Tessaradoma boreale Busk 1860

Tessaradoma boreale (Busk, 1860)

(Figs 84–90, Table 15)

Onchopora borealis Busk, 1860: 213, pl. 28, figs 6–7.

Tessaradoma boreale: Hayward 1979: 60 (listed); Hayward & Ryland 1999: 182, figs 68C, D, 69.? Tessaradoma boreale: Winston 2005: 55, figs 146–154.

Material examined. MNCN 25.03/3958, 3959, locality V01; MNCN 25.03/3960, locality DR02; MNCN 25.03/ 3961, locality DR01; MNCN 25.03/3962, locality V01; MNHN IB- 2013-628, locality DW117; MNHN IB- 2013- 629, locality DW106; OLL 2015/909, 910, 924 a, b, c, locality DW117; OLL 2015/911, locality V01; NHMUK 1899.7.1.2611, Shetland, the Kerries, Barlee, Busk Coll., type; NHMUK 1899.7.1.2614, Shetland, type, Barlee, Busk Coll.

Description. Colony erect, rigid, dichotomously branching, arising from a short biserial encrusting part. Encrusting zooids spindle-shaped, arranged in two alternating series as branching runners; zooidal morphology similar to zooids in erect colony part but with less-conspicuous areolar pores, spiramen positioned closer to peristome, slightly more rugose peristomial surface and round (not oval) peristomial aperture.

Erect branches cylindrical, slender, formed by 2 alternating back-to-back pairs of autozooids; these elongaterectangular to hexagonal, separated by indistinct sutures soon obscured by secondary calcification; frontal shield slightly convex, abruptly rising around spiramen and peristome, surface with longitudinally aligned wrinkles, imperforate except for spiramen and a single row of numerous round and relatively large marginal areolar pores that become slit-like and often closed during ontogenetic calcification; round spiramen on summit of short but prominent tube, situated proximal to and some distance from peristome. Primary orifice rounded or transversely Dshaped, obscured by long, tubular peristome with wrinkled or tuberculate surface, distinctly projecting from frontal plane at angle of about 110–120, thus directed somewhat distally; peristomial aperture round or more commonly transversely oval with distal and proximal edges occasionally more elevated than lateral ones. During ontogeny, the ooecium, and to a lesser extent the spiramen and peristome, become increasingly immersed in thickening frontal shield calcification.

Ovicell peristomial (ooecium incorporated into distal part of peristome), formed by distal zooid. Visible part of ooecium globular, broader than long; ectooecium membranous, surface of calcified endooecium like that of zooidal frontal shield.

Avicularia adventitious, small, paired, oval, substituting for marginal pores at same level as, or slightly proximal to, spiramen; cystid slightly raised at first and usually somewhat oblique to zooidal surface, later immersed in secondary calcification; additional avicularia may be developed anywhere on frontal surface during Ontogeny; rostrum short, semi-elliptical, directed proximally to laterally, distolateral margin slightly raised and somewhat serrated; crossbar complete, without columella, palatal foramen semi-elliptical, postmandibular opesia distinctly smaller.

Ancestrula elongate-oval, highly convex, smooth distal gymnocystal margins narrow and low, proximolateral gymnocystal walls extensive and steeply sloping, with narrow V-shaped central area composed of fused and nonarticulated mural spines (costae) that are extremely broad, short-branched and horizontally aligned, the area between spine bases filled by growth of tips of neighbouring costae; 2 distalmost costae raised, with flattened or cyclindrical branch tips fusing in centre, proximally framing a prominent and slightly bent peristome with nodular interior-walled surface as in autozooids; suborbicular peristomial aperture flared, primary orifice immersed, with calcified distolateral rim. Sole daughter zooid from ancestrula budded distally.

Remarks. Tessaradoma boreale is a widely reported species from the outer shelf and slopes of the North and South Atlantic from the Arctic to the tropics, from depths of 50 to 3700 m, respectively (Cheetham 1972; Hayward & Ryland 1999, p. 182). However, when describing specimens from the Caribbean and noticing certain differences, Winston (2005, p. 56) suggested that T. boreale is a species complex and in need of revision. Morphological differences between populations from different regions are, nevertheless, extremely subtle, which would render morphological redefinition of species difficult. Intracolonial variability adds further to this problem as secondary calcification during ontogeny greatly alters surface morphology and disguises some of the characters in proximal colony parts.

The description given above is based on Galicia Bank specimens, which differ from the type of T. boreale in some quantitative characters. Part of the type material (NHMUK 1899.7.1.2611 and NHMUK 1899.7.1.2614) was photographed using SEM for purposes of comparison. Thorough analysis and redescription of the type specimens is, however, beyond the scope of this paper. In our specimens areolar pores are not as conspicuous, and the peristome, spiramen and avicularia are slightly larger/longer than in types of T. boreale. Despite these differences we have decided to assign the Galicia Bank specimens to T. boreale, inasmuch as 1) the degree of intraspecific variability in T. boreale is unknown, and 2) because the species has been recorded more or less continuously from Shetland, its type locality, along the European continental slope to northern Iberia (Hayward 1978, 1979; Hayward & Ryland 1999; Reverter-Gil & Fernández-Pulpeiro 2001; Reverter-Gil et al. 2014). A thorough redescription of the type material, in conjunction with a genetic analysis, is certainly necessary.

Cheetham’s (1972) work on the depth distribution of this species suggests that T. boreale is dependent on relatively cool temperatures, and therefore occurs in increasingly deeper waters towards the equator. Nevertheless, the depth distribution on Galicia Bank is deeper than Cheetham’s estimate for this latitude (c. 120 m at 42º N). This deviation could be related to higher temperatures around Galicia Bank, which is under the influence of Mediterranean Outflow Water, which has a temperature of 10.5–11º C (Cartes et al. 2014). This difference may possibly stem from the fact that Cheetham's work was based on more than one species.

The ancestrula of T. boreale is very striking, and is here described for the first time (given that the Galicia Bank population is indeed conspecific). To our knowledge, the only previous record of an ancestrula in this genus was given by Hayward (1981, 48, figs 25C, D) when describing Tessaradoma circella Hayward & Cook, 1979 from South Africa. It was reported as tatiform with a series of branched marginal spines overarching an opesia that occupies the entire frontal surface, with the branches fusing along the midline of the zooid. In fact, it differs significantly from a typical tatiform ancestrula, which has simple, straight, cylindrical spines surrounding an oval opesia. Although not described, Hayward's (1981, fig. 25C) drawing of the ancestrula shows the development of a peristome that is either formed by the distalmost pair of flattened and branching spines or by a vertical extension of the gymnocystal, distolateral, vertical walls, which is not evident from the drawing.

The ancestrula of T. boreale from Galicia Bank is similar in that the few mural spines are flattened and branching. In our specimens, however, they are positioned horizontally, are extremely short, tightly fused in the middle, and are reminiscent of cribrimorph costae. The frontal membrane is thus entirely overarched by the extremely well-developed proximolateral gymnocyst and the central costate frontal shield. A gymnocystal peristome, as present in T. circella, does not exist in the Galicia Bank specimens. Instead, there is a long tubular peristome made of cryptocystal-like fabric identical to those of later autozooids. The combination of a gymnocystal (‘spinocystal’) and an interior-walled frontal shield seems to be a specific and unique character, and can be interpreted as one of the complex morphologies characterising the evolution of the umbonulomorph frontal shield (Gordon & Voigt 1996).

Tessaradoma boreale was recorded from 21 stations over the entire depth range sampled (between 675 and 1697 m), encrusting mainly corals but also rocks and shells.