Ophelina cf. abranchiata Stop-Bowitz , 1948

Wiklund, Helena, Neal, Lenka, Glover, Adrian G., Drennan, Regan, Muriel Rabone, & Dahlgren, Thomas G., 2019, Abyssal fauna of polymetallic nodule exploration areas, eastern Clarion-Clipperton Zone, central Pacific Ocean: Annelida: Capitellidae, Opheliidae, Scalibregmatidae, and Travisiidae, ZooKeys 883, pp. 1-82 : 1

Ophelina cf. abranchiata Stop-Bowitz, 1948

General comments on Ophelina abranchiata and similar morphotypes.

Three small, abranchiate morphospecies found in the UKSR material, Ophelina ganae sp. nov., O. cf. abranchiata NHM_1769 and O. cf. abranchiata NHM_2017, are very similar to Ophelina abranchiata Støp-Bowitz, 1948. This species has its type locality as East Greenland, 200 m depth, but has subsequently been reported worldwide, from predominantly deep waters [see references in Kongsrud et al. (2011), Parapar et al. (2011)]. Sene-Silva (2007) proposed Polyophthalmus translucens Hartman, 1960 and Ophelina farallonensis Blake, 2000, both described from deep waters off California, as junior synonyms of O. abranchiata . Blake (2000b) had separated O. farallonensis from O. abranchiata due to presence of long anal tube, but he most likely did not realize that the original description of O. abranchiata by Støp-Bowitz (1948) was based on an incomplete specimen without an anal tube as already suggested by Sene-Silva (2007), Kongsrud et al. (2011) and Parapar et al. (2011). The absence of branchiae also likely led to confusion of O. abranchiata with the much larger (30 mm long, 30 chaetigers) abranchiate species Ophelina nematoides by some workers (e.g. Maciolek and Blake 2006).

Such confused taxonomic history is further complicated by the fact that published ( Neal et al. 2018) and unpublished (Kongsrud pers. comm.) molecular data suggest presence of several species within an O. abranchiata complex. Kongsrud et al. (2011) provided a re-description of type material, together with report on material from North Sea and North Atlantic, but without providing genetic data. One possible, and previously overlooked, character may be the number of chaetigers as Kongsrud et al. (2011) reported an invariable number of 18 chaetigers for O. abranchiata , and considered specimens bearing17 chaetigers from Skagerrak in need of further evaluation, not ascribing these to O. abranchiata . In other reports, variation of 17-19 chaetigers (4-10 mm long) was given by Barroso and Paiva (2013) and Parapar and Moreira (2008) referred to 16-22 segments with detectable annulation, rather than chaetigers. Hartman (1960) reported 18 chaetigers for P. translucens and Blake (2000b) reported 18 or 19 chaetigers (5-7 mm long) for O. farallonensis .

The anal tube, an important feature upon which opheliid species have been differentiated in the past is mostly missing in these morphotypes even where hundreds of specimens are available (Neal pers. obs.). Where the anal tube has been observed ( Blake 2000b; Parapar and Moreira 2008; Kongsrud et al. (2011); Parapar et al. 2011; Barroso and Paiva 2013; Neal pers. obs.) its form appears to be very similar.

Parapar and Moreira (2008) provided the first SEM examination of O. abranchiata morphotypes and reported presence of lateral organs between notopodia and neuropodia for Iberian specimens. Subsequently, examination of Icelandic morphotypes by Parapar et al. (2011) led to the report of presence of lateral organs in all chaetigerous segments as well as in the anterior segments of the body which are achaetigerous and devoid of parapodia, suggesting the presence of three such segments. However, this feature was not observed in SEM examination of bathyal material from Brazil by Barroso and Paiva (2013). Despite this variation, none of these authors suggested lateral organs as a useful taxonomic character and previously Purschke and Hausen (2007) only considered that they might be useful for higher level systematics of annelids. Although the presence and distribution of lateral organs was only investigated in non-type specimens ( Parapar and Moreira 2008; Parapar et al. 2011; Barroso and Paiva 2013), their presence reported is some specimens and absence in others could possibly be result of interspecific variability. However, it is important to stress that these structures are extremely small (around 5 µm) and their observation depends on the quality of the specimens (i.e. in shriveled specimens, this feature may be impossible to observe). Two UKSR specimens were investigated under SEM for lateral organs. Lateral organs were confirmed in both specimens examined. Three lateral organs were associated with three prechaetigerous segments and then one observed between the noto- and neuropodium of each chaetiger. Such distribution is consistent with previous observations ( Parapar and Moreira 2008; Parapar et al. 2011). The pattern which pore openings form in each morphotype appears to vary, but currently we cannot confirm consistency of such observation due to the low number of specimens examined so far.

Clearly, additional morphological characters are needed to distinguish small abranchiate species currently lumped under O. abranchiata , P. translucens and O. farallonensis .