| Summary: | Leptosomatum bacillatum, a marine nematode that occurs in high densities in the sponge Halichondria panicea, has an annual reproductive cycle at Dutch latitudes. During the period that the population has been studied, the smallest juveniles were present from July to September. Females continue growing after the adult stage has been reached, resulting in both 6,5 and 14 mm long females in May and June. Pre-adult males loose their cephalic capsule, which serves to adhere the pharynx, during the last moult; subsequently the somatic musculature and digestive tract starts to atrophy (Chapter 4). The material described by de Man (1893) from the type-locality of L. elongatum has been compared with the Dutch nematodes and with the material described from the Black Sea by Filipjev (1918); they are identical. Because L. elongatum Bastian, 1865 (Syn. L. bacillatum (Eberth, 1863)) represents the type-species of the genus Leptosomatum, all species in which the character-complex (sexual dimorphism of the amphid) is absent, and which were previously considered to be a species of Leptosomatum, have been removed from that genus. Males of Leptosomatum species differ from females and juveniles by the aberrant amphid structure expressed in long amphidial glands and obvious ganglia. In males I have never been able to observe the excretory pore or renette; the pore is always present in females, whereas the renette may or may not be present. A ventral row of coelomocytes is often present in both sexes; sometimes, however, individuals occur in which these coelomocytes are undeveloped. The function of these cells is unknown as are the factors that induce their development; the latter applies to the renette as well. An T.E.M-study of the cephalic capsule revealed that Timm's interpretation (1953), that the capsule is composed of six pairs of sclerotised pieces, is incorrect; the capsule has to be considered a ring. The species of the genus Leptosomatum have neither prominent glands in the lateral chord nor a vaginal ovejector (Chapter 2). Given the above considerations, L. ranjhai Timm, 1960, was at first considered a species incertae sedis. Later, when new material became available, a new genus Orthophallonema was erected to receive this species (Chapter 5). L. caecum Ditlevsen, 1923 has been transferred to Pseudocella. Species provided with a vaginal ovejector and glands in the lateral chord of the vulvar region, i.c. L. arcticum and L. grebnickii, both Filpjev, 1916; L. elongatum and L. tetrophthalmum both sensu Platonova, 1967; and L. gracile sensu Allgén, 1954, have been transferred to Leptosomatides. I have not been able to trace the male on which Inglis (1971) based the description of L. micoletzkyi. The presence of subventral precloacal papillae, shape and position of the amphid prevents placement in Leptosomatum. Based on the description only, it is impossible to decide to which genus it belongs and, therefore, it is considered a species incertae sedis. More information is desired regarding L. abyssale Allgén, 1951; L. bathybium Allgén, 1954; L. behringicum Filipjev, 1916; L. breviceps Platonova, 1967; L. groenlandicum Allgén, 1954; L. indicum Stewart, 1914; L. pedroense Allgén, 1947; L. sabangense S teiner, 1915 and L. tetrophthalmum Ssaweljev, 1912. These are considered species inquirenda. Although more information is desired regarding L. sabangense sensu Micoletzky, 1930 nec Steiner, 1915 this species was renamed as L. sundaense Bongers, 1983. At present the genus Leptosomatum is composed of three groups. To the L. bacillatum-group belong L. bacillatum (Eberth, 1863) with its synonyms (L. elongatum Bastian, 1865 (the type-species); L. gracile Bastian. 1865; L. tuapsense Sergeeva, 1973; and L. filipjevi Schuurmans Stekhoven, 1950) L. acephalatum Chitwood, 1936; L. clavatum Platonova, 1958; and L. sachalinense Platonova, 1978 with its synonym (L. diversum Platonova, 1978). This group is characterized by the absence of a cephalic capsule in the male; in females and juveniles this structure is present. In the L. punctatum-group, the cephalic capsule is absent in all stages and sexes. To this group belong L. punctatum (Eberth, 1863), L. keiense Micoletzky, 1939 and possibly L. breviceps Platonova, 1967. The material described by Micoletzky as L. keiense might be composed of two species distinguishable by the presence or absence of a precloacal papilla. The third 'group' is composed of one species only namely L. kerguelense Platonova, 1958 (synonym L. crassicutis Platonova, 1958) in which all stages and sexes are provided with a cephalic capsule. The nematodes that were considered by Mawson (1958) to represent L. arcticum also belong to this group. Restudy of male paratypes of Syringonomus typicus Hope & Murphy, 1969 confirmed the supposition that, in addition to the characters originally described, long amphidial glands are also present in this genus. Leptosomatum and Syringonomus are closely related and should form a nominal taxon (Leptosomatinae ?) together (Chapter 2). Based on the genital system the genus Leptosomatides is closely related to Thoracostoma and Deontostoma. Females of Leptosomatides can easy be distinguished from those of Leptosomatum by the presence of a vaginal ovejector in the former; a character hardly used in the past. After transferring some of the Leptosomatum species to Leptosomatides the latter genus is composed of the following species: Leptosomatides euxinus Filipjev, 1918 (the type-species) with its synonym L. caucasiensis Sergeeva, 1973; L. reductus Timm, 1959; L. arcticus (Filipjev, 1916) with its synonyms L. steineri Filipjev, 1922 pro Leptosomatum gracile sensu Steiner, 1916 nec Bastian, 1865 and Leptosomatum tetrophthalmum s ensu Platonova, 1967 nec Ssaweljev. 1912; L. filiformis Filipjev, 1946; L. grebnickii (Filipjev, 1916) with its synonyms L. steineri s ensu Filipiev, 1946, L. crassus Platonova, 1967 and Leptosomatum e longatum sensu Platonova. 1967; L. marinae Platonova, 1976 with its synonyms L. acutipapillosus Platonova, 1976 and L. brevisetosus Platonova, 1976 (Plat. pers. comm.); and finally L. antarcticus Mawson, 1956 which, if provided with an onchium as has been described, might belong to Deontostoma. L. conisetosus Schuurmans Stekhoven & Mawson, 1955 has been placed in Deontostoma. More information is desired regarding L. microlaimus (Allgén, 1957) Platonova, 1976, and in the interim it will be considered a species inquirenda. L. inocellatus Platonova, 1967 does not fit in Leptosomatides and is considered a species incertae sedis (Chapter 3). Special attention is paid to the Leptosomatum bacillatum-complex (Chapter 6). Species of this complex can hardly be distinguished with traditional methods in which allometric growth is not taken into account. If log body length is plotted against log tail length, for example, a linear function appears to exist. For every specimen the log ratio can be computed against the corresponding regression lines of a reference population. These log ratio's are not length dependant and therefore offer possibilities for a multivariate analysis. In this paper such a procedure is demonstrated resulting in a splitting of the L. bacillatum-complex into five groups. Rearrangement of the genera in the family Leptosomatidae is desired. For this purpose, however, the genera must be revised and well defined. This thesis is an attempt to that study.
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