A comparison of scolex morphology between the plerocercoid and the adult of Calliobothrium verticillatum (Tetraphyllidea: Onchobothriidae)

Abstract

The morphological changes associated with the ontogenetic transformation of the scolex from larva to adult were investigated in the tapeworm Calliobothrium verticillatum (Rudolphi, 1819) van Beneden, 1850 by comparing the morphology of plerocercoids collected from the hermit crab Pagurus pollicaris Say, 1817 with that of adult worms collected from the shark Mustelus canis (Mitchell, 1815), using scanning electron microscopy. The apex of the scolex in young plerocercoids bore a conspicuous sucker. Each bothridium was flat and divided into four regions: an anterior oval region with two marginal bumps, and three posterior loculi clearly marked by two costae. The microthrix pattern was not determined for very young plerocercoids. The most conspicuous feature of older plerocercoids was the sucker on the apex of the scolex. The bothridia were concave, and each bore an apical region subdivided into three shallow suckers and three conspicuous loculi. Densely packed blade-like microtriches intermingled with filiform microtriches were visible on all surfaces of the scolex except the surface directly adjacent to the opening of the apical sucker of the scolex, which was entirely devoid of microtriches, and the proximal surfaces of the bothridia, which had only blade-like microtriches. The apical sucker was absent from the scolex adult specimens. The only remnant of this structure was a small knob on the apex of the scolex. The bothridia were concave and bore, from anterior to posterior, three conspicuous apical suckers arranged in a horizontal row adjacent to one another, two pairs of single-pronged hooks, and three conspicuous loculi. Densely packed blade-like microtriches were present on all proximal surfaces of the scolex. Microtriches were not present on the distal surfaces of the apical suckers of the bothridia, on the hooks, or on the distal surfaces of the loculi of the bothridia. The latter regions were covered with very small, round structures. The differences in the microthrix components of the various surfaces of the scolex between the plerocercoid and the adult suggest that complex surface changes occur during this phase of ontogeny. Comparison of the development of this species with that of other onchobothriids suggests that, in general, the number of loculi to be expected in the plerocercoid of an onchobothriid is one plus the number of posthook loculi in the adult.