Megalomma bioculatum ( Ehlers, 1887 )

Tovar-Hernández, María Ana & Carrera-Parra, Luis F., 2011, Megalomma Johansson, 1925 (Polychaeta: Sabellidae) from America and other world-wide localities, and phylogenetic relationships within the genus 2861, Zootaxa 2861 (1), pp. 1-71 : 17-18

Megalomma bioculatum ( Ehlers, 1887)

Figures 3A–C , 5A–G

Branchiomma bioculatum Ehlers, 1887: 260–263 , Pl. 53, Figs 1–9 .— Hartman, 1938: 18.

Megalomma bioculatum .— Johansson, 1927: 132.— Hartman, 1965: 234.— Perkins, 1984: 352–354, Fig. 38.

Type material examined. [ MCZ] 669, holotype and six syntypes partially dried and incomplete (only crowns with anterior thorax), US coast survey, C. P. Patterson, Gulf Stream and Gulf of México Exploration , S. Blake, Alex Agassiz, 1877–78, Straits off Florida, 24° 37.5’ N, 83° 36’ W, 201 m, May 04, 1868, 166 m. [ MCZ] 824, lot with an onuphid and an oenonid from the original series of syntypes.

Additional material examined. [ MCZ] 47066, Expedition Gerda, UMML, Sta. 1035, US Florida Straits , 24° 36.1377’ N, 80° 56.6574’ W, 139–185 m, February 26, 1969 (2 specs) .

Diagnosis. Eyes only in dorsalmost pair of radioles (spherical); dorsal margins of collar not fused to faecal groove; anterior peristomial ring exposed totally dorsally; thoracic chaetae Type B.

Description. Branchial crown longer than thorax with 14–18 pairs of radioles. Radioles with broad reddish or brown bands on middle third. Outer surfaces of radioles quadrangular basally, rounded distally. Sub-distal compound eyes present only in dorsalmost pair of radioles, large, spherical. All radioles with short tips ( Fig. 3C ). Dorsal collar margins diagonal, not fused to faecal groove ( Figs 3A , 5A ). Dorsal lappets absent. Dorsal pockets absent ( Figs 3A , 5A ). Anterior peristomial ring exposed totally dorsally ( Fig. 3A ). Ventral lappets triangular, not overlapped, as long as the ventral shield of collar ( Figs 3B , 5B–C ). Lateral collar margins not covering the basal union of radioles. Dorsal lips erect, triangular, very long (about ¼ of branchial crown length), with mid-rib ( Fig. 5C ). Dorsal pinnular appendages present. Ventral lips about half of dorsal lips length, broadly rounded. Ventral sacs present. Caruncle and keel absent. Body cream colored, depressed. Total thorax-abdomen length 47–94 mm, maximum width 4 mm throughout most of thorax. Seven thoracic chaetigers. Tori in chaetigers 2–3 short, occupying a half of the distance between notopodia and ventral shield margins, not contacting shields ( Figs 3B , 5B–C ). Inferior thoracic chaetae Type B ( Fig. 5G ). Thoracic uncini with main fang surmounted by 9–10 rows of numerous minute teeth, handles 2x length of main fang ( Fig. 5D ). Companion chaetae with teardrop-shaped membranes ( Fig. 5E–F ). Abdomen with 74–88 chaetigers. Abdominal chaetae unknown. Abdominal uncini with main fang surmounted by 7–8 rows of numerous minute teeth. Pygidium unknown. Tubes from syntypes and additional materials covered with shell fragments and gravel.

Remarks. Perkins (1984) noted that Ehlers’ figure (1887: pl. 53, fig. 9) of a thoracic avicular hook is incorrect because after re-examination of type specimens the handles are much shorter than illustrated originally. On the other hand, neither Ehlers nor Perkins described or illustrated the shape of abdominal chaetae, and new dissections for the types were not possible since these have only thoracic segments.

Among the species of Megalomma , only M. bioculatum , M. gesae , M. kaikourense , M. modestum and M. pigmentum , share the presence of eyes only in the dorsalmost radioles (also present in M. vigilans , but not included in this study, see methods section) and the anterior peristomial ring exposed in some degree (except in M. modestum in which the peristomium is not exposed). In M. bioculatum and M. kaikourense the anterior peristomial ring is exposed totally dorsally (in M. gesae and M. pigmentum it is exposed dorsally and laterally). Megalomma bioculatum have thoracic chaetae Type B (Type C in M. gesae and M. pigmentum ; Type A in M. modestum and M. kaikourense ). In M. modestum the dorsal collar margins are fused to the faecal groove (not fused in the other four species). Megalomma pigmentum has a caruncle (absent in the other four species). Megalomma bioculatum , M. modestum and M. kaikourense do not have basal ventral flanges (present in M. pigmentum and M. gesae ) and the branchial crown in M. bioculatum and M. pigmentum is longer than thorax (shorter in M. gesae , as long as thorax in M. modestum and M. kaikourense ).

Megalomma bioculatum is a lower shelf to upper slope species ( Perkins 1984). It has been found in depths of 90 to about 200 m. Perkins (1984) stated that records from shallow water of tropical West Africa and Brazil are incorrect and he emphasized also that all of the specimens reported as M. bioculatum from shallow waters of the Caribbean area belong to other species as well.

Figures

FIGURE 1. Variations on dorsal margins of collar and inferior thoracic chaetae in Megalomma. A–C) Margins fused to faecal groove; D) margins not fused to faecal groove; E) chaeta type A; F) chaeta type B; G) chaeta type C. A–D) Schematized; E–G) modified from Fitzhugh (1989). Abbreviations: dlc dorsal lappets; fg faecal groove. The partial exposition of the anterior peristomial ring is marked in gray.

FIGURE 2. Megalomma acrophthalmos (Grube, 1878). A) Thorax, dorsal view; B) thorax, ventral view; C) collar and anterior segments, ventral view; D) peristomium, frontal view; E) thoracic chaetigers; F) dorsal and ventral lips; G) collar and anterior segments, dorsal view; H–I) inferior thoracic chaetae type B; J) dorsalmost radiolar eye; K) eye from a lateral radiole; L) eye from a ventral radiole. A–L) Specimens from Philippines, USNM, 38683. Abbreviations: apr anterior peristomial ring; ca caruncle; d dorsum; dl dorsal lip; dlc dorsal lappets of collar; nl notopodial lobe; v ventrum; vl ventral lobe; vs ventral sacs; vsc ventral shield of collar. Scale bars: A–D, G) 1 mm; E) 0.5 mm; F) 1 mm; H–I) 50 µm; J–L) 0.3 mm.

FIGURE 3. Megalomma bioculatum (Ehlers, 1887). A) Thorax, dorsal view; B) same, ventral view; C) dorsalmost radiolar eye. A–C) Specimen from Florida, MCZ, 47066. Abbreviation: apr anterior peristomial ring. Scale bars: A–B) 1 mm; C) 0.3 mm.

FIGURE 4. Megalomma circumspectum (Moore, 1923). A) Thorax, dorsal view; B) same, ventral view; C) same, lateral view; D) ventral sacs as indicated with arrows, branchial crown removed; E) posterior end; F) eyes from dorsal radioles; G) dorsalmost radiolar eye; H) eye from 2nd dorsal radiole; I) eye from a lateral radiole; J) ventralmost ocular spot. A–J) Specimens from La Paz, México, ECOSUR. Scale bars: A–C, E) 1 mm; D) 0.5 mm; F) 2 mm; G–J) 0.3 mm.

FIGURE 5. Megalomma bioculatum (Ehlers, 1887) and M. claparedei (Gravier, 1908). A, I) Thorax, dorsal views; B–C, H) thorax, ventral views; D, P) thoracic uncini; E–F, N) companion chaetae; G) inferior thoracic chaeta type B; J) dorsalmost radiolar eye; K) abdominal uncinus; L) superior thoracic narrowly hooded chaeta; M) abdominal broadly hooded chaeta; O) inferior thoracic chaeta type B. A–G) Megalomma bioculatum; H–P) Megalomma claparedei. A) Specimen from Florida, MCZ, 47066; B–G) modified from Perkins, 1984; H–I) syntype from Djibouti, MNHN 559; J–P) modified from Gravier, 1908. Abbreviations: apr anterior peristomial ring; dl dorsal lip; dp dorsal pockets; vsc ventral shield of collar. Scale bars: A) 1 mm; H–I) 0.5 mm; B–G, J–P) not scaled.

FIGURE 6. Megalomma circumspectum (Moore, 1923). A, D, G) Thorax, lateral views; B) thorax, ventral view; C, E–F) thorax, dorsal views; H) base of the dorsalmost radiole, lateral view; I) dorsalmost radiolar eye; J–K) eyes from lateral radioles; L) eye from a ventral radiole; M) base of 2nd dorsal radiole, lateral view; N) spermatozoon; O) oocytes; P, V) thoracic uncini; Q) abdominal uncinus; R–S) superior thoracic narrowly hooded chaetae; T–U) inferior thoracic chaetae type C. A–E, H–V) specimens from Nayarit, México, ECOSUR; F–G) specimen from Santa Cruz, California, LACM, 003489. In F–G parasitic copepods are indicated with arrows. Scale bars: A–G) 1 mm; H–M) 125 µm; N) not scaled; O) 25 µm; P–V) 20 µm.

FIGURE 7. Megalomma coloratum (Chamberlin, 1919). A) Body, lateral view; B) thorax, lateral view; C) glandular ridge on chaetiger 3 as indicated with arrow; D) thorax, ventral view; E) branchial crown and thorax, dorsal view; F) ventralmost radiole; G) dorsalmost radiolar eye; H) eye from 5th dorsal radiole; I) abdominal tori; J–L) pygidiums. A–L) Specimens from Mazatlán, México, ECOSUR. Scale bars: A, E) 10 mm; B–D) 3 mm; F–H) 0.3 mm; I–L) 3 mm.

FIGURE 8. Scanning electronic micrographs of body and caruncle. A) Lateral view of Megalomma coloratum (Chamberlin, 1919) from Mazatlán, México, ECOSUR; B–C) lateral and dorsal views of caruncle of Megalomma lobiferum (Ehlers, 1887) from Ria Lagartos, México, ECOSUR. Scale bars: A) 1 mm, B–C) 0.2 mm.

FIGURE 9. Megalomma coloratum (Chamberlin, 1919). A) Body, lateral view; B) branchial crown, dorsal view; C) thorax, dorsal view; D) dorsal and ventral lips; E) peristomium, frontal view; F) oocytes; G) abdominal uncinus; H–I) thorax, ventral views; J) superior thoracic narrowly hooded chaeta; K) inferior thoracic chaeta type C; L–M) abdominal broadly hooded chaetae; N) dorsalmost radiolar eyes; O–P) eyes from from 5th dorsal radiole in adult and juvenile stages, respectively; Q) ventral radiole; R–S) thoracic uncini and companion chaetae. A–S) Specimens from Mazatlán, México, ECOSUR. Abbreviations: bls basal lateral skeleton; d dorsum; dl dorsal lip; ep epithelium; no nuchal organ; rs radiolar skeleton; v ventrum; vl ventral lip. Scale bars: A–E, H–I) 1 mm; F) 15 µm; G, J–M, R–S) 20 µm; N) 125 µm.