Cephalic tubercles, a new character useful for the taxonomy of desmoscolecidae (Nematoda)

Abstract

The cephalic regions of 10 nematode species of the family Desmoscolecidae were examined by scanning electron microscopy (SEM). They belonged to two genera, Tricoma and Desmoscolex, which were collected from off the coasts of Japan, Caribbean Sea, Clew Bay (Ireland), and Thailand. The basic structure of the lip region was the same within Tricoma regardless of geographic region, but a distinct intergeneric difference was found. In species of this genus, six conspicuous outer labial sensilla protruded from the smooth lip surface, and the circular ridges corresponding to the boundary of the closed oral aperture, the external rim of the labia, and the anterior rim of the cephalic concretion ring were distinctive. On the other hand, the labial surface of specimens of the genus Desmoscolex was covered by a variety of small, tuberculate structures. These cephalic tubercles were 70-240 nm in diameter and recognizable only by SEM examination. The shape of the cephalic tubercles and the area of the head covered by them were constant within the same species, but differed among species. The labial sensilla were surrounded and obscured by specialized cephalic tubercles in Desmoscolex. The cephalic tubercles were found only in adults of Desmoscolex. These results suggested that the presence of cephalic tubercles is a synapomorphic character of Desmoscolex, and that it is useful for phylogenetic studies of the Desmoscolecidae. The family Desmoscolecidae is a unique group of free-living nematodes characterized by desmens, which are thick, transverse rings considered to consist of sedimentary particles and cementing materials (desmos). Earlier taxonomic revisions of the family by Timm (1970) and Freudenhammer (1975) were based mainly upon external structures, in particular the shape and number of the desmens and the shape and arrangement of somatic setae on each desmen. Decraemer (1974, 1975a,b, 1977, 1978a,b, 1979, 1984) studied this group extensively; recently, she has revised the order Desmoscolecida using the methods of phylogenetic systematics (cladistics) and character states of both external and internal morphology (Decraemer, 1985). Scanning electron microscopy (SEM) is a most useful technique in taxonomic studies of nematodes (Hirschmann, 1983). This is especially true in studies of Desmoscolecidae because precise observations of external structures are essential for classification within the family. SEM also can reveal details that are not resolved by ordinary light microscopic techniques. During a SEM investigation of deep-sea nematodes collected from the western 1 This research was supported in part by a postdoctoral fellowship awarded to Y. Shirayama from the Smithsonian Institution. The authors thank Abbie Yorkoff, Brian Kahn, Susann Braden, and Walter Brown for their technical assistance. Drs. Robert P. Higgins and Chittima Aryuthaka provided part of the material used in this research. 2 To whom correspondence should be directed. 3 Present address: Ocean Research Institute, University of Tokyo, Minami-dai, Nakano-ku, Tokyo 164, Japan. TRANS. AM. MICROSC. Soc., 111(3): 211-222. 1992. ? Copyright, 1992, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.162 on Thu, 11 Aug 2016 04:49:29 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. Pacific, we found previously unknown ultrastructure in the labial region of three undescribed species of Desmoscolex. We also examined other desmoscolecids collected from shallower depths of the Atlantic and Indian Oceans to determine the general occurrence of the structure. The primary objective of this paper is to describe labial ultrastructure in all of the species of Desmoscolex examined. The distribution of such structure in desmoscolecid genera also is addressed, and the value of the structure for the taxonomy of the family is discussed. MATERIALS AND METHODS Five collections of desmoscolecid nematodes were studied (Table I). These nematodes were collected from various parts of the world, including the northwestern Pacific, north Atlantic, and eastern Indian Oceans, and the Caribbean Sea. The collections represented depths from subtidal to bathyal deep sea. Specimens studied represented two genera, four subgenera, and 10 species (Table II). Four species of the genus Tricoma were selected to represent the subfamily Tricominae; six species of Desmoscolex were chosen to represent the Desmoscolecinae. Both genera included representatives of species collected from different geographic areas. Representatives of both genera were taken from the area off Sanriku, northeastern Japan. Specimens represented one species of the subgenus Tricoma, three species of the subgenus Quadricoma, six species of the subgenus Desmoscolex, and one species of the subgenus Desmolorenzenia. Specimens were extracted from sediment with a 63-,m mesh sieve and were fixed and preserved in 10% formalin in seawater. They were cleaned in deionized water using a sonicator for 10-30 sec and gradually transferred into 100% ethanol using a vapor diffusion technique. Specimens were critical-point dried and coated with carbon, gold, and palladium. SEM observations were made with a Hitachi 570 (LAB6) scanning electron microscope, usually operated at an acceleration voltage of 15 kV.