The temporal and spatial occurrence of Tetrastemma fozensis (Hoplonemertini) in intertidal bivalves

The bivalve Scrobicularia plana, which iscommonly found on tidal flats in northwestern Europe,has disappeared from many locations during the pasttwo decades. Its local disappearance has beensuspected to be a result of pollution, but otherfactors such as parasites have also been discussed. The occurrence of Tetrastemma fozensis, a smallhoplonemertine inhabiting the mantle cavity of S. plana,was examined in bivalves from six differentestuaries in northern Spain. The bivalves are easilycollected, and the nemertines leave the bivalves whenplaced in a small jar with seawater for 72 h. Regularexaminations at 6–10 wk intervals of S. planafrom the estuary of Plencia revealed that thepercentage of bivalves containing nemertines T. fozensisdecreased during the summer months, but wasabove 70% during the rest of the year. In mostestuaries about 40–60% of the bivalves containednemertines, but in March 1994, 100% of the bivalvesfrom the estuary of Hendaye were hosts for nemertines.Nemertines were not found in S. plana from thehighly polluted estuary of Bilbao in either March orSeptember 1994. The number of nemertines withinindividual bivalves was positively correlated with thesize of the bivalves in locations where the percentageof bivalves containing nemertines was above 75%. Inone estuary where the nemertines disappeared almostcompletely from the bivalves between March 1994 andSeptember 1994, parasitic copepods appeared withinsome bivalves in September 1994. The ecology of thenemertine T. fozensis still remains largelyunknown, but we suspect that the nemertines may bebeneficial for their hosts.