The Enigmatical Trematode “Dictyocotyle coeliaca”

Abstract

Dictyocotyle coeliaca Nybelin, 1941, the only nionogenetic trematode ever to be found in the cœlom of a vertebrate, was described from two specimens, one from Raia lintea near Skagen and the other from R. radiata at Trondheim. Ten years ago one of us described two other specimens from R. clavata and expressed the hope that a look-out would be kept for the trematode in zoological laboratories¹. This appeal brought nine specimens from J. E. N. Sloan, formerly of the Zoology Department, University of Edinburgh, and it seems to have led to the publication of two original papers. In neither paper was the total number of parasites specified, but 16 out of 55 Raia naevus and 31 out of 73 R. radiata taken on Scottish fishing grounds² and 35 out of 135 R. naevus landed at Plymouth³ were infected. Mean numbers of parasites per fish were 7 and 4 respectively, and in each instance rays of other species did not yield any parasites. This trematode closely resembles the common cloacal parasite Calicotyle kroyeri, differing chiefly in the nature of the posterior adhesive organ (opisthaptor), which is a delicate hookless disk showing faint markings or irregular shapes and sizes. These simulate but are not suctorial compartments, or loculi, such as are found in Calicotyle and other Monocotylidae, and mainly for this reason the taxonomic status of Dictyocotyle remains undetermined. Llewellyn and Green³ believe it “distinctly possible” that the host landed at Brixham “had been mis-identified”. When at the time confirmation was sought, Dr. E. B. Francis was emphatic, stating, “the species of Raia (as mentioned in my previous letter !) was ‘clavata’, source Brixham” (letter dated March 26, 1947).