Arichlidon reyssi, Watson Russell, 2000

Watson, Charlotte, Chivers, Adam J., Narayanaswamy, Bhavani E., Lamont, Peter & Turnewitsch, Robert, 2014, Chrysopetalidae (Annelida: Phyllodocida) from the Senghor Seamount, north-east Atlantic: taxa with deep-sea affinities and morphological adaptations, Memoirs of Museum Victoria 71, pp. 311-325 : 319-321

Arichlidon reyssi metatrochophore planktonic larvae

Figures 5A–F .

Material examined: NE Atlantic, France, Arcachon , from plankton outside Marine Station, Nov 1987, coll. C. Cazaux, 3 entire specimens all 6 segments .

Description based on planktonic specimens. 1: Length 480 µm, width 440 µm; 2: length 640 µm, width 440 µm; 3: length 720 µm, width 520 µm; NTM W25385.

Broad, ovoid bodies filled with dense oily droplets; conspicuous fascicles of long, brown, latero-anteriorly directed transitory notochaetal spines in first chaetigerous segment (fig. 5A). Smallest larva 1 with notochaetal fans more folded and bare mid dorsal line; larvae 2 and 3 with notochaetal palaeal fans spread over dorsum from segments II–VI; compound falcigerous neurochaetae from segments II–VI. All larvae possess large rounded epispheres with three pairs of eyes; largest pair in anterodorsal position with apparent lenses, smaller pairs more dorsal. Larva 1 prostomium with small, unpaired, anterolateral cirrus (developing lateral antenna?); larva 3 with circular hyaline patch mid-episphere and developing mouth. No median antennae, palps or nuchal organs visible.

Small ciliate ‘buds’ present each side of body at posterior latero-dorsal edges of episphere at dividing line between head and trunk (nascent adult segment 1). Larval segment I with two pairs of larval tentacular cirri, longer than following cirri; inserting at the same level as the transitory notochaetae. Transitory notochaetae insert in large, rounded dorso-lateral lobe with 2 aciculae; number ~15, with larger spinelets along entire lateral edge and minor spinelets in another plane along part of length (figs 5A–D).

Segment II, ventral view: very small neuropodial rami present and directed towards mid-body ventral line i.e. not laterally; with fascicles of spinigerous neurochaetae, ventral cirri absent. Neuropodia III–V1 with subulate ventral cirri (fig. 5C).

Notopodia of segments II–III with larval primary palaea types only: with 2–4 lateral palaea, 1–3 short spines, 4 large symmetrical main palaea and 2 broad asymmetrical palaea in medial-most position. Notopodium of segment III with 2 lateral palaea with 8–12 ribs, 5–7 main palaea with 17–21 ribs and 4 symmetrical median palaea. Larval main palaea distally rounded. Subsequent notopodia with 1 small spine overlying dorsal aciculum; notopodia of segments II– VI with relatively short, subulate dorsal cirri (fig. 5B) .

Segments IV – VI notopodium with adult chaetal types replacing larval types. Notopodia of segment IV with 4 lateral palaea with 6–14 ribs; 4–5 main palaea with 15–19 ribs, including large, slightly asymmetrical subunit 1 palae with 19–20 ribs and 3 raised serrated ribs; 5–6 median palaea grading in size and degree of asymmetry with 7–14 ribs, including tall lateral-most one with 2–3 raised and serrated ribs as tall as or taller than main palaea group) (fig. 5E). Notopodia of segment V with 3 lateral, 4 main and 5 median; notopodia 6 with small notosetal fascicle comprising 1–2 slender lateral, 2 main and 3 short median palae. Adult main palae distally squarer (fig. 5F) .

Segment III neuropodia mainly with falcigerous neurochaetae with slender, narrow blades; recognisable adult chaetae and adult types from neuropodia of segments IV– VI. Neuropodium of segment IV with 1 superior spiniger, 2–4 mid-superior falcigers with long blades, 4 mid-inferior falcigers with shorter blades and 6–8 inferior falcigers with typical adult smooth, short, curved blades (fig. 5F). Neuropodium of segment VI with 1–2 superior long, narrow-bladed falcigers and 2–3 lower falcigers with shorter, slender blades. All neuropodia with 1 short, simple spine overlying ventral acicula. Pygidium composed of ventral median conical protruberance and dorsal rounded structure with two lateral anal cirri .

Remarks. Cazaux (1968) provided detailed figures of the early development of a species he identified as Chrysopetalum debile , collected at different stages from the plankton at Arcachon, NE Atlantic. The ‘ C. debile ’ identification was based on one of a number of chrysopetalid species present in the region, and original material was subsequently lost. Study of recent material of metatrochophore chrysopetalid larvae from the same locality and described in this paper, confirms Cazaux’s material as most likely belonging to the species Arichlidon reyssi .

Behavioural observations in Cazaux’s 1968 paper include a description of the planktonic larvae not feeding but living on their reserves and at the slightest touch rolling into a ball, becoming bristly like a ‘Chaetosphera’ larvae. He observes there is a planktonic duration of at least three weeks between metatrophore 1 to nectochaete 1, and their presence in stations located between the ocean and inner estuary of the Bay of Arcachon between October and December. Bhaud in litt. mentions their presence in the Western Mediterranean between August and October.

Distribution. Planktonic larvae of Arichlidon reyssi have been reported from the Mediterranean and NE Atlantic coast.

Figures

Figure 5. A–F: Arichlidon reyssi 6-segmented larva, Arcachon, NE Atlantic, NTM 25385; A, D–F: slide preparations. A, Entire larva, dorsal view; B, anterior end, dorsal, left side detail (transitory chaetae drawn in part); C, anterior end, ventral view, left side detail; D, detail of anterior end of fig. 5A; E, notopodium segment IV; F, neuropodia segments IV and V. Scalebars: A, 50µm; B-C, 100µm; D-F, 10 µm.