CHARACTERIZATION OF THE EXTERNAL FEATURES OF SCHISTOCEPHALUS-SOLIDUS (MULLER, 1776) (CESTODA) FROM DIFFERENT GEOGRAPHICAL REGIONS AND AN ASSESSMENT OF THE STATUS OF THE BALTIC RINGED SEAL PHOCA-HISPIDA-BOTNICA (GMELIN) AS A DEFINITIVE HOST

A comparative study of some morphological (segment number, scolex morphology and biometry, length and weight) and biological (maturation in different hosts) features of Schistocephalus solidus plerocercoids and adults from different geographical regions (Baltic Sea and the British Isles) was carried out. The length of the plerocercoids from Gasterosteus aculeatus was shown to be the variable that best correlated with segment number. A very clear bimodal distribution of segment numbers separated the majority of British and Baltic plerocercoids (British n = 21, mean length 25.48, SD 5.63, range14-34 mm; mean segment number 66.33, SD 8.68, range 51-86. Baltic n = 30, mean length 33.23, SD 4.64, range 23-48 mm; mean segment number 117.27, SD 10.30, range 99-138). Adult S. solidus from the intestines of Baltic ringed seals Phoca hispida botnica and from a Welsh cormorant Phalacrocorax cal bo cat-be were also compared, and a similar bimodal distribution of segment numbers was found (Baltic n = 70, mean segment number 106.16, SD 10.60, range 77-136; Welsh n = 98, mean segment number 73.13, SD 8.78, range 54-97). Neither the morphology nor measurements of the scolex from apical-view scanning electron microphotographs provided distinguishing features for taxonomic purposes. Of 580 adult worms from Baltic ringed seals only 2.9% were gravid, 2.1% from spring and 10.5% from autumn samples. By contrast, of 98 adults from the Welsh cormorant 46.7% were gravid. The proportion of gravid worms did not increase with increasing worm numbers in seals. Reasons for poor maturation are discussed. Plerocercoids of British S. solidus were from leiurus (gymnurus) forms of G. aculeatus, which were relatively small, whereas in the northern Baltic plerocercoids were from semiarmatus or trachurus forms, which were larger. As segment number was definitively established during the growth of the plerocercoid in the stickleback, the hypothesis is proposed that segment number is a phenotypic variable related to stickleback length (size).