, Gabriele Speckels pers. comm.).The present record in 2008 is the first record of a fertilized female blue crab from German coastal waters of the North Sea.Elsewhere ovigerous females have been recorded from the northern coasts of France, Belgium and the Netherlands (ICES WGITMO 2001, 2006, 2007;Wolff 2005) (Figure 2).Hill et al. (1989) noted that adult blue crabs are excellent swimmers and can migrate several hundred kilometers (Callinectes, from the Greek calos = "strenuous", nectes = "swimmer", sensu Stimpson 1860).Migrations of females are usually associated with mating, gonadal maturation and spawning (Hill et al. 1989).And so an active migration of the captured fertilized female from Benelux, where the occurrence of permanently established populations of C. sapidus is very likely (Kerckhof et al. 2007;Wolff 2005), and to the German North Sea coast is plausible.However, passive dispersal of larvae with water currents or the transport of juvenile or adult specimens as hitchhikers on a ships' hull are also possible pathways of introduction.The use of the barnacles on the carapace as a biogeographically marker is not really helpful in this case (Figure 1) because the circumpolar species B. crenatus indicates only that, since its last molt, the captured blue crab has lived in European waters or elsewhere in the Northern hemisphere (Hutchins 1952).However, it is conceivable that the captured blue crab has lived for a more or less prolonged period on the German North Sea coast before it was caught.It is very likely that C. sapidus can survive German winters with lowest water temperatures of about 2°C (Otto et al. 1990) because blue crabs can tolerate even acute periods of freezing temperatures (Bauer 2006).At temperatures below 5°C, blue crabs enter a dormant phase during which they bury into the sediments, by which means they are insulated from cold water to some extent (Hill et al. 1989;Hill 2004).
Within the native range of C. sapidus at the Atlantic coast of America blue crabs occupy water ranging from a near-ocean salinity of 34 PSU to freshwater in rivers as far as 195 km upstream from the coast (Hill et al. 1989).However, male and female blue crabs gather in different areas of salinity, with adult males tending to remain at lower salinities (Hill et al. 1989).It is interesting that the known German records show a similar pattern (Figure 2, Table 1).Males were found in the upper estuaries of the River Elbe and of the River Weser in a salinity range of 1 to 23 PSU (Grabemann et al. 1993), while both females were captured in adjacent marine waters at salinities >30 PSU (BLMP 2005).Due to the infrequent occurrence of crabs in German waters the potentially invasive C. sapidus is not yet considered to be established (cf.Gollasch andNehring 2006, cf. Nehring et al. 2008).
Nevertheless, the recent record of a fertilized female specimen of Callinectes sapidus in German waters is remarkable as an introduced mature female specimen has theoretically a higher potential for the establishment of a founder population compared to an introduced male specimen because a single female crab may release millions of fertilized eggs.Therefore the management instructions by the appropriate authority were suitable and appropriate and must be permanent directives laid down in the trilateral alien management plan for the Wadden Sea, which is actually in current preparation (Nehring et al. 2009).

Figure 1 .
Figure 1.Mature female of Callinectes sapidus Rathbun, 1896 caught in the German Wadden Sea on 26th of May 2008.The carapace is covered by specimens of the barnacle Balanus crenatus Bruguière, 1789 (photo by M. Kröger).
a Coordinates are not exactly known.