Megalomma acrophthalmos ( Grube, 1878 )

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 : 15-17

Megalomma acrophthalmos ( Grube, 1878)

Figure 2A–L

Sabella acrophthalmos Grube, 1878: 258–259 .

Megalomma acrophthalmos .— Knight-Jones, 1997: 316, Fig. 2A–L .

Material examined. [ USNM] 38683, topotypes, Philippines, Negros Island, Bashford Dean (20 specs).

Diagnosis. Eyes in most radioles (spiraled and spherical); dorsal margins of collar fused to faecal groove; dorsal lappets and dorsal pockets present; anterior peristomial ring exposed; caruncle present; thoracic chaetae Type B.

Description. Branchial crown longer than thorax with 22–27 pairs of radioles. Radioles brown colored over outer and lateral margins and adjacent pinnules. Outer surfaces of radioles quadrangular basally, rounded distally. Subdistal compound eyes present in most radioles. Four dorsalmost pairs of radioles with large spiraled eyes ( Fig. 2J ). Lateral and ventral pairs of radioles with spherical eyes distinctly smaller than dorsalmost ones ( Fig. 2K–L ). All radiolar tips long ( Fig. 2J–L ). Dorsal collar margins rounded anteriorly, fused to faecal groove ( Fig. 2A ). Dorsal lappets rounded, longer than lateral collar margins, slightly overlapped distally, brown colored ( Fig. 2A, G ). Dorsal pockets reaching division for second chaetiger ( Fig. 2A, G ). Anterior peristomial ring exposed dorsally between dorsal pockets ( Fig. 2G ). Ventral lappets rounded, as long as ventral shield of collar, not overlapped ( Fig. 2C ). Lateral collar margins covering basal union of radioles. Dorsal lips brown colored, triangular with mid-rib ( Fig. 2F ). Dorsal pinnular appendages absent. Ventral lips about half as long as dorsal lips, broadly rounded ( Fig. 2F ). Ventral sacs present ( Fig. 2D ). Caruncle as long as dorsal lappets, erect, triangular with rough base and smooth distal end ( Fig. 2D ). Keel absent. Body cream colored, depressed. Total thorax-abdomen length 36–47 mm, maximum width 5 mm throughout most of thorax. Seven thoracic chaetigers. Thoracic tori longest on chaetigers 2–3. Tori in chaetigers 2–3 occupy the entire distance between notopodia and ventral shield margins ( Fig. 2B ), contacting shields. Other thoracic tori not contacting shields. Inferior thoracic chaetae Type B ( Fig. 2H–I ). Thoracic uncini with handles more than 2x length of main fang. Companion chaetae with teardrop-shaped membranes. Abdomen with 62–98 chaetigers. Abdominal chaetae unknown. Abdominal uncini with main fang surmounted by 8–10 rows of numerous minute teeth. Pygidium tetralobed with purple eyespots. Tubes and gametes unknown.

Remarks. The specimens reviewed here agree with the information provided about the distribution and shape of radiolar eyes by Knight-Jones (1997) for the type of M. acrophthalmos described from Philippines (located at the Natural History Museum, University of Wroclaw, Poland, T–MPW 364). Although in her description, information was not provided for the presence of a caruncle; in her figure 2G, a small lobe is illustrated placed in middle of peristomium. However, in the unpublished notes by Knight-Jones sent to the first author during 2007, she suggest that M. acrophthalmos has a median organ as those seen in sabellariids. As stated by Tovar-Hernández and Salazar- Vallejo (2008), both, the innervation and the hyaline cartilage in the median organ on sabellariids are similar to those present in the caruncle in species of Megalomma ; consequently these structures could be considered homologous. Thus, the lobe illustrated by Knight-Jones (1997) could correspond to the distal end of the caruncle. The caruncle in topotype material of M. acrophthalmos examined in this study is as long as dorsal lappets, and placed above the mouth, between dorsal lips, erect, triangular with a smooth distal end and a rough base.

Capa and Murray (2009) recorded two specimens as Megalomma cf. acrophthalmos from Western Australia. Their description matches partially with the topotypes examined in this study except for the following features. Firstly, their specimens have a smooth keel (thickened and non-lamellate) projecting ventrally between dorsal lips, arising from raised triangular mound situated middorsal to dorsal lips. They suggested that the keel does not appear to be homologous to the caruncle, rather it is a smooth projection of the peristomium arising between dorsal lips, although the location of the keel is the same as the caruncle in M. carunculata , M. lobiferum and M. pigmentum . Thus, a studyof the innervations of the keel is needed in order to confirm that it is only a projection of the peristomium or a smooth caruncle. Secondly, in their specimens, thoracic tori are not in contact with ventral shields (at least tori from chaetigers 2–3 are in contact with the ventral shields in specimens examined here). Finally, in their material, dorsal lappets are as long as lateral collar margins whereas these are longer than lateral collar margins in specimens recorded here.

In the genus Megalomma ten species share the presence of dorsal lappets: Megalomma acrophthalmos , M. fauchaldi , M. lanigera , M. mushaense , M. nechamae , M. perkinsi , M. phyllisae , M. quadrioculatum , M. sp., and M. vesiculosum . However, M. acrophthalmos , M. sp. and M. quadrioculatum are easy distinguishable from all others by having a caruncle (absent in all others). Megalomma acrophthalmos differs from M. sp., by having the branchial crown longer than thorax (shorter than thorax in M. sp.), dorsal radioles with spiraled eyes (dorsal radioles with spherical eyes in M. sp.), anterior peristomial ring exposed between dorsal pockets (not exposed in M. sp.) and longest thoracic tori in chaetigers 2–3 (same length in all thoracic segments in M. sp.). Megalomma acrophthalmos differs from M. quadrioculatum by having compound eyes in most radioles (only in dorsalmost in M. quadrioculatum ), all radioles with long tips (all radioles with short tips in M. quadrioculatum ), ventral sacs present (absent in M. quadrioculatum ) and handle of thoracic uncini more than two times the length of main fang (shorter than length of main fang in M. quadrioculatum ).

Figures

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.