Megalomma Johansson, 1925

Capa, María & Murray, Anna, 2009, Review of the Genus Megalomma (Polychaeta: Sabellidae) in Australia with Description of Three New Species, New Records and Notes on Certain Features with Phylogenetic Implications, Records of the Australian Museum 61 (2), pp. 201-224: 204-205

publication ID 10.3853/j.0067-1975.61.2009.1529

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Megalomma Johansson, 1925


Megalomma Johansson, 1925  

Type species. Branchiomma kollikeri Claparède, 1869   , a junior synonym of Amphitrite vesiculosum Montagu, 1815   (see Tovar-Hernández & Salazar-Vallejo, 2008).

Synapomorphy. Presence of subdistal, sessile and compound radiolar eyes, at least on the internal margin of the dorsalmost pair of radioles ( Fitzhugh, 1989).

Remarks. There are other morphological characters that have been included in the diagnosis of the genus (see last emendation by Tovar-Hernández & Salazar-Vallejo, 2008) and are herein assessed.

Megalomma   has been attributed with rounded external margins of radioles, but some of the species described in the present study clearly show distinct externally quadrangular margins along some length of their radioles ( Fig. 1G). However, it is not easy to distinguish between the two states (see also Fitzhugh, 1989, p. 21), and this may not be a significant character anyway. The species studied have longitudinal bands of cilia on the outer lateral edges ( Fig. 1H) as stated by Perkins (1984) but this features is not unique to Megalomma   and they have been described in other genera [e.g., Laonome ( Capa, 2007)   ].

The radioles are supported by rows of vacuolated cells (referred to as the radiolar skeleton). The number of rows is variable within the genus and also within some of the species. It has been stated by Tovar-Hernández & Salazar-Vallejo (2008) that the maximum number of rows is 16, but some of the specimens described in this study (e.g., M. phyllisae   n.sp., Fig. 4A View Figure 4 ) exhibit up to 30 cells in transverse section (see variability of rows in radiolar skeleton in Fig. 5 View Figure 5 ).

In Megalomma   , the ventral lips terminate ventrally in parallel lamellae, as defined by Nicol (1931) and Fitzhugh (1989), seen between the ventral lappets of the peristomial collar, and some species also possess vesicles “formed by the outpocketing of the parallel lamellae” called ventral sacs (as in Fitzhugh, 1989) ( Fig. 1I,J).

Tovar-Hernández & Salazar-Vallejo (2008) described for the first time the existence of a caruncle in some Megalomma species   , as an erect, triangular ciliated lobe, between the dorsal lips. The examination of Australian specimens has determined the presence of a smooth “keel” in some species but its detailed morphology, ultrastructure or function has not been studied. It does not appear to be homologous to the caruncle, rather it is a smooth projection of the peristomium arising between the dorsal lips, forming a ventrally-directed ridge.

Characters and states relating to chaetae and uncini should also be reconsidered, as there is probably more variation in the genus than has been recorded so far. Previous studies agree that notochaetae of the first chaetiger are elongate and narrowly hooded, as are the rest of the superior thoracic chaetae ( Perkins, 1984; Fitzhugh, 1989, 2003; Tovar- Hernández & Salazar-Vallejo, 2008), and that the inferior thoracic notochaetae are broadly hooded ( Perkins, 1984; Fitzhugh, 1989, 2003; Tovar-Hernández & Salazar-Vallejo, 2008) but with hoods variable in length (Tovar-Hernández & Salazar-Vallejo, 2008). Abdominal neurochaetae have been described as elongated, narrowly hooded chaetae ( Perkins, 1984; Fitzhugh, 1989, 2003; Tovar-Hernández & Salazar- Vallejo, 2008) ( Fig. 1K) but they could be interpreted as broadly hooded (but with long tips) in some species ( Fig. 1L). The length of the thoracic uncini handle could also be considered as useful for discriminating between species as this varies from medium length (same length as the distance from main fang to breast) to long (twice the length of the distance from main fang to breast).

The thoracic companion chaetae possess a proximal shaft or handle, generally similar in length to the thoracic uncini handle, and a distal membrane that has been described as teardrop-shaped by most authors ( Perkins, 1984; Fitzhugh, 1989, 2002, 2003), or as roughly symmetrical ( Fitzhugh, 1989; Tovar-Hernández & Salazar-Vallejo, 2006; Giangrande et al., 2007), or as asymmetrical ( Knight-Jones, 1997). After studying the position of the small rows of teeth and the orientation of the tip of the distal membrane, we have concluded that the membrane is asymmetrical ( Fig. 1M). There is some variation in the form of the companion chaetae among different species, so this character may be useful from a systematic point of view.

Note: generic diagnostic characters have been omitted from species descriptions, In Table 2, we have incorporated the new species into Knight-Jones’ operational “Groups”, and we have re-interpreted collar fusion, lappets and presence of pockets for some taxa.