Sertularella gayi ( Lamouroux, 1821 )

Peña Cantero, Álvaro L. & Horton, Tammy, 2017, Benthic hydroids (Cnidaria, Hydrozoa) from bathyal and abyssal depths of the Northeast Atlantic held in the modern Discovery Collections, Zootaxa 4347 (1), pp. 1-30 : 14-16

Sertularella gayi ( Lamouroux, 1821)

( Fig. 6A–E )

Sertularella gayi —Schuchert, 2001: 98–99, fig. 83A–B (synonymy).

Material examined. 9752#1, one stem 48 mm high, on coral, no gonothecae; 13881#1, a strongly polysiphonic stem 200 mm high, basibiont of Antennella secundaria , Filellum sp. and Zygophylax levinseni , no gonothecae; 51217#1, numerous colony fragments up to 70 mm high, with a gonotheca, and a few incipient stems on tube of benthic organism; 54907#3, a few stems up to 32 mm high, on coral, no gonothecae.

Description. Polysiphonic, rigid stems consisting of a main axis and a series of accompanying stolons. Main axis divided by little-marked nodes into smooth internodes arranged in zigzag.

Branching usually regular. Main axis giving rise to pinnae alternately arranged in one plane. Pinnae originating at base of hydrothecae (some colonies with accessory pinnae emerging from accompanying stolons) and arranged in sub-opposite pairs with one hydrotheca between both pinnae and three hydrothecae between pairs of subopposite pinnae (i.e. there are two hydrothecae between those forming branches). Branching may also occur alternately every third hydrothecae (i.e. there are no sub-opposite pairs).

Each internode with a hydrotheca ( Fig. 6A–D ). Free adcauline wall smooth ( Fig. 6B ) or with up to four undulations ( Fig. 6C–D ). Rim provided with four very little-marked cusps.

Gonotheca fusiform with three distal low cusps ( Fig. 6E ).

Measurements. Sertularella gayi f. gayi: Free adcauline wall 380–500, adnate adcauline wall 450–550, adcauline wall 830–1020, abcauline wall 700–790, diameter at aperture 290–340, maximum diameter 400–450. Gonothecae: length 2200, maximum diameter 740. Sertularella gayi f. robusta: Free adcauline wall 520–650, adnate adcauline wall 400–500, adcauline wall 1020–1150, abcauline wall 740–800, diameter at aperture 350–380, maximum diameter 500, diameter at diaphragm 250. Nematocysts: 8 x 2.

Remarks. The polysiphonic condition so characteristic in this species develops quite early during stem development, as it can be seen in incipient stems. We have observed stems with just four hydrothecae that already have an accessory stolon originating at the most basal hydrotheca.

Some colonies have accessory pinnae originating in different planes from accompanying stolons, which gives the colony the appearance of irregular branching. We have been able to find out the actual structure of those stems with irregularly arranged pinnae. The material from Stn 13881#1 and Stn 54907#3 have unbranched pinnae (only two with a secondary one) in several planes, as typically described for S. gayi f. robusta (see Ramil & Vervoort 1992: 225). A careful examination of this material shows that most pinnae originate from the accompanying stolons. However, it is also possible to see, hidden by those stolons and the accessory pinnae originating from them, a main axis which has a regular, alternate branching in one plane every third hydrothecae (i.e. there are two hydrothecae between those forming branches). In these colonies, when a pinna of the alternate pattern develops, a stolon is also formed. This runs downwards on the side of the stem and forms an accessory pinna at a constant distance, more or less perpendicular to the plane formed by the alternate branching of the main stem, thus resulting in the irregular branching.

According to Ramil & Vervoort (1992: 222) “The degree of annulation of the (free) adcauline hydrothecal border is quite variable, being scarcely noticeable in some hydrothecae, while others (at the same colony) may be almost fully annulated”. We found a similar variation in the material examined ( Fig. 6A–D ).

Ramil & Vervoort (1992: 225) kept separated S gayi f. gayi and S. gayi f. robusta indicating that there are differences in the ramification pattern, size of hydrotheca and gonothecal aperture. According to these authors, the pinnae are disposed in the same plane in S. gayi f. gayi , while they are on all sides of the axis in S. gayi f. robusta. They also indicated that in the younger parts of the colonies of both forms the branches are pinnately arranged generally in the same plane, but that there are three hydrothecae between two consecutive branches in S. gayi f. robusta, whereas pinnae occur in pairs in S. gayi f. gayi . As for the size of the hydrotheca, the authors indicated that it is larger in S. gayi f. robusta. Finally, they pointed out that the gonothecal aperture has three low cusps in S. gayi f. robusta, but it is bilabiate in S. gayi f. gayi .

According to Schuchert (2001: 99) ‘as the morphology of both forms intergrade, they are more probably variants only’. The study carried out here does not justify them either. As indicated above, colonies with irregular branching (traditionally considered to belong to S. gayi f. robusta) actually have a main axis that gives rise to pinnae alternately arranged in one plane. The “irregular” branching is consequence of the accessory pinnae originating from the accompanying stolons. We also found, in material from Stn 54907#3, two stems with irregular branching, therefore assignable to S. gayi f. robusta, that have a pair of sub-opposite pinnae, at the end of one of the stem in one case, and at the base in the other. In material from Stn 51217#1, there are both types of branching, namely with or without sub-opposite pairs, even in the same fragment. In this material, there are also irregularities in the branching pattern, such as several consecutive pinnae directed to the same side and with five hydrothecae in between.

The single gonotheca observed here ( Fig. 6E ) was found in the material from Stn 51217#1. This material is fragmentary, but branching is clearly in one plane and, as indicated above, pinnae are arranged either alternately every third hydrotheca or, more frequently, in sub-opposite pairs. The branching in one plane and the presence of sub-opposite pairs would indicate that we are dealing with S. gayi f. gayi . The gonotheca, however, is morphologically closer to those found in S. gayi f. robusta, having three distal blunt projections, contrary to the typical two lip-like processes of characteristic shape (bilabiate) described in S. gayi f. gayi . On the other hand, Vervoort & Watson (2003) considered that the development and number of apertural cusps on the gonothecae have no real taxonomic value.

Finally, the size of the hydrotheca, apparently larger in S. gayi f. robusta, is highly variable and the differences between both forms do not seem large enough to justify keeping them.

Ecology and distribution. This species is found at depths between 10 (Medel et al. 1991) and 1200 m ( Ramil & Vervoort 1992); the present material comes from the Porcupine Seabight at depths from 150 to 1060 m. Widely distributed in the Atlantic Ocean, but also recorded from the northern Pacific Ocean and New Zealand waters (cf. Vervoort & Watson 2003).

Figures

FIGURE 6. Sertularella gayi (Lamouroux, 1821): A – D: hydrothecae showing variation in hydrothecal shape; E, distal part of gonotheca. Diphasia margareta Hassall, 1841: F, hydrotheca and origin of branch; G, distal part of hydrotheca showing hydrothecal aperture; H, characteristic perisarc thickening (arrow). Scale bars: 200 µm (A – F), 100 µm (G – H).