Adhesion in Polyopisthocotylean Monogeneans
Fewer comprehensive studies of gland cells in larval polyopisthocotyleans exist but Whittington et al. (2000b) review these. Figure 19 illustrates the arrangement and content of gland cells for the larva of Grubea cochlear (Mazocraeidae) to demonstrate the main features for a polyopisthocotylean as determined using light microscopy. A set of gland cells on either side of the pharynx and a second set laterally on either side of the body posterior to the pharynx, both with gland ducts leading anteriorly to open around the terminal mouth, all contain needle-like secretion in oncomiracidia of this family (Figure 19; Whittington and Kearn, 1990). No ultrastructural studies have been made to determine whether these needle-like secretions are electron-dense rod-shaped bodies like those reported from the anterior glands of most larval and adult monopisthocotyleans (Section 5.2). Most polyopisthocotylean larvae possess anterior gland cells that contain either needle-like secretion or granular secretion, but the latter is the most common condition reported so far (Whittington et al., 2000b). Only a single species studied, Tonkinopsis trans-fretanus (Bychowskicotylidae), has been reported at the level of the light microscope to have both of these different types of secretions, but this requires confirmation (Whittington et al., 2000b).
The ultrastructure of the anterior glandular secretions of a single polyopisthocotylean species, Zeuxapta seriolae (Axinidae), has been studied and this revealed that there are two different types of granular secretions based on granule size (Whittington et al., 2000b). There are, however, no descriptions of the anterior gland cells of Z. seriolae using the light microscope and, until these are forthcoming, the ultrastructural studies on these anterior glands are of limited value (Whittington et al., 2000b).
Posterior gland cells with granular contents were described in the larvae of two hexabothriid species by Whittington (1987). Peripheral ducts containing
Figure 19 Oncomiracidium of Grubea cochlear (Monogenea: Polyopisthocotylea: Mazocraeidae) showing two groups of anterior gland cells (ag) on each side of the larva containing needle-like secretory bodies as observed using phase contrast light microscopy. Scale bar = 50 pm. (Modified from Whittington and Kearn, 1990.)
Figure 19 Oncomiracidium of Grubea cochlear (Monogenea: Polyopisthocotylea: Mazocraeidae) showing two groups of anterior gland cells (ag) on each side of the larva containing needle-like secretory bodies as observed using phase contrast light microscopy. Scale bar = 50 pm. (Modified from Whittington and Kearn, 1990.)
granular secretion were observed at the haptor margin close to hooklets in the larvae of one of these species, Hexabothrium appendiculatum, and strands of material, most likely secreted by the posterior gland cells, were noted in specimens that had attached to glass and Perspex surfaces (Whittington, 1987).
There is a startling lack of information about gland cells in larval poly-stomatid monogeneans, which is especially surprising because the anatomy and chaetotaxy of numerous species is otherwise well documented (Whittington et al., 2000b). Anterior gland cells are certainly present in the larvae of Poly stoma integerrimum because Combes (1968) demonstrated that oncomiracidia migrate from the branchial cavity of tadpoles to the bladder via the ventral skin using the anterior glands. Furthermore, he noted that the glands in larval P. integerrimum became enlarged and their contents became refringent before migration occurred, indicating an emphasis on their importance during the journey. Observations on the biology of Protopolystoma xenopodis by Tinsley and Wynne Owen (1975) suggest that larvae and juveniles can attach and move freely on or in the host toad, Xenopus, during migrations via the cloaca, urinary ducts and kidneys to the urinary bladder, but no mention was made of anterior glandular secretions that may be adhesive. The unique invasion route of the desert toad, Scaphiopus, by larvae of Pseudodiplorchis americanus and Neodiplorchis scaphiopodis and described by Tinsley and Earle (1983), would also appear to require looping locomotion, but whether anterior attachment is achieved by suction or by adhesives was not mentioned. Tinsley and Jackson (1986), however, did describe briefly the behaviour of four week old juveniles of P. americanus during migration to the bladder via the gut of the toad. Alternate attachment by haptor and 'oral sucker' was mentioned, but again, no reference was made to whether anterior attachment is accomplished by secretions or suction (Tinsley and Jackson, 1986). The oncomiracidia and especially the juvenile, pre-migrant specimens of P. americanus are well endowed with gland cells in many regions of the body, but they have not been studied in detail (Richard Tinsley and Jo Cable, peronal communication). Recent studies on the host recognition behaviour of polystomatid larvae by Du Preez et al. (1997) also noted that oncomiracidia loop once contact has been made with the surface of a tadpole, but no details were given of the method of anterior attachment.
As mentioned in Section 5.1, the general perception is that adult polyopistho-cotyleans in their typical habitat, the gills of fishes, are sedentary. Indeed many families possess an asymmetrical haptor that bears clamps only on one side and previous studies demonstrate that 'right-' or 'left-footed' individuals can occur in the same species. This indicates that asymmetry may depend on which side of the gills the parasites settle as larvae. This aspect of poly-opisthocotyleans is reviewed by Kearn (1994, 1998). Kearn (1998) comments that these asymmetrical worms are most likely sedentary and notes that they 'do not possess conspicuous anterior adhesive glands characteristic of mobile monogeneans'. It is a fact that most taxonomic descriptions of polyopsitho-cotyleans make no mention of anterior gland cells or ducts that may play a role in adhesion. Llewellyn (1966) suggested that the adoption of blood feeding by polyopisthocotyleans may have contributed to their sessile life style because their food supply is readily accessible and inexhaustible. Polyopisthocotyleans such as microcotylids and gotocotylids (Figure 2D) that have numerous hap-toral clamps seem unlikely to have the capacity to coordinate the movement of so many clamps. Plectanocotyle (Plectanocotylidae), however, may be mobile (Llewellyn, 1966). Rees (1986) has confirmed that adult P. gurnardi are able to move using anterior adhesives, but there is a need for a detailed analysis in this and other adult polyopisthocotyleans to assess the presence of anterior adhesive secretions and to study their ultrastructure.
It is difficult to determine the relative contributions to anterior attachment by suction and by adhesives. An anterior organ that can generate suction is known in many polyopisthocotyleans (Section 5.1), but the roles of secretion and suction in attachment may not be independent for two reasons: 1) a secretion may aid suction by sealing the peripheral rim of a sucker (such as the false oral sucker of some polyopisthocotyleans); 2) suction can be generated by pulling apart two objects separated only by a thin, watery secretion (Nachtigall, 1974). Clearly, a significant amount of investigation is required to determine more about anterior attachment, and especially the role of adhesives or other secretions, in polyopisthocotylean monogeneans.
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