Monhysteridae

Monhysteridae de Man 1876

Although Xyalidae are associated mostly with euhaline areas, it is perceived that species of Monhysteridae are confined mainly to marine littoral, estuarine, brackish, limnic and even terrestrial environments (Lorenzen 1978, 1979; Jacobs 1987). Marine Monhysteridae also dwell in supertidal and tidal zones as well as in decaying plant material and on body surface of some marine animals, usually crustaceans, with a few species living in the sublitoral zone and thus are considered as true marine species (Lorenzen 1979). The only species thought to occur off coastal zones was believed to be Odontolaimus ceti, a symbiont of finback whales. However, investigation of deepsea meiofauna, revealed the presence of many small and minute species of Monhysteridae. Monhysteridae species have been recorded in deepsea sediments, with the use of more accurate methods of sampling and isolation of abyssal meiofauna such as multicorers and successive sieving through meshes with minimal mesh size of 30 µm (Bussau 1993; Vincx et al. 1994; Vopel & Thiel 2001).

The generic taxonomy of Monhysteridae is now rather confused. Lorenzen (1978) clearly separated the family Monhysteridae s. str. from the related Xyalidae. Andrássy (1981) revised all inland aquatic and terrestrial species of Monhysteridae and provided emended diagnoses for Monhystera s. str., Monhystrella, Sinanema and the newly established Anguimonhystera, Eumonhystera and Geomonhystera. Unfortunately, Andrássy (1981) ignored brackish and marine species which actually constitute more than half species of Monhysteridae (Gerlach & Riemann 1973). Jacobs (1987) updated the taxonomy of Monhysteridae with the provision of complete lists of marine species and the erection of the new genus Thalassomonhystera. However, demarcation between genera remains obscure. Thus, Jacobs (1987) established the tribes Monhysterini (comprising of two genera, Eumonhystera Andrássy 1981 and Monhystera Bastian 1865) and Thalassomonhysterini Jacobs 1987 (comprising of Thalassomonhystera Jacobs 1987). The tribes and genera are characterised either by partly overlapping features, e.g. V­shaped and wide stoma, or are hardly discernible by routine optical microscopical study, e.g. peri­oral platelets present or absent, lips amalgamated or not amalgamated. Riemann (1995) also indicated that some very fine characters used by Jacobs (1987) were visible only in the electron microscope.

Currently, we assume all marine “ Monhystera ” species to belong either to Geomonhystera (G. d i s j u n c t a with related species) or to Thalassomonhystera (all other marine “ Monhystera ” species not related to G. disjuncta). With new techniques in describing the diversity of marine and especially deep­sea monhysterid species, e.g. molecular barcoding (Floyd et al. 2002), the genera of Monhysteridae will have to be reconsidered.