Monogenean gill parasites – monopisthocotyleans

The buccal and gill chambers of fishes have been colonised several times by monogeneans, suggesting that these sites offer substantial improvements in life style for skin parasites. However, it is by no means clear what these advantages are. Ready access to host blood does not seem to have been a significant factor, since, with the exception of the polyopisthocotylean monogeneans, most monogenean gill parasites are epidermis feeders. Predation pressure from ‘cleaner’ fishes and other organisms that subsist on a diet of fish ectoparasites, may be less than on the body surface, especially in the gill chamber, and there will be more opportunity to meet conspecifics for mating in the more compact living space. In general, no special challenge faces monogenean colonisers from the outer skin surface. The buccal and branchial cavities offer extensive, relatively flat areas of epidermis for attachment and feeding, and ambient water currents are not necessarily stronger than they would be on the skin surface of a rapidly moving fish, although gill-ventilating currents are likely to be more or less continuous. This situation is reflected in the lack of obvious specialisations in many monopisthocotylean monogeneans (gyrodactylids; capsalids) living in the buccal and gill chambers. In contrast, the haptors of the ‘dactylogyroideans’ (see Appendix 2) and the polyopisthocotyleans have undergone extensive morphological adaptations for life on the gills. Like the capsalids, ‘dactylogyroideans’ have two pairs of large hooks or hamuli, but in typical ‘dactylogyroideans’ only two of the four hamuli point in a ventral or ventro-lateral direction, the other two pointing dorsally or dorso-laterally (Figure 5.2). Some ‘dactylogyroideans’ have undergone extensive speciation in parallel with the enormous expansion and diversification of teleost fishes. Particularly spectacular is the ‘explosive’ expansion on cyprinid fishes of the dactylogyrines (Dactylogyrus, Neodactylogyrus) in which the ventrally orientated hamuli are reduced. In fact Gibson, Timofeeva & Gerasev (1996), following Gusev (1985) in rejecting Neodactylogyrus in favour of Dactylogyrus, listed over 900 nominal species of Dactylogyrus worldwide, compared with a mere 400 or so in Gyrodactylus (see Chapter 4). This makes Dactylogyrus the largest helminth genus. The polyopisthocotyleans have evolved unique organs functioning as clamps for attachment to the gills and, in addition, have undergone changes related to their exploitation of blood as a source of food. All of these changes set the polyopisthocotyleans apart from the monopisthocotyleans and earn them a separate place in this book (Chapter 6).