Monotheca bergstadi Gil & Ramil 2021, sp. nov.

Monotheca bergstadi sp. nov.

urn:lsid:zoobank.org:act: 3BFDE359-6A4B-4910-9AB5-226193B6584E

Figs 7C–D, 8; Table 12

Plumularia pulchella – Millard 1957: 232; 1962: 300; 1966: 493; 1975: 398–399, fig. 125c–d [not Monotheca pulchella (Bale, 1882)].

Diagnosis

Colonies monosiphonic, mostly unbranched. Hydrocaulus divided into internodes by straight nodes, each internode bearing one apophysis and three nematothecae. Hydrocladia composed of two internodes: one athecate proximal without nematothecae, with two internal perisarcal rings, and one thecate distal bearing a hydrotheca and three nematothecae. Hydrotheca deep campanulate and abcauline wall concave. Mesial inferior nematotheca long and lateral nematothecae short. Gonothecae arising frontally, large, barrel-shaped and smooth-walled.

Etymology

The specific name bergstadi honours Dr. Odd Aksel Bergstad, Institute of Marine Research (IMR), Bergen, Norway, leader of the SEAFO 2015 cruise, in recognition of his wide contribution to deep-sea research.

Material examined

Holotype SOUTH ATLANTIC OCEAN • colony, 10 mm high, with gonothecae; Vema Seamount, stn BT5; 31°37′16″–31°36′58″ S, 8°22′37″–8°23′06″ E; 71–94 m depth; 31 Jan. 2015; SEAFO-2015 leg.; LZM- UV slide R. 582; SAMC-A092083.

Paratypes SOUTH ATLANTIC OCEAN • 7 colonies, 8–13 mm high (2 growing on algae), 3 colonies, with gonothecae; same collection data as for holotype; SEAFO-2015-40042, SEAFO-2015-40167, SEAFO-2015-40572, SEAFO-2015-40768.

Additional material

SOUTH ATLANTIC OCEAN • 1 colony, 10 mm high growing on algae, with gonothecae; Vema Seamount, stn PT4; 31°39′43″–31°38′10″ S, 8°22′37″–8°23′42″ E; 50–108 m depth; 31 Jan. 2015; SEAFO-2015 leg.; SEAFO-2015-40137 • 1 colony, growing on bryozoan, without gonothecae; Vema Seamount, stn GRAB9C; 31°36′09″ S, 8°22′29″ E; 84 m depth; 31 Jan. 2015; SEAFO-2015 leg.; SEAFO-2015-40080 • 1 colony, without gonothecae; Vema Seamount, stn GRAB12B; 31°37′56″ S, 8°23′12″ E; 89 m depth; 1 Feb. 2015; SEAFO-2015 leg.; SEAFO-2015-40245.

Description

Colonies composed of a reticulate hydrorhiza growing on algae and a bryozoan, supporting erect, monosiphonic and mostly unbranched, occasionally branched once, hydrocauli (Fig. 7C). Stem regularly divided into internodes by straight nodes, each bearing a latero-distal apophysis and three nematothecae: two axillar, flanking the apophysis and one on the basal half on the opposite side. Apophyses alternately directed left and right and disposed almost in the same plane (Fig. 8A). Hydrocladia inserted on apophyses and composed of two internodes: one athecate basal and one thecate distal. Basal internode short, without nematothecae and with two internal perisarcal rings, one basal and the other distal. Thecate internode slightly longer than athecate, with one hydrotheca and three nematothecae: one mesial inferior and a pair of laterals (Figs 7D, 8A).

Hydrotheca deep campanulate, adcauline wall fully adnate to internode, abcauline wall concave, margin straight, smooth and slightly flared. Mesial nematotheca long, reaching or even surpassing the middle of the abcauline wall of hydrotheca. Lateral nematothecae comparatively shorter and placed on small, yet distinct apophyses reaching the hydrothecal rim (Fig. 7D). All nematothecae two-chambered and movable; rim of upper chamber even throughout.

Gonothecae arising frontally from apophyses of hydrocladia; large, barrel-shaped, smooth-walled, truncated apically; aperture wide and circular; operculum not observed (Figs 7C, 8B–C).

Remarks

The validity of the genus Monotheca or its synonymy with Plumularia Lamarck, 1816 has been widely discussed during the last few years in the literature. Watson (2011) and Calder (2019) indicated that, despite the traditional interpretation of the genus, Monotheca might involve a polyphyletic group. Indeed, some molecular analyses (Leclère et al. 2007, 2009; Moura et al. 2008; Maronna et al. 2016) revealed that the type species of both genera, Monotheca margaretta Nutting, 1900 and Plumularia setacea (Linnaeus, 1758), respectively, did not cluster together. In addition, the latest molecular study of the superfamily Plumularioidea (Moura et al. 2018) supports the validity of Monotheca. Taking into account that the new species is closely allied to M. margaretta, we have decided to assign it to this genus, under the name Monotheca bergstadi sp. nov.

Our material is closely related to four nominal species of Monotheca, namely M. margaretta Nutting, 1900, M. pulchella (Bale, 1882), M. flexuosa (Bale, 1894) and M. femina (García, Aguirre & González, 1978). The latter is currently accepted as a junior synonym of M. margaretta (Calder 1977; Ansín Agís et al. 2001; Schuchert 2020; as Plumularia margaretta), and we agree.

The remaining valid species are easily recognizable by the morphology of their gonothecae. In M. margaretta, they are barrel-shaped, with well-developed transverse ridges and a broad, apical aperture (Calder 1997; Ansín Agís et al. 2001); in M. pulchella the gonothecae are ovate, with an obliquely truncate aperture with a submarginal row of large, internal teeth surrounded by large, internal teeth (Bale 1882; Ralph 1961; Watson 1973, 2011); finally, in M. flexuosa, the gonothecae are fusiform, with

slightly undulated walls and a rather small, apical aperture produced into a neck of variable height (Bale 1894; Watson 2011).

In addition, M. margaretta is an amphi-Atlantic species, whereas M. pulchella and M. flexuosa are predominately Indo-Pacific.

Nevertheless, M. pulchella was reported several times from the Atlantic Ocean and Mediterranean Sea (for a review, see Calder 1997 and Ansín Agís et al. 2001), and the actual status of these records have been subjected to different interpretations in the literature. The records from the Northeast Atlantic and the Mediterranean Sea, all with annulated and barrel-shaped gonothecae, were included in M. margaretta by Calder (1997) and Ansín Agís et al. (2001), and we agree with this conclusion, despite the fact that Watson (2011) considers that they are conspecific with M. flexuosa. The records of M. pulchella from the Mediterranean (Bouillon et al. 2004) are based on the material studied by García Corrales et al. (1978, as Plumularia femina) and Medel & Vervoort (1995), and belong to M. margaretta, despite some figures (Bouillon et al. 2004: fig. 92h, j) being based on Millard (1975: fig. 125c–d) and representing a different species (see below). The records of M. pulchella from the Argentinian coast (Blanco 1973, 1994; Genzano 1990, 1994; all as Plumularia pulchela), were provisionally placed under M. margaretta by Ansín Agís et al. (2001) because the involved colonies were sterile.

The morphology of the colonies collected at the Vema Seamount studied in this report, with respect to their tropho- and gonosome, completely coincides with those described by Millard (1975) as M. pulchella from South Africa and the Vema Seamount. This material, characterized by its barrel-shaped, smooth-walled gonothecae, is clearly distinct from the current concept of M. pulchella, and also from other previously discussed species.

In fact, Watson (2011) excluded the South African records from the synonymy of M. pulchella, but did not assign them to any known species of Monotheca. Consequently, we consider that this material represents a new species, for which we propose the name M. bergstadi sp. nov.

Distribution

Monotheca bergstadi sp. nov. has previously been reported from Vema Seamount (Millard 1966, as Plumularia pulchella) and South Africa, from the west coast of Cape Peninsula to Natal (Millard 1957, 1962, 1975; all as P. pulchella). Its bathymetric distribution extends from the littoral zone to a depth of 100 m (Millard 1975, as P. pulchella).