Microphthalmidae

Watson, Charlotte, Gunton, Laetitia M. & Kupriyanova, Elena K., 2024, Three new species of bacterivorous Chrysopetalidae and Microphthalmidae (Annelida) inhabiting a whale fall off eastern Australia, Records of the Australian Museum 76 (5), pp. 249-264 : 257

Family Microphthalmidae

Hartmann-Schröder, 1971

Type species. Microphthalmus sczelkowii Mecznikow, 1865 .

Diagnosis (from Salazar-Vallejo et al., 2019). Body small, delicate, rarely longer than five mm. Prostomium with 0–2 eyes. Antennae filiform. Median antenna position variable. Palps filiform, sometimes missing. Tentacular cirri usually on 2–3 distinct segments. Dorsal cirri thin, smooth, threadshaped or cirriform. Parapodia biramous or sub-biramous, lateral, rarely directed ventrally or dorsally. Pygidium comprising a pair of dorso-lateral anal cirri and ventral anal structures, either anal membrane or a ventral cirrus and a pair of anal cirri. No jaws. Free-living, rarely symbiotic with sand dollars (sea urchins), polychaetes, or sipunculans.

Remarks. Bold text indicates known characters present in taxa within the family Microphthalmidae . The family currently comprises seven genera and 53 accepted species ( Read & Fauchald, 2024a). The diagnosis above is emended to include Pleijelius because the results of phylogenetic analysis here necessitated transfer of Pleijelius to Microphthalmidae , which is also justified by morphology. Shared morphology is observed in the new species of Pleijelius and Microphthalmus . The main character shared by species of Pleijelius and Microphthalmus is the position of the median antenna. When present in adults, it is positioned mid-dorsum at the posterior edge of the prostomium at the level of segment I, as described for Microphthalmus bifurcatus Hartmann-Schröder, 1974 (fig. 10) and observed in M. hvalr sp. nov. ( Fig. 3B ) and Pleijelius keni sp. nov. ( Fig. 4B ).

According to Salazar-Vallejo et al. (2019), the monophyly of the Microphthalmidae is supported by the pygidium transformed into an anal membrane, clearly seen in various Microphthalmus spp. The importance of this character needs re-assessment as the pygidium of Pleijelius does not possess a ventral anal membrane but instead bears a ventral cirrus and a pair of anal cirri. Similar numbers of body segments, three achaetous anterior segments with six pairs of tentacular cirri, simple notochaetae and falcigerous neurochaetae further support close relationship of Pleijelius and most Microphthalmus species.

Majority of Microphthalmus species are interstitial in sandy and muddy sediments in shallow waters. The deep-sea free-living taxa Microphthalmus and Pleijelius are found in bathyal to abyssal chemosynthetic habitats in bacterial mats associated with whale and wood fall communities in the Atlantic and Pacific Oceans.

Figures

Figure 3. Microphthalmus hvalr sp. nov. A, entire body, dorsal view; B, detail of anterior end, dorsal view, showing anterior achaetous segments I-III and prostomium; C, mid and posterior body, dorsal view, showing lack of notochaetae and pygidium.

Figure 4. Pleijelius keni sp. nov. A, entire body, dorsal view; B, detail of anterior end, dorsal view; C, detail of anterior end, ventral view; D, neuropodium mid-body, ventral view; E, detail of falcigerous neurochaetae; F, detail of flattened and slenderer notochaetae. Abbreviations: ac anal cirri; la lateral antennae; p palps; achaetous segments I-III (note shorter ventral tentacular cirri on segment III), chaetigerous segment IV, ventral view; go - genital organ; sn - simple neurochaeta; vc - ventral cirrus.