Monostaechoides providentiae Gil & Ramil 2021, gen. et comb. nov.

Monostaechoides providentiae (Jarvis, 1922) gen. et comb. nov.

Figs 5–6, 7A–B; Table 11

Plumularia providentiae Jarvis, 1922: 347–348, pl. 26 fig. 21.

Antennella quadriaurita – Millard 1966: 492–493. — Calder 1997: 27–29, fig. 6 (not Antennella quadriaurita Ritchie, 1909).

Material examined

SOUTH ATLANTIC OCEAN • 3 colonies, 19–23 mm high, on sponge (1 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-2015-40063, LZM-UV slide R. 576 • 4 colonies, 5–7 mm high (1 colony, growing on algae, 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.; SEAFO-2015-40227, SEAFO-2015-40497, SEAFO-2015-40768, LZM-UV slide R. 581.

Description

Hydrorhiza composed of a cluster of perisarcal tubes covered by a sponge growing on old gorgonian axis. In some cases, isolated hydrocladia are born directly on the hydrorhiza but, in most cases, several monosiphonic primary hydrocladia arise in tufts from a short, polysiphonic axis composed by several entangled stolons protruding from the sponge. The basal part of the primary hydrocladium is composed of one to five internodes separated by straight nodes, provided with a variable number of scattered nematothecae separated from the remainder of hydrocladium by an oblique node. This part is formed by a regular succession of hydrothecate and ahydrothecate internodes, delimited by alternating oblique and straight nodes; hydrothecate internodes with proximally oblique and distally straight nodes; ahydrothecate internodes with a reversed position of nodes (Figs 5E, 6C, 7B). Almost all primary hydrocladia carry lateral ramifications randomly disposed, always originating from their posterior side. In most cases, the subsidiary hydrocladia arise from the distal end of ahydrothecate internodes, just on the back side of the oblique nodes within the heteromerous part of the colony (Fig. 6A); occasionally, some ramifications are found on the basal part of primary hydrocladia (Figs 5F–G, 7A). All subsidiary hydrocladia (i.e., of the second, third and even fourth order) are born on small apophyses and are composed of a basal ahydrothecate internode of varied length carrying between one and four nematothecae (Figs 6A, D, 7A), followed by a regular succession of hydrothecate and ahydrothecate internodes with the same structure as the primary hydrocladia. Usually, all subsidiary hydrocladia originating from the same hydrocladium are directed to the same side in a linear succession, but irregularities also occur.

Hydrothecate internodes with one hydrotheca and five nematothecae: one mesial inferior and two pairs of laterals. Hydrotheca cup-shaped, widening towards rim; adcauline wall adnate for about half its length; abcauline wall straight; hydrothecal rim circular, even and slightly everted. Mesial nematothecae not reaching hydrothecal base. Two pairs of lateral nematothecae; first pair borne on well-developed apophyses adpressed to the hydrothecal wall, and as long as the nematothecae proper, the latter reaching the hydrothecal rim; second pair small, inserted on bases of apophyses (Fig. 5D). Ahydrothecate internodes usually with two frontal nematothecae in a row, although the number may vary between one and three. All nematothecae bithalamic, movable and conical, with adcauline wall of distal chamber scooped.

Colonies monoecious; gonothecae of both sexes found on same hydrocladia, arising from below the hydrothecal bases, just above the mesial nematothecae (Figs 6A–B, 7B). Male gonotheca small, sackshaped, with small and circular aperture located at the rounded top, basal part slightly curved and carrying one nematotheca, and narrowing into a short pedicel composed of one internode. Female gonotheca

pear-shaped, rather curved, with a distal, slightly tilted, circular aperture, closed by lid; basally provided with two nematothecae and narrowing into a two-segmented pedicel.

Variability

In some hydrocladia, the regeneration processes when ahydrothecate internodes are damaged result in two ahydrothecate internodes, each one with one or two nematothecae, between two consecutive hydrothecate internodes. We have also observed a subsidiary hydrocladium originating from the back side of a hydrothecate internode (Fig. 6D), but this type of ramification is exceptional and probably related to regeneration processes.

Remarks

Our material agrees with the main features described by Jarvis (1922) as Plumularia providentiae. In both cases the colonies are ramified, with the subsidiary hydrocladia originating from the back side of a true axis (or hydrocladia) shifted on to one side, but that does not adopt the shape of a scorpioid sympodium. Moreover, the morphology of hydrothecae and the number and arrangement of the nematothecae are also similar.

The main difference is found in the presence, in our colonies, of subsidiary hydrocladia originating from the basal part of some primary hydrocladia; however, this is an occasional feature and not the norm. Moreover, Jarvis (1922) described P. providentiae with homomerously segmented hydrocladia, but in our colonies the segmentation is heteromerous. Nevertheless, the existence of intermediate ahydrothecate internodes is clearly visible only in subsidiary and younger hydrocladia. In older parts of the colony, and mainly in primary hydrocladia, the perisarc of the wall is thick, masking the heteromerous segmentation.

In our opinion, these differences do not justify the description of a new species and, therefore, we identify our material as Monostaechoides providentiae (Jarvis, 1922) gen. et comb. nov.

In addition, the material described from the Vema Seamount by Millard (1966) as Antennella quadriaurita (Ritchie, 1909), with hydrocladia clustered together basally and ramified following the same pattern as our colonies, also belongs to this species.

Colonies found in Bermuda with a similar morphology and with the same ramification pattern were described by Calder (1997) as A. quadriaurita (see Calder 1997: 28, fig. 6a). That material, excluded from A. quadriaurita by Galea & Ferry (2015), is also included here in M. providentiae gen. et comb. nov.

Differences between M. providentiae gen. et comb. nov. and M. bertoti gen. et comb. nov. were discussed by Galea & Ferry (2015), and refer to the ramification pattern, with hydrocladia more or less alternately arranged in M. bertoti gen. et comb. nov., and a different number of nematothecae on both cauline and cladial internodes.

Distribution

This species has been reported from Providence Atoll, the Seychelles (Jarvis 1922, as Plumularia providentiae), Vema Seamount (Millard 1966; Berrisford 1969, both as A. quadriaurita) and Bermuda (Calder 1997, as A. quadriaurita) in depths from 42 to 85 m.