Freshwater nematodes of the Galápagos

Abstract

Freshwater nematodes of the Galápagos archipelago were studied from four samples collected in 1988. Eighteen species, two of which are new to science, belonging to five orders, Enoplida, Dorylaimida, Mononchida, Monhysterida and Chromadorida, are described. Alaimus wittmeri n.sp. is characterised by having a smooth cuticle, a more posteriorly enlarged pharynx, a posteriorly situated amphidial fusus, obscure but anteriorly situated excretory pore, a mono-opisthodelphic ovary, and by the absence of sexual dimorphism in the lip region, the absence of pre-vulval uterine sac, the presence of males that have few (4–7) and weak ventromedian supplements and absence of mucro on tail tip. Monhystera floreanae n. sp. is distinguished by its females having a small body (L = 492–634 µm) with a narrow lip region, narrow posteriorly located amphidial openings, a small value for the c'-ratio (c' = 6.0–7.5), weak vaginal sphincter, by the absence of post-vaginal gland cells, absence of males and absence of crystalloid bodies. The difficulty in the use of crystalloid bodies as diagnostic character in the genus Monhystera is discussed. Two forms, a big form and a small form, which are similar except in measurements are reported for Achromadora micoletzkyi. SEM photos of ruptured cuticle of A. micoletzkyi and A. semiarmata show unique intra-cuticular structures. Comparison of these structures in the two species is made. Papilliform instead of setiform outer labial sensilla are also observed in A. micoletzkyi. The presence of the male and fine crystalloid bodies in Achromadora semiarmata, dorsal body pore in Prismatolaimus dolichurus and Prismatolaimus kenyensis, forty longitudinal cuticular ridges around the mid-body of Stomachoglossa pachyderma, crystalloid bodies in Ironus dentifurcatus are reported here for the first time. SEM photos of twelve species: Monhystera floreanae n. sp., Monhystera somereni, Alaimus wittmeri n. sp., Monhystrella hastata, Paramphidelus dolichurus, Eutobrilus annetteae, Ironus dentifurcatus, Chronogaster cameroonensis, Rhabdolaimus terrestris, Stomachoglossa pachyderma, Achromadora micoletzkyi and Achromadora semiarmata, are also presented.

The richest nematode fauna was found in El Junco (San Cristobal), the only permanent freshwater lake on the archipelago. The nine species recorded from the lake are probably only a fraction of the total nematode fauna. The very recent man made reservoir La Toma has a relatively high diversity, due to its vicinity to El Junco. It is argued that the freshwater nematodes from San Cristobal probably arrived there through passive transport by birds.

The freshwater bodies encountered on Floreana are very different from those on San Cristobal and so is their nematode fauna. Part of the species found on these islands may have been transported by land birds, others may have invaded the freshwater from terrestrial habitats.

Apart from the two new species, the other ones have a wide to very wide distribution (cosmopolitan, southern hemisphere, ...), so that endemism of freshwater nematodes is very low, if it exists at all. This is due to easy dispersal capacities of nematodes on the one hand and the recent colonization (not more than 10 000, max. 40 000 years) on the other hand.