Monostaechoides, Gil & Ramil, 2021

Gil, Marta & Ramil, Fran, 2021, Hydroids (Cnidaria, Hydrozoa) from the Vema and Valdivia seamounts (SE Atlantic), European Journal of Taxonomy 758, pp. 49-96 : 71-72

Genus Monostaechoides gen. nov.

urn:lsid:zoobank.org:act:E7E0F40E-8573-4EDF-98F5-D3CD4EAA3ED2

Type species

Plumularia providentiae Jarvis, 1922 , designated herein.

Additional species

Monostaechoides bertoti ( Galea & Ferry, 2015) gen. et comb. nov. (= Monostaechas bertoti Galea & Ferry, 2015 ).

Diagnosis

Halopteridids with monosiphonic hydrocladia arising directly from creeping stolons. Hydrocladia branched, with several cladia originating dorsally from the distal parts of its ahydrothecate internodes. All cladia directed towards the same side or arranged either alternately or irregularly left and right along the stem. Branches of second and third order frequent in, at least, one species. Hydrothecate internodes with one hydrotheca, two pairs of lateral nematothecae and one mesial inferior nematotheca. Ahydrothecate internodes with a variable number of nematothecae. Hydrotheca partly adnate to its corresponding internode, cup-shaped, with untoothed rim. All nematothecae conical, bithalamic and movable. Gonothecae provided with nematothecae on the basal part.

Etymology

The generic name Monostaechoides is derived from a combination of the generic name Monostaechas Allman, 1877 , and the latinized form of the Greek word-forming element ‘- eidés ’, meaning ‘like, resembling’ and referring to the affinities of the new taxon with the genus Monostaechas . The gender of the name is masculine.

Remarks

The presence of hydrothecae on the hydrocaulus is the main defining character of the family Halopterididae Millard, 1962 ( Millard 1962, 1975; Schuchert 1997), and the generic limits within the family are largely based on the shape of the colonies and their ramification patterns ( Schuchert 1997).

The new genus described herein is characterized by having monosiphonic stems or primary hydrocladia arising from hydrorhiza and supporting irregularly pinnate or unilaterally-arranged secondary hydrocladia that, in turn, can originate hydrocladia of second and third order, in at least the type species. Another distinctive feature is the origin of the subsidiary hydrocladia from the postero-distal parts of ahydrothecate internodes, on the backside of an oblique distal node.

The general habit of the colonies, with single monosiphonic stems carrying laterally-placed hydrocladia, resembles those of Halopteris Allman, 1877 , Monostaechas Allman, 1877 and the recently described Thamnopteros Galea, 2020 .

Resemblances with Halopteris are found in the ramified nature of the colonies belonging to both genera, but in Halopteris the hydrocladia are routinely arranged in either alternate or opposite pairs, and originate from the hydrothecate internodes of the stem, laterally to the hydrothecae. Thamnopteros builds polysiphonic colonies giving rise to monosiphonic branchlets bearing pinnate hydrocladia with the same origin as in Halopteris ( Galea & Maggioni 2020) .

The new genus shows more affinities with Monostaechas Allman, 1877 in both the origin of subsidiary hydrocladia on the postero-distal part of ahydrothecate internodes, just behind the distal oblique node, and the tendency to a unilateral disposition of subsidiary hydrocladia. Nevertheless, in Monostaechas the ramification pattern is a helicoid or scorpioid sympodium, in which each subsidiary hydrocladium originates from the postero-distal part of the first ahydrothecate internode of the previous hydrocladium ( Billard 1913; Millard 1975; Schuchert 1997), resulting in a false axis composed of the basal parts of successive hydrocladia ( Billard 1913; Millard 1975). In Monostaechoides gen. nov., there is a ‘true axis’ represented by a stem or primary hydrocladium bearing several secondary hydrocladia irregularly disposed along the same axis. This branching pattern is clearly different from that displayed by Monostaechas , supporting the creation of a new genus.

The colonies of Monostaechas fisheri Nutting, 1906 , recently redescribed by Galea & Maggioni (2020), show another ramification pattern, different from that met with in Monostaechoides gen. nov. In this case, the stem is devoid of hydrothecae and the lateral ramification builds a true sympodium (see Billard 1913: fig. 7).

The same type of ramification found in Monostaechoides gen. nov. was also described in specimens of Antennella secundaria (Gmelin, 1791) collected from Indonesia ( Billard 1913: 8, pl. 1 figs 2–3), the Seychelles (Millard & Bouillon 1973: 78) and South Africa (Millard 1975: 334), suggesting the existence of other undescribed species within this genus. Both Billard (1913) and Millard (1975) pointed out that, in these colonies, the main axis is formed by the first hydrocladium, and does not originate from the basal part of successive hydrocladia, excluding these materials from Monostaechas . Ramified colonies assigned to A. secundaria were also described by Vervoort & Vasseur (1977: 66, fig. 28b), Ryland & Gibbons (1991: 526, fig. 1a) and Calder (1997: 30, fig. 7a), but, in all cases, the ramification fits well with a sympodial pattern and was clearly different from that in Monostaechoides gen. nov.