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94 Parasitic Copcpoda of British Fishes

Male: Unknown.

Comments: Previous descriptions of this species were too generalized to provide morphological details useful for comparative purposes. The only illustration of the habitus known to the author is thai given by the discoverer (von Nordmann, 1832). Markevich (1956) reproduced his earlier figures ihowing the second antenna and the posterior extremity. The former did not reveal clearly the nature of the basal swelling. The latter exaggerated the lateral ends of the rows of denticles on the last ibdomlnal srgni'nt showing them as quite prominent spines. This is an illusion caused by llj rlmposition of several denticles (not unknown in other copepods). The "spines" disappear on \lirhl rotation of the abdomen. Markevich also differed from this author in his armature formula of

the iwlmming legs.

/ fibbm is a parasite living on the gills of the common European eel, Anguilla imguillu. Most of its m "'I • come from the coasts of the Baltic and the North Sea. from brackish coastal regions. In or iif.n tin- (lulf of Neva about 50% or all the eels examined were found to be infected (Markevich, 1956) The claim of Gadd (1904) thai he found £. gibhus on Leuciscus nttilus( = Rutilus ruiiltts) must L i oniidered suspect, since it was not confirmed by extensive subsequent searches in the same area. In Britain H has been recorded very recently (Canning ct al., 1973) on an eel in Slapton Ley Nature

the iwlmming legs.

/ fibbm is a parasite living on the gills of the common European eel, Anguilla imguillu. Most of its m "'I • come from the coasts of the Baltic and the North Sea. from brackish coastal regions. In or iif.n tin- (lulf of Neva about 50% or all the eels examined were found to be infected (Markevich, 1956) The claim of Gadd (1904) thai he found £. gibhus on Leuciscus nttilus( = Rutilus ruiiltts) must L i oniidered suspect, since it was not confirmed by extensive subsequent searches in the same area. In Britain H has been recorded very recently (Canning ct al., 1973) on an eel in Slapton Ley Nature

lit erve.

Genus Thersitina Norman, 1905

Closely related to Ergasilus, this genus is monolypic. Its diagnosis, therefore, is the same as that of its only species and will be found below. The name Thersitina was first used by Norman (1905) as a replacement for the preoccupied name Thtrsftes, originally given to it.by Pagenstechcr (1861). The (cnctal similarity between Thersitina and Ergasllus prompted some authors to consider them ss

>•■■•,••......II, .....inoi phological differences between them (e.g. structure of the second antenna)

IN large enough to warrant their retention as independent taxa.

Thersitina gasterostei (Pagenstechcr, 1861) (Figs. 108-128) Syn.: Thersites gasieroiteiPagenstechcr. 1861 Ergasllus gasterostei K roycr. 186J Ergasllus hiuncinulus Gadd. 1901 ThcalUi gasumuw T. Scott. 1901 Thersitina biuneinataidsii, 1901).orYamnguli (1963)

Female (Fig. 108): Ccphalothorax incorporating first and second leg-bearing segments, much inflated, sublenticular, its long axis at right angles to that of remainder of body; anterior surface olicn, bearing prominent hump; antennary region ventral, mouth'*wrrfvr\*tv$,A>f\\i>K,(rwn_r

Parasitic Copcpoda of British Fishes

95

single row of peg-like denticles. First maxilla similar to that of Ergasilus. Second maxilla (Fig. 115) with robust base and narrow but sturdy distal process, turning up at tip and armed apically with five to nine denticles. Maxillipcd absent.

Four biramous swimming legs with three-segmented rami, except for two-segmented fourth exopod (Fig. 116). Sympods two-segmented; first segment unarmed, second with slender seta lateral to base of exopod; in first leg also single row of denticles on posterior margin medial to endopod (Fig. 117). in remaining pairs two such rows (Fig. 1 IS). Armature formula of rami as follows.

Endopod Fxopod

12 3 12 3

Lcgl .....1-0 1-0 4.11 0-1 1-0 3.11

Leg 2 .....1-0 24 4,1 0-1 1-0 6

Leg 3 ...'.. 1-0 2-0 4.1 0-1 1-0 6

Lcg4 .....1-0 2-0 4.1 0-16 —

According Hi Gurney (1933) the second segment of the fourth exopod carries only five setae. Spines and lateral margins of some segments with strips of serrated membrane. Fifth leg (Fig. 119) papilliform, with one seta at apex and one al base.

Male (Figs. 109. 110) (fide Gurney. 1933): Ccphalothorax fusiform, not inflated, with rounded anterior and truncated posterior margin; only first leg-bearing segment incorporated. Borders between free leg-bearing segments rather indistinct. Genital complex longer than broad, widening posteriorly, with incipient posterolateral lobes. Abdomen live-segmented (Gurney, 1933) or four-segmented (Markevich, 1956), Uropod as in female, but long seta comparatively much longer. Total length without uropod setae about 1.0 mm.

Appendages similar to those of female. Maxillipcd (Fig. 110) subchclalc, with very long and slender subchcla. Single seta on posterolateral corners of genital complex (possibly representing vestigial sixth legs). _v

According Hi Gurney (1933) the second segment of the fourth exopod carries only five setae. Spines and lateral margins of some segments with strips of serrated membrane. Fifth leg (Fig. 119) papilliform, with one seta at apex and one al base.

Male (Figs. 109. 110) (fide Gurney. 1933): Ccphalothorax fusiform, not inflated, with rounded anterior and truncated posterior margin; only first leg-bearing segment incorporated. Borders between free leg-bearing segments rather indistinct. Genital complex longer than broad, widening posteriorly, with incipient posterolateral lobes. Abdomen live-segmented (Gurney, 1933) or four-segmented (Markevich, 1956), Uropod as in female, but long seta comparatively much longer. Total length without uropod setae about 1.0 mm.

Appendages similar to those of female. Maxillipcd (Fig. 110) subchclalc, with very long and slender subchcla. Single seta on posterolateral corners of genital complex (possibly representing vestigial sixth legs). _v

Comments: T. gasterostei is a common species and is probably better known than most parasitic poecilostomcs. Its biology has been studied in detail mainly because its common hosts, the sticklebacks, make very good laboratory animals. Its life history comprises four nauplius stages (Figs. 120-123). followed by five copepodid stages of increasing complexity (Figs. 124-128). The ontogeny of T. gasterostei was investigated by Gurney (1913, 1933). the physiology of its host-parasite relationships by Walkcy el al. (1970). Kroyer (1863) and Gadd (1901) placed this species in lirgasilus, from which it must be excluded :;';;;;*^'^»'^»*^^*v»i«>l'5.iif^{,res,iJMlx,,bayin_{>(5 described the species independently from Pagens-}}