Veel misvormde Mactra’s op Texel aangespoeld

A high percentage of approximately 70% of shells of Mactra corallina cinerea Montagu 1803, were found to be malformed in a sample of shells freshly drifted ashore on the northern part of Texel, December 1998/January 1999. All shells were deformed at the posterior side, they were more or less truncated, with more pronounced growth lines at that side. Shell growth also was retarded as compared with normal shells of the same cohort (fig. 1). A cause for such deformities is difficult to give without experimental work. As deformation started after about 15 mm shell length, the deformation was acquired later during life. Somewhat comparable shell monstrosities were found by T. Voortman in 1994 on the beach of the nearby Frisian island Ameland (unpublished, sample in Zoological Museum Amsterdam). However, these shells were less deformed and deformation occurred in a lower percentage of the shells (25-29%). Shell malformations in Mactra shells collected near Zandvoort were different, showing shell flexures at the anterior side or blisters inside the shell, but no deformation at the posterior side. As a possible cause for the Texel (and Ameland) monstrosities one might suggest crowding. Effects of crowding resulting in shell deformation have been reported before in Cerastoderma edule (Kreger, 1940). Also Spisula subtruncata shells freshly stranded on the Dutch coast often show an indentation of the ventral shell margin which I relate to crowding. Such malformations have apparently not been reported before in Mactra, certainly not in such a high percentage. No earlier records exist although the shells often stranded in millions on the Dutch coasts, particularly after a cold period in winter. Another possible cause might be a pollutant. TBT e.g. has caused anomalies in shell formation in some oyster species. TBT became in use in antifouling paints for ships on a larger scale only after 1970. It would be very interesting to find out whether such malformations in Mactra started after 1970. If they did occur already before 1970, TBT can be excluded as a cause. The author should like to get more information on comparable shell deformations found elsewhere, in Mactra or other bivalves.