Dysponetinae, Aguado, Nygren and Rouse, 2015

Watson, Charlotte & Faulwetter, Sarah, 2017, Stylet jaws of Chrysopetalidae (Annelida), Journal of Natural History 51 (47 - 48), pp. 2863-2924 : 2911-2912

Subfamily Dysponetinae

The buccal cavity of Dysponetus species is similar to that seen in taxa of Chrysopetalinae . It comprises a small proboscis with terminal papillae, a muscular pharynx and a pair of tanned, pointed stylet jaws with a longitudinal groove and a small mid-way flange which anchors the jaws at the proboscidial/pharynx juncture. The rod-like jaws are morphologically very similar between species with a common orientation within the pharynx: each stylet sitting apart facing each other, and, where visible, the inner groove better developed in the anterior half of the stylets. Stylets may possess serrations with dentition of inner distal stylet jaw margin recorded for Dysponetus caecum by Dahlgren and Pleijel (1995). The function of the Dysponetus simple stylet jaw is considered similar to that seen in jaws of Chrysopetalinae taxa: puncturing prey tissue, achieving traction, then in concert with a muscular pharynx sucking fluids down the jaw groove.

All Dysponetus species possess notochaetal fans of brown, golden or silvery coloured, serrate spines that are held in tight erect fascicles leaving the dorsum completely bare, or in more robust, flattened notochaetal fascicles that cover the lateral sides of the dorsum in part, e.g. Dysponetus pygmaeus ( Figure 1b ); long-shafted neurochaetae extend out beyond the body. Most species possess bodies that very easily fragment and deciduous appendages; body and chaetal morphology are simple and often hard to differentiate, but species differences are discernible (Watson et al. in prep.).

Sensory characteristics of Dysponetus species include a median antenna, lateral antennae and palps (latter two structures differentiated); large, small or absent eyes and a nuchal organ in the form of ciliated patches. Species possess single, double or absent buccal cirri in comparison to a full mouth flap, and also simpler body wall musculature and sensory characters in comparison to those of Chrysopetalinae taxa. Ciliate body bands and lateral organs are absent. The small body size (2–6 mm L), the simplification of body structures, and the reproductive modes (male copulatory organs and hermaphroditism) observed in Dysponetus species are cited as adaptations to a meiofaunal and endobenthic lifestyle ( Tzetlin et al. 2002).

Dysponetus has the broadest latitudinal (Arctic–Antarctic) and depth range (intertidal to> 6000 m) within the Chrysopetalidae . The lifestyle of particularly the light-bodied, very small species (2 mm L) is considered endobenthic; species may find cover deeper within flocculent sediments. Small, temperate dwelling Dysponetus species (~ 6 mm L) are usually epibenthic and are found living amongst algal substrates. Dysponetus species appear entirely free-living, with no collection data or published records indicating a symbiotic lifestyle.

Figures

Figure 1. Live photos: (a) Chrysopetalinae. Arichlidon gathofi Watson Russell, 2000a (~5 mm L); (b) Dysponetinae. Dysponetus pygmaeus (~3 mm L); (c–e) Calamyzinae: (c) free-living Micospina aureobohnorum Watson et al., 2016 (~4mm L); (d) ectoparasite Calamyzas amphictenicola (~ 2 mm L); (e) symbiont Shinkai longipedata (~ 250 mm L). Copyright: a, c, e: © Greg Rouse; b: © Monika Kedra/ Slawomira Gromisz; d: © Fredrik Pleijel. All photos reproduced with permission by the copyright holders.