Dysponetus Levinsen, 1879

Watson, Charlotte & Faulwetter, Sarah, 2017, Stylet jaws of Chrysopetalidae (Annelida), Journal of Natural History 51 (47 - 48), pp. 2863-2924 : 2894-2895

Genus Dysponetus Levinsen, 1879

( Figures 1b , 18a–f ; Tables 1, 2)

Type species: Dysponetus pygmaeus Levinsen, 1879

Material examined

One specimen Dysponetus pygmaeus: NTM W. 26207, Greenland, North Atlantic (mCT- 00038); 1 specimen Dysponetus macroculatus: SIO-BIC , specimen sacrificed for sequencing, Madang, New Guinea, South West Pacific .

Distribution

Dysponetus species occur in all world oceans, from the Arctic to Antarctica, 78°N to 64°S.

Habitat

Dysponetus species occupy diverse habitats and depths ranging from the intertidal to ~ 6000 m. An endo-/epibenthic lifestyle is led in habitats encompassing: shallow sands of the Mediterranean (e.g. Dysponetus bipapillatus ); abyssal mud and clay sediments, NE Atlantic (e.g. Dysponetus caecus recorded by Böggemann (2009) and Watson et al. (2014)); under kelp holdfasts (e.g. Dysponetus pygmaeus in Laminaria J.V. Lamouroux, 1813 , southern Greenland and Dysponetus populonectens in Macrocystis C. A. Agardh, 1820 , southern California); cryptofauna among calcareous algae on coral reefs, Papua New Guinea (e.g. Dysponetus macroculatus ).

Dysponetus species are very small, often fragile and in many instances are so lightbodied that they either need to take refuge amongst more complex, hard substrates or are swirled along in the water movements immediately above the substrate or within flocculent sediments. Tzetlin et al. (2002) observed a Dysponetus individual inhabiting the upper layer of liquid bottom mud, using its thin chaetae for soaring and floating in the mud.

General morphology

The two Dysponetus species examined here are small: Dysponetus pygmaeus , 2 mm L for 15 segments and Dysponetus macroculatus , 2.5 mm L for 21 segments. Dysponetus pygmaeus possesses robust, dark brown noto- and neurochaetae ( Figure 1b ), oval palps, and a pair of buccal cirri; eyes are absent and ventral cirri are absent on segment 2. Dysponetus macroculatus possesses slender, pale notochaetal fascicles, conical palps, a single buccal cirrus and two pairs of complex eyes; ventral cirri are absent on segment 3 ( Figure 18e ). Both species possess ciliated dorsolateral nuchal patches, although ciliate body bands and lateral organs are absent ( Pleijel and Dahlgren 1998; Tzetlin et al. 2002).

Pharynx and jaws

Our CT scan of Dysponetus pygmaeus shows unknown, dense gut contents obscuring the full view of the buccal cavity and pharynx ( Figure 18a ). Microscope pictures of Dysponetus macroculatus reveal a short proboscis and a muscular, relatively short, undifferentiated pharynx which is often barrel-shaped but which can also appear slightly more elongate and extending to about segment 6 ( Figure 18b, c ). Caeca are not discernible in CT scans. The two stylets are situated close together but set apart, or, possibly due to handling pressure, slightly touch at the anterior ends. Each stylet is straight and slender along its length and displays a very slight to less slight, swelling mid-way ( Figure 18d–f ). All species of Dysponetus possess an entirely tanned, russetbrown coloured pair of stylets ( Figure 18e, f ; CW pers. obs.).

Remarks

Due to the very small size of the Dysponetus pygmaeus jaws it is difficult to ascertain exact details of their morphology including that of the mid-stylet swelling, which in this studied individual is almost invisible. Other specimens of Dysponetus pygmaeus , and stylet jaws examined in a number of other Dysponetus species , indicate a slight midstylet swelling (CW pers. obs.).

Dysponetus macroculatus has stylet jaws composed of a distinct mid-way flare which forms a triangular, pointed distal half and a slender, rod-like, basal half. The longitudinal groove is most developed in the anterior half of the stylet, and is exceptionally wide with shallow marginal sides visible ( Figure 18e, f ). The jaw groove in Dysponetus species is usually very slender and faces inwards, as observed in Chrysopetalinae stylets (CW pers. obs.). The anterior view of the groove in Dysponetus macroculatus is possibly the result of pressure on the slide during photographing. In comparison to the majority of jaws in other Dysponetus species examined, Dysponetus macroculatus jaws appear exceptional in their possession of the greater degree of mid-way flare (CW pers. obs.).

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.

Figure 18. Dysponetus pygmaeus (mCT-00038), imaged through micro-CT. (a) entire body, volume rendering, dorsal view, semi-transparent rendering to show position of jaws; (b) cross-section, dorsal view; (c) cross-section, lateral view; (d) jaws, dorsolateral view and cross sections through the jaw along its length; Dysponetus macroculatus, micro-photographs: (e) anterior end of animal, dorsal view; (f) stylets, dorsal view.