Hincksina sceletos ( Busk, 1858 )

Berning, Björn, Spencer Jones, Mary E. & Vieira, Leandro M., 2021, Revision of the European species of the genus Hincksina Norman, 1903 (Bryozoa, Cheilostomatida, Flustridae), Zootaxa 5081 (3), pp. 333-352 : 340-342

Hincksina sceletos ( Busk, 1858)

( Fig. 3 )

Membranipora sceletos Busk, 1858: 262 , pl. 60, fig. 3; Hincks 1880b: 73; Waters 1898: 677, pl. 49, fig. 2–6.

Membraniporella sceletos (Busk) : Norman 1909: 289, pl. 36, fig. 8.

part Hincksina flustroides (Hincks) : Norman 1909: 286.

Hincksina sceletos (Busk) : Reverter-Gil et al. 2012: fig. 3C–E.

not Hincksina sceletos (Busk) : Harmelin & d’Hondt 1992: 32, pl. 1, fig. A–C.

Material examined. Lectotype (here designated): NHMUK 1899.7.1.1144, Busk coll., one colony fragment without ovicells, free of a substrate, mounted on slide, Madeira (no further information on depth and location provided) . Non-type material examined using SEM: NHMUK 1908.3.23.1, Norman coll., two colonies on bivalve shells, on slide, Madeira (no further information on depth and location provided); NHMUK 2016.6.9.1 (formerly part of 1911.10.1.346), previously identified as Hincksina flustroides, Norman coll., one abraded colony on bivalve shell, Madeira (no further information on depth and location provided); NHMUK 2016.6.9.4 (formerly part of 1911.10.1.662), Norman coll., one colony on mytilid fragment, Madeira (no further information on depth and location provided) . Non-type material examined optically: NHMUK 1911.10.1.662, Norman coll., several colonies on bivalve shells, Madeira (no further information on depth and location provided) .

Description. Colony encrusting, unilaminar, multiserial ( Fig. 3A, F ). Autozooids oval to rectangular (ZL 495±53, 399–597, 20; ZW 266±24, 237–322, 20), arranged in irregular series, zooids separated by shallow grooves ( Fig. 3B ), whitish translucent when dried. Vertical walls with two or more uniporous pore plates per neighbouring zooid. Development of zooecial gymnocyst in zone of astogenetic repetition restricted to proximolateral corner(s), cryptocyst practically absent, opesia therefore almost as long as zooecium (OpL 446±54, 325–543, 20; OpW 197±19, 159–235, 20) ( Fig. 3A ), marginal walls relatively thin and zooids densely packed. Opercular region in adult zooids laterally framed by a pair of flattened fan-shaped or palmicorn oral spines ( Fig. 3B, G ), aligned oblique to zooidal midline and converging distally, ramifying into 2–5 extremely short branches, terminations straight with uncalcified fissures. Frontal membrane in mature zooids overarched by 10–14 stout vertically compressed mural spines of falciform shape ( Fig. 3A, E, G ), initially growing vertically, abruptly bending inwards at a 90° angle and becoming vertically broader towards the zooid interior (i.e. the thickness remaining the same when viewed from above) while vertically thinning again towards midline, with the tip slightly upraised; horizontal and vertical thickness variable between zooids, lateral spines exceeding beyond the zooidal mid line, interdigitating or occasionally colliding with the opposite spines without fusing, proximal spines shorter ( Fig. 3B, G ); both oral and mural spines in late astogenetic zooids apparently unjointed.

Ovicell endozooidal, positioned in distal avicularium, ooecium continuous with the gymnocyst of the avicularian cystid forming a short but broad hemispherical cap (OvL 80±13, 52–102, 20; OvW 119±18, 78–148, 20), proximal margin raised to produce a central peak ( Fig. 3D, F ).

Avicularia interzooidal, distal to most autozooids ( Fig. 3A, F ), usually pointing in distal directions but often laterally or even proximally, distinctly longer than wide, widest proximal to condyles (AL 213±21, 169–245, 20; AW 119±12, 102–139, 20); rostrum semielliptical or with straight margins narrowing slightly distally ( Fig. 3C ), at an acute angle to colony surface, mandible vaguely bell-shaped (ML 117±16, 81–143, 20; MW 96±14, 73–127, 20), hinged on a pair of short triangular condyles delimiting an approximately semicircular proximal area, entire opesia oval, framed by an immersed calcified shelf slightly thinning laterally and increasing in width distally, interior mandible marked by a large subcircular to oval lucida ( Fig. 3C ); gymnocyst reduced to cystid corners.

Ancestrula tatiform; periancestrular zooids with slender cylindrical spines, the pair of oral spines vertically oriented, 8–12 mural spines arching over the frontal membrane, fully developed mural spines not present before fifth- to eighth-generation zooids ( Fig. 3H ).

Remarks. Busk (1858) did not indicate the number of specimens he examined. The only specimen present in the Busk collection at the NHMUK today is marked as “type”, which is here designated as lectotype. Additional material was deemed to be present in the Waters collection at Manchester Museum. According to Reverter-Gil et al. (2012, p. 163), however, these specimens seem to have been mislaid.

In contrast to the other Hincksina species along the European continental shelf, which have simple cylindrical or flattened mural spines, H. sceletos has laterally compressed (i.e. vertically broadened), falciform mural spines in mature zooids. As in the species from the Azores (see below) the pair of oral spines in auto- and ovicellate zooids is also compressed, fan-shaped and branching. The only other species with flattened oral spines is Hincksina calpensis from the Strait of Gibraltar and southern France, although in this species the flattened spines only occur in ovicellate zooids, are smaller, and bifurcate only once ( Harmelin & d’Hondt 1992; Reverter-Gil et al. 2012, fig. 3C–E).

Moreover, it seems that the oral and mural spines in H. sceletos (as well as those of H. synchysia n. sp., Hincksina neptuni and Hincksina alice , see below) are unjointed. It may be that the spines in H. sceletos have a predetermined breaking point at the base as broken spines hang on to the lateral walls, suggesting that there is sclerotised interior tissue connecting the upper part of the spine with its base. The point of break is irregular, however, and both the spine and the base are thickly calcified. Also, broken mural spines are frequently observed, which would be difficult to explain if they were jointed.

Shape and thickness of the mural spines of H. sceletos depend to some degree on the position in the colony and/or substrate surface topography. Moreover, the early astogenetic zooids are identical to those in other species, i.e. they are equipped with thin cylindrical spines. It is only in the fifth- to eighth-generation of autozooids that the laterally compressed spines develop, and the dense zooid packing (i.e. absence of gymnocyst from the lateral zooidal margins) is also reached only in the zone of astogenetic repetition.

The geographic occurrence of H. sceletos is restricted to the Madeiran archipelago. The colonies were found to grow on shells, while their depth of occurrence is unknown.

The species Hincks (1880b, p. 71) newly described as Membranipora nodulifera from Madeira is very likely a Hincksina colony that has lost its spines. The avicularia he figured (pl. 9, fig. 2) are typical, and he also mentioned cap-shaped ooecia. Whether the species is synonymous with H. sceletos or specifically distinct needs yet to be ascertained. Moreover, only part of the material from Madeira that Norman (1909) referred to H. flustroides belongs to Hincksina sceletos ; the remaining two specimens (NHMUK 1911.10.1.346) rather resemble H. synchysia n. sp.

Figures

FIGURE 3. Hincksina sceletos (Busk, 1858).A. Overview of lectotype, the autozooids having relatively thin mural spines (NH- MUK 1899.7.1.1144). B. Close-up of autozooid; note the flattened, fan-shaped oral spines (lectotype, NHMUK 1899.7.1.1144). C. Two avicularia, the lower one showing the interior mandible with its muscle attachment scar and median lucida (lectotype, NHMUK 1899.7.1.1144). D. Ooecium (NHMUK 1908.3.23.1). E. Mural spines in lateral view (lectotype, NHMUK 1899.7.1.1144). F.Auto- and ovicellate zooids (NHMUK 1908.3.23.1). G. Several autozooids with thicker mural spines and the flattened oral spines viewed at a slightly oblique angle (NHMUK 1908.3.23.1). H. Periancestrular area showing the transition from thin, cylindrical, monomorphic spines in early astogenetic zooids to fully differentiated oral and mural spines in adult zooids (NHMUK 1908.3.23.1). Scale bars: A, F, H, 400 µm; B, C, 100 µm; D, E, 50 µm; G, 200 µm.