Arichlidon Watson Russell, 1998

Watson, Charlotte & Faulwetter, Sarah, 2017, Stylet jaws of Chrysopetalidae (Annelida), Journal of Natural History 51 (47 - 48), pp. 2863-2924 : 2873-2876

Genus Arichlidon Watson Russell, 1998

( Figures 1a , 4a–d , 5a–c ; Tables 1, 2)

Type species: Arichlidon hanneloreae Watson Russell, 1998

Material examined

One specimen Arichlidon hanneloreae: NTM W. 22823, Great Barrier Reef, Queensland, South West Pacific , (mCT-00171); 2 specimens Arichlidon reyssi ( Katzmann, Laubier and Ramos, 1974) : NTM W .13180, Crete, Mediterranean Sea (mCT-00036; mCT-00245).

Distribution

Arichlidon species are found in the majority of world oceans between 40°N and 39°S.

Habitat

Arichlidon species can occur in moderately large numbers when associated with rich invertebrate micro-faunas and have been recorded from an extremely broad range of habitat types, from tropical to temperate waters in association with seagrasses and algae; crevices in rocky and coral reefs; rotting wood in mangrove channels; coarse shell habitats on seamounts and every grade of sand, clay and mud substrate from intertidal to abyssal depths, ~ 4000 m. Arichlidon species have been observed in association with sessile invertebrates such as hydroids and sponges, as well as inside the tubes of sedentary tube-dwelling polychaetes, tube dwelling amphipods and among the gills of Pinctada Röding, 1798 bivalve molluscs ( Watson Russell 1998, 2000a; Watson et al. 2014).

General morphology

Arichlidon species are relatively broad, small-bodied (~ 5 mm length) chrysopetalids with a dorsum completely covered by silver to gold coloured and often brown flecked, lateral, main and median paleael fans ( Figure 1a ). Interlocking paleael types form a type of carapace and there is a slight dorsal convexity formed by a mid-dorsal ridge of overlapping ornate, median paleael fascicles ( Figure 4a ). Species possess main paleae with heavily ornamented ribs and raised tubercules, uniquely shaped inferior neurochaetal falcigers and epitokous swimming neurochaetae ( Watson Russell 2000a, Watson et al. 2014). Sensory structures include a prostomium fused with anterior segments, two large pairs of complex eyes often fused into a rectangular block, large cylindrical palps and a glandular nuchal fold ( Watson Russell 1998, 2000a).

Pharynx and jaws

Arichlidon possesses an elongate proboscis, a broad, undifferentiated pharynx and large posterior caeca. The elongate stylets are broad, triangular, robustly calcified and occupy almost the entire proboscis and anterior pharynx ( Figure 4b–d ). The jaws are situated close to each other but do not overlap, as seen in cross section ( Figure 5c ). The jaw margins appear straight but a very slight outer, mid-way sloping projection is present. Distinct longitudinal, raised ridges run along the inner stylet margins with their basal ends extending and forming small spurs ( Figure 5a–c ). Serrations are often visible on the distal jaw tips under a compound microscope. Although faint, it is possible that the ‘granular’ band posterior to the jaws is composed of calcifications ( Figure 4c, d ).

Remarks

A previously unremarked structure in the pharyngeal region was recently observed and found to be present in all Arichlidon species. It does not show up on micro-CT images, being visible only under a compound microscope. The structure is composed of two triangular shaped glands situated opposite one another, either side of each stylet. Upon dissection, tiny golden-brown globules spill out; their function is currently unknown ( Watson et al. 2014, figure 4a).

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 4. Anterior end of Arichlidon hannelorae (mCT-00171) imaged through micro-CT and rendered in 3D. (a) dorsal view; (b) dorsal view, semi-transparent rendering to show position of jaws; (c) dorsal view, virtually dissected to expose pharyngeal structures; (d) lateral view, virtually dissected to expose pharyngeal structures.

Figure 5. Jaws of Arichlidon hannelorae (mCT-00171) imaged through micro-CT. (a) dorsal view; (b) lateral view; (c) dorsal view and cross sections through the jaw along its length.