Sabellomma, Nogueira, João Miguel De Matos, Fitzhugh, Kirk & Rossi, Maíra Cappellani Silva, 2010

Nogueira, João Miguel De Matos, Fitzhugh, Kirk & Rossi, Maíra Cappellani Silva, 2010, A new genus and new species of fan worms (Polychaeta: Sabellidae) from Atlantic and Pacific Oceans — the formal treatment of taxon names as explanatory hypotheses, Zootaxa 2603, pp. 1-52 : 4-5

Sabellomma gen. nov.

Type species. Parasabella minuta Treadwell, 1941 .

Definition. A phylogenetic hypothesis, accounting for presence of unpaired, simple eyespots along entire lengths of outer radiole margins. Unpaired, simple eyespots originated by an unspecified mechanism(s) in a reproductively isolated population of individuals with no eyespots, subsequent to which the character became fixed in the population by an unspecified mechanism(s), followed by population splitting events, leading to individuals to which specific hypotheses S. minuta gen. nov., comb. nov., S. harrisae gen. nov., sp. nov., and S. collinae gen. nov., sp. nov., also apply.

Sabellomma gen. nov., is an ad hoc hypothesis (homoplasy), as the presence of unpaired, simple eyespots distributed along the entire lengths of radioles is also an autapomorphy for the specific-level hypothesis, Sabella cerasina Grube, 1870 (see The Phylogenetic Hypothesis, Sabellomma , below). An additional synapomorphy, also an instance of homoplasy, is subsumed by this hypothesis in one transformation series – interramal eyespots – which is also a synapomorphy for the clade containing Stylomma , Bispira , Sabella , Sabellastarte , Branchiomma , Pseudobranchiomma (see The Phylogenetic Hypothesis, Sabellomma , below), and also present in Myxicola .

Collective characteristics of individuals to which Sabellomma gen. nov., applies. Small- to mediumsized individuals, with 4–9 pairs of radioles; most specimens with 4–5 thoracic chaetigers. Radioles without palmate membrane; outer margins with numerous, irregularly distributed, lensed eyespots, usually more numerous in areas with pigmented bands across radioles. Dorsal lips with radiolar appendages, associated radiolar skeleton absent; pinnular appendages present or absent; distally rounded ventral lips, continuing ventrally as parallel lamellae with ventral sacs, and lamellae terminating between collar ventral lappets. Dorsal-most margins of branchial lobes with thickened ridges. Posterior peristomial ring collar only covering branchial lobes; dorsal margins well separated from faecal groove; one pair of triangular, non-overlapping ventral lappets. Interramal, simple ocelli in all chaetigers, especially evident in abdomen. Collar chaetae elongate, narrowly hooded, arranged in two oblique rows, ventral row chaetae shorter than dorsal; remaining thoracic notopodia with superior arc of elongate, narrowly hooded chaetae, inferior chaetae as two rows of mucronate paleae. Thoracic neuropodia with avicular uncini about as long as high, crest with 4–5 irregular rows of teeth, extending for less than one-half extension of main fang; companion chaetae with rounded denticulate heads and long, drop-shaped, slightly asymmetrical membranes. Abdominal notopodia with avicular uncini similar to thoracic or with shorter handles; abdominal neuropodia with two rows of elongate, narrowly hooded chaetae, anterior row of chaetae shorter than posterior. Pygidium triangular, distally blunt; eyespots present.

Remarks. While the synapomorphy, albeit homoplasious (see Definition above) to which Sabellomma refers is the presence of unpaired, simple eyespots along the outer margins of radioles, and possibly also interramal eyespots (see The Phylogenetic Hypothesis, Sabellomma , below), it is conceivable that the thorax composed of less than eight chaetigers might also be accounted for in this hypothesis. The latter condition is, however, also known among individuals to which Branchiomma and Pseudobranchiomma also apply, but causal accounts of this condition have not yet been presented among Sabellinae . The presence of thickened ridges at the dorsal margins of branchial lobes might be a synapomorphy of Sabellomma , but this character is only visible using SEM ( Figs 2 C, F–G; 8A, D; 12D, I) and we have not been able to confirm these observations with light microscopy.

The radiolar eyespots are lensed units, irregularly distributed along each outer, lateral radiolar margin, usually more numerous in areas with pigmented bands across radioles. Among individuals to which the three specific hypotheses will be applied below, no significant differences in the number and distribution of eyespots were observed.

In his revision of Demonax, Perkins (1984) described individuals of D. microphthalmus (Verrill, 1873) as having radiolar eyespots arranged similar to that found in Sabellomma . However, all specimens of D. microphthalmus we examined, including material from close to the type-locality, have pigment spots along radioles, but no indication these are eyespots. If, on the other hand, members of D. microphthalmus do have eyespots, this condition would be an autapomorphy, albeit homoplasious, accounted for by the specific hypothesis. Giangrande (1994: 230, fig. 1) described members of D. tommasi as having “large lenticular eyes showing an irregular arrangement”. In the absence of phylogenetic hypotheses for Demonax , it is unknown if radiolar eyes is plesiomorphic for the genus.

In addition to the presence of interramal eyespots among members of Myxicola , Sabellomma , and the clade comprising Stylomma , Bispira , Sabella , Sabellastarte , Branchiomma , and Pseudobranchiomma , Tovar- Hernandez and Salazar-Vallejo (2008: Figs 1 E-G, 2C-D; see also Capa and Murray [2009]) described interramal eyespots among members of Megalomma carunculata , and M. Capa (pers. comm.) has observed this character among members of an undescribed Demonax species. Based on the phylogenetic hypotheses inferred by Capa and Murray (2009: fig. 13), interramal eyespots is not a plesiomorphic condition for Megalomma . As with radiolar eyes, noted earlier, there is no way at present to know if interramal eyespots is plesiomorphic for Demonax .

Figures

FIGURE 1. Sabellomma minuta gen. nov., comb. nov. A: entire specimen, left ventro-lateral view; B – D: entire specimen, ventral, ventral and left lateral views, respectively; E, G: anterior end, ventral views; F: anterior end, right lateral view; H: posterior end, right ventro-lateral view. A, C – D, F – H from spec. 18 (MZUSP 1050), B, E from holotype (AMNH 2894). Scale bars: A – D = 2 mm; E – G = 0.7 mm; H = 0.5 mm.

FIGURE 2. Sabellomma minuta gen. nov., comb. nov. A: total worm, right lateral view; B: anterior end, left lateral view; C: anterior end, dorsal view; D: anterior end, left lateral view; E: dorsal lip (dissected crown); F – G: details of base of crown; H: anterior end, right lateral view; I – J: details of anterior end, ventral view. br = branchial ridge; ci = ciliated patch; dl = dorsal lip; pa = pinnular appendage; pl = parallel lamellae; ra = radiolar appendage; vs = ventral sacs. Photos A – D, F – J from spec. 10 (LACM-AHF 2400); photo E from spec. 20 (LACM-AHF 2403). Scale bars: A – B = 0.5 mm; C = 0.3 mm; D, H = 0.4 mm; E, I = 75 µm; F = 0.1 mm; G = 40 µm; J = 80 µm.