Sponge invaders in Dutch coastal waters

Continuous monitoring by scientists and volunteers of the Biological Working Group of the Dutch SubAqua Union and the 'Anemoon' Foundation over the past four years, yielded a surprising six new records of sponges for Dutch coastal waters. Oscarella lobularis, Celtodoryx girardae, Suberites virgultosus, Haliclona (Haliclona) simulans, Halisarca aff. dujardini, and a species identified as Leucosolenia somesii were unknown from Dutch coastal waters before 2000. The latter is a giant calcareous sponge, seemingly belonging to the common Leucosolenia variabilis, but here assumed to be an invader as well, as it has spicular characters well outside the variation found in the majority of Dutch L. variabilis specimens. It is likely a member of a 'forgotten' species, L. somesii. Habit photographs, SEM images of the spicules, and for O. lobularis and H. aff. dujardini, photographs of histological sections are provided to substantiate these new records. With the exception of C. girardae, most of the species resemble previously described widespread north-east Atlantic species, occurring in the area to the south and west of the Netherlands, so it is assumed pending future genetic research that at least several of the invaders comprise range extensions related to rising winter temperatures. Possibly, recent shellfish imports may be an additional causal agent. We also report the occurrence of unprecedented spicular deviations observed in three sponge species commonly occurring in Dutch waters, Halichondria (Halichondria) panicea, Hymeniacidon perlevis and Haliclona (Soestella) xena, which grew in small inland water bodies. Possibly, the limited space in these inland waters with possible stress factors for sponges such as reduced water exchange, and deviating chemistry, have caused the sponges to form stunted growth in spicules varying from rhabds with rounded endings to silica spheroids. We provide an updated list of sponges found in Dutch waters and a list of suspected or proven invaders of Dutch waters.