J. Llewellyn

Larvae and larval development of monogeneans.

Publisher Summary This chapter focusses on a group of parasitic platyhelminths generally referred to as monogenetic trematodes. A few monogeneans are viviparous and the newly-born young resemble the parents, but most are oviparous, and the newly hatched young is a larva and called an oncomiracidium. Most oncomiracidia are partly covered externally with cilia, but some are without cilia. At its posterior end, the oncomiracidium invariably bears an adhesive organ or haptor in the form of a ventrally-directed disc or cup. A gut is present in all of the oncomiracidia. The osmo-regulatory system of larval monogeneans has flame cells and efferent ducts. The larval alimentary canal and nervous system foreshadow the adult condition, but the patterns of external ciliation and of the osmo-regulatory system may reflect phylogenetic development. The study of development of haptor yielded useful clues to evolutionary trends in monogeneans. Further development now took one of two main directions––that is, to elaborate the hamuli and abandon the marginal hooks and to replace some of the marginal hooks with new muscular adhesive organs that are essentially adult features, leaving the hamuli as largely postoncomiracidial, preadult organs. The elaboration of the hamulus apparatus took place by doubling the number of hooks, by incorporating accessory sclerites, or both. The replacement of marginal hooks by adult muscular organs took place in a variety of definite patterns, and if the parasites are arranged according to these patterns, there is a conspicuous correlation between these groupings and groups arranged according to hosts.

The host-specificity, micro-ecology, adhesive attitudes, and comparative morphology of some trematode gill parasites

In recent years there have been several accounts of the occurrence of diclidophoroidean trematodes parasitic on the gills of fishes, e.g. Price (1943), Sproston (1946), Dawes (1947), Brinkmann (1952), and Chauhan (1953), and these accounts have included descriptions of the morphology of the parasites. The distribution records have revealed a generally high degree of host specificity and, in some species, a preference even for certain gill arches of the particular hosts, while the morphological descriptions have shown that there is a considerable variation in the form of the parasites, extending to various degrees of deviation from bilateral symmetry (PI. I, figs. 1–11). These morpho-logical variations are present in spite of the fact that the different parasites occupy such broadly similar habitats in the gill chambers of their respective hosts. But, as far as I am aware, this is the first attempt to investigate the distribution and morphology of the parasites in relation to their micro-habitats.

Amphibdellid (monogenean) parasites of electric rays (Torpedinidae).

Electric rays (Torpedinidae) are known to harbour on their gills certain monogenean parasites (Amphibdella spp. and Amphibdelloides spp.) whose taxonomy is in a very confused state. Muchof this confusion has centred around a single morphological feature: a ‘transverse bar’ has been variously held to be present or absent from the adhesive apparatus, or sometimes two such bars have been stated to be present. Among those who have regarded the trans-verse bar as having taxonomic importance, Price (1937) recognized two genera of amphibdellids distinguishable by the presence or absence of the bar, but Palombi (1949) and Bychowsky (1957) believe that the bar may be present in young specimens but absent or inconspicuous in older specimens. No-one has investigated the function of thistransverse bar, or, indeed, any functional aspect of the adhesive apparatus. Again little attention has been paid in taxonomy to the genitalia or host specificity, or, in spite of a record of an amphibdellid from the heart of its host, to micro-habitat.

The adhesive mechanisms of monogenetic trematodes: the attachment of species of the Diclidophoridae to the gills of gadoid fishes

An investigation of the adhesive mechanisms in all nine species of the genus Diclidophora has shown that there is a common structure consisting of a pair of hinged jaws operated by intrinsic muscles and also by a more powerful extrinsic muscle which acts on a diaphragm to produce a suction pressure that is converted into a clamping action. This investigation has revealed errors in Cerfontaines and other descriptions of the anatomy of Diclidophora adhesive organs, but substantiates Cerfontaine account of the adhesive mechanism, and therefore disagrees completely with the more recent account given by Sproston.

The life histories and population dynamics of monogenean gill parasites of Trachurus trachurus (L.)

Gastrocotyle trachuri and Pseudaxine trachuri infect young Trachurus trachurus at Plymouth as soon as the 3- or 4-month-old adolescent fishes descend to the sea bottom in October. The parasites normally mature in 3 or 4 months, but, exceptionally, in about 1 month, and the life-span is normally no longer than 1 year. Trachurus specimens at the beginning of their second year pick up a largely new infection of parasites. G. trachuri and P. trachuri are much less frequent on 2- and 3-year-old specimens of Trachurus and probably occur only very rarely on still older fishes, the limiting factor being not an age-immunity but a post-spawning migration of the host from the concentration of free-living infective stages of the parasites in coastal waters. The parasites have adapted themselves to a seasonal change in the feeding habits of Trachurus by ceasing to produce larvae in anticipation of the summer disappearance of scad from the sea bottom in pursuit of pelagic food-organisms.

The attachment of the monogenean Discocotyle sagittata Leuckart to the gills of Salmo trutta L.

The monogenetic trematode Discocotyle sagittata attaches itself to the gills of Salmo trutta with its long axis inclined to the long axis of the primary lamella to which it is attached, the direction of inclination being determined by the incidence of the gill-ventilating current of the host. The inclination is to some extent brought about by an asymmetrical disposition of the posterior adhesive organs, but Discocotyle appears to retain a primitive unspecialized character in that the asymmetrical disposition is facultative and not obligatory and permanent as in Axine. The adhesive apparatus consists of four pairs of clamps and one pair of hooks. The hooks appear to be the persistent posterior hooks of the larva. Each clamp is operated by an extrinsic muscle-tendon-fair-lead-hinged-jaws system generally similar but possibly less efficient than the corresponding system in Plectanocotyle gurnardi. Scheurings and Sprostons interpretations of the clamp-closing mechanism of Discocotyle as a spring system are wrong.

Spermatophores in the Monogenean Entobdella diadema Monticelli from the skin of sting-rays, with a note on the taxonomy of the parasite

Entobdella diadema produces spermatophores from a pair of follicular glands lying one on each side of the body alongside the main nerve cord, and the secretion of these glands is stored in a two-chambered reservoir within the cirrus-sac. The vagina is made up of two regions, a wide distal part to receive, accommodate and digest the spermatophores, and a proximal narrow part connected through the vitelline reservoir with the oviduct. E. diadema Monticelli, 1901, is shown to be distinct from E. bumpusii Linton, 1900, and has been recorded from two new hosts, Dasyatis pastinaca at Plymouth, and Myliobatis aquila at Naples. We wish to thank the directors and staffs at the Plymouth Marine Laboratory and at the Stazione Zoologica, Naples, for providing excellent facilities for the above studies to be carried out. The work was assisted by a grant to one of us (J. L.) from the Trustees of the Browne Fund, to whom grateful acknowledgement is made.

The adhesive mechanisms of monogenetic trematodes: the attachment of Plectanocotyle gurnardi (v. Ben. & Hesse) to the gills of Trigla.

Little is known of the mechanisms whereby trematode parasites of fish gills attach themselves to their hosts. The adhesive apparatus consists of a posterior set of suckers or clamps supported by skeletal bars (sclerites) whose arrangement varies considerably in the different species. Yet though the pattern of these sclerites forms the main basis for the classification of the 200 or so species of the Diclidophoroidea into its six families, the only attempts to describe the mechanism of attachment of these parasites to their hosts appear to be those of Cerfontaine (1896), who described the attachment of Diclidophora denticulata to the gills of Gadus viretis , and of Sproston (1945a) and Llewellyn (1956a), both of whom described the attachment of Kuhnia scombri to the gills of Scomber scombrus . In the present paper the mechanism by which Plectanocotyle gurnardi (v. Ben. & Hesse) adheres to its hosts Trigla cuculus L., and T. lineata Gmelin will be described.