Arichlidon gathofi Watson Russell, 2000: 465–477

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 : 321-323

Arichlidon gathofi Watson Russell, 2000: 465–477

Figures 1–5 .

Material examined: Paratypes. USA, off North Carolina, western Atlantic, Stn. 2606, 34° 35'N 75° 52'W, 45 m, coll. RV Albatross , 18 Oct 1885, USNM 186017 . Note: 148 individuals were collected; among these were 36 juveniles and 4 late nectochaete larvae, the latter described herein .

Description based on benthic specimens. 7 segments: length 520 µm, width 500 µm (fig. 6A, B); 7 segments: length 460 µm, width 460 µm; 8 segments: length 540 µm, width 460 µm; 10 segments: length 700 µm, width 500 µm; 11 segments: length 840 µm, width 52 µm; 14 segments: length 920 µm, width 520 µm.

Larvae of 7 segments with broad, ovoid body shape with palaeal fans fully extended over dorsum, neurochaetae extending out beyond palaea; dense oil globules in gut. Rounded prostomium with faint red eye pigment visible; short, stout median antenna inserts on anterior edge of prostomium; lateral antennae, palps and nuchal fold absent. Segments I–III in adult configuration (figs 6A, B). Segment I more visible in ventral view, with two pairs of dorsal and ventral tentacular cirri (fig. 6B).

Notopodium of segment II with 2 narrow palaea, 6–8 ribs. Notopodium of segments II–IV include primary, expanded palaea in medial position with 15–16 ribs (fig. 6C). Segments V–VII with adult type, slimmer, asymmetrical median palaea, numbering 2–3, shorter than main fan, with 11–14 ribs (fig. 6D). Broad, asymmetrical medial-most, subunit 1, main palae ( A. gathofi species character) present posterior segments VI– VII. Prominent, curved notochaetal spine originating from lateral group (continues into adult); subulate dorsal cirri present on all notopodia (figs 6C, D).

Neurochaetae of segment II all spinigers; neurochaetae of segments III–VII include 2–3 superior spinigers; adult groupings of mid-superior and mid-inferior falcigers; typical short, curved articles of inferior falcigers. Pygidium composed of slender ventral cone and dorsal structure with two filiform anal cirri.

Post-larvae and juveniles 8–14 segments with body slightly tapered anteriorly and posteriorly; neurosetae not extending out beyond palae. Prostomium smaller with two pairs of eyes, longer, subulate median antenna, two lateral antenna and two ventral, long, cylindrical palps. Triangular mouth fold posterior to palps, pair of stylets evident in pharynx; raised glandular nuchal fold present posterior to prostomium. Increasing numbers of adult main palaeal notochaetae and neurochaetae with increasing body segments.

Remarks. Chrysopetalid notochaetal palaea, spines and neurochaetal shafts are composed of internal longitudinal ribs and horizontal diaphragms ( Westheide and Watson Russell, 1992). The appearance of the first chaetae arises in the trochophore after initiation of the first larval segment. These long, brown, spinulose provisional chaetae are internally striated. The metatrochophore 4-segmented larvae develop compound falcigerous neurochaetae with striated shafts, and the generation of the sixth segment initiates primary, laterally folded, notochaetal palaeal fans and spines, all striated internally ( Cazaux, 1968; Watson Russell, 1987).

This construction of internally striated chaetae creates maximum strength and lightness for larvae and adults found mid-water. Adult chrysopetalids may also possess epitokous, swimming neurochaetae, as first described for Arichlidon gathofi collected from the plankton ( Watson Russell, 2000, Fig. 5A , and reproduced in this paper as fig. 6F).

Mileikovsky (1962) observed that the long provisional chaetae found in chrysopetalid, sabellariid and some ‘Chaetosphaera’ spionid trochophore larvae are probable convergent structures suited to a similar pelagic mode of living, with larvae able to be transported very long distances. There is no record of chrysopetalid teleplanic larvae, but chrysopetalid metatrochophore larvae have been collected from vertical plankton tows from the surface down to 100 m, in 3000–4000 m depth in the Gulf Stream, NW Atlantic ( Mileikovsky, 1962). Original material was lost but its identity is inferred from his figures as belonging to either the genus Arichlidon or the deepsea-dwelling Strepternos (see Watson Russell, 1997).

Distribution and habitat. Arichlidon gathofi is found from North Carolina, USA to Panama, Central America, western Atlantic. Benthic habitat varies from silty sands in the Gulf of Mexico to algal, sea-grass, shell and coral rubble substrates of the islands of the north and south Caribbean; 1– 106 m.

Remarks on the larval morphology and development of Arichlidon reyssi and A. gathofi . Metamorphosis at the 6–7-segment stage occurs at benthic settlement and includes loss of larval notopodia 1 (comprising larval pair of cirri and transitory, provisional chaetae, figs. 5A–D) and development of adult segment I in a dorsal/ventral plane. The episphere reduces in size as it differentiates into a more adult prostomium and its appendages develop. Concurrent with these changes is development of adult notopodia II and III with forward rotation and part fusion, particularly evident in dorsal view; larval primary palaea are lost on notopodia II and replaced by a few, short adult palaea (fig. 6A). The nuchal fold begins to take shape as a result of these former changes and forms part of the retraction mechanism of the anterior end. A discreet caruncle, as postulated by Cazaux (1968), is found primarily in Chrysopetalum and is not present in Arichlidon species.

Adult segment I is developed from the ciliate buds seen in the larvae at the conjunction of the episphere and trunk (fig. 5A–D). From a 7-segmented larvae onwards, this segment I appears reduced and fused in part to the prostomium. It supports a pair of dorsal and ventral cirri that are often more visible in ventral view. These later-formed adult cirri are shorter than the larval pair and are approximately the same size as those dorsal cirri seen in segment II (fig. 6A, B). At no developmental stage are chaetae present on adult segment I in Arichlidon species, and the term ‘tentacular’ is therefore retained as a descriptor for the cirri of this segment.

A similar series of morphological changes has been described for the larval deep-sea chrysopetalid Strepternos didymopyton Watson Russell, 1991 , which has the same anterior end schema, i.e. segment I with two pairs of tentacular cirri, segment II with notopodia, chaetae, neuropodia with chaetae, ventral cirri absent ( Watson Russell, 1997). In Strepternos and Arichlidon , the small neuropodia 1 does not at any time possess ventral cirri (fig. 5C, 6B). It has been the contention of some authors, e.g. Perkins, 1987, that there has been loss of ventral cirri from this segment during ontogeny.

Identification of Atlantic Arichlidon larvae to species. Chaetal patterns in the midposterior body of chrysopetalid larvae can be used for identification to genus and species ( Watson Russell, 1987). The shape of the main palaea (and particularly the inferior-most curved, falcigerous neurochaetae from posterior segments) identify the above larvae as belonging to the genus Arichlidon (fig. 5F). Adult lateral, main and median palaeal types are present from segments IV–VI, with the overall highest numbers of adult chaetal types present in segments IV–VI in A. reyssi and segments V–VII in A. gathofi . The tall lateral-most median palae—a distinguishing species character for A. reyssi — is clearly visible from segment IV (fig. 5E); the shorter, broader median palae visible from segment V in A. gathofi (fig. 6D).

Figures

Figure 1. A, Map of Senghor Seamount, located in the Cape Verde Archipelago, NE Atlantic. Data extracted from Smith and Sandwell (1997); dataset created by A. Dale (SAMS). B, Senghor Seamount with the location of transects. Data and map created by Thor Hansteen and Alexander Schmidt, GEOMAR. (A, B, reproduced from Chivers et al., 2013.)

Figure 2. A–D: Thrausmatos senghorensis sp. nov., Senghor Seamount, SMF 22963. A, Anterior end, dorsal view, slide preparation; B, anterior end, dorsal view; C, mid-body parapodium, slide preparation; D, detail of superior-most neurochaeta (asterisked in fig. 2C). Scalebars: A-C, 100 µm; D, 10µm.

Figure 3. A–B: Dysponetus caecus, Senghor Seamount, SMF 22964. A, Anterior end, dorsal view, slide preparation; B, neuropodium XII with superior swimming neurochaetae. Scalebars: A-B, 100µm.

Figure 4. A–B: Arichlidon reyssi, adult, Senghor Seamount, NMS.Z.2013.160.09, slide preparations. A, Anterior end; B, mid-body notopodium from anterior end. Scalebars: A, 100 µm; B, 50µm.

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.