Porella tenuis

Porella tenuis (Calvet, 1906)

(Figs 4 C–G; 5A, B; Table 6)

Palmicellaria tenuis Calvet, 1906: 431, pl. XXVIII, figs 7, 8.

Palmicellaria skenei tenuis: d’Hondt 1974: 18.

Material examined. MNCN 25.03 /4202: DA05, Gazul MV, 418–422 m depth, 22 June 2010, INDEMARES CHI- CA Project, IEO coll., no information on the original substrate; MNCN 25.03 /4203: BT26, Almazán MV, 930–940 m depth, 4 March 2011, INDEMARES CHICA Project, IEO coll., no information on the original substrate; MNCN 25.03 /4204: BT06, Chica MVD, 660–667 m depth, 19 February 2011, INDEMARES CHICA Project, IEO coll., on MDACs.

Description. Erect colony with cylindrical branches and zooids opening only on the frontal side (Fig. 4C). Autozooids elongate, arranged in two or three longitudinal alternated series, orifices from the outer zooids facing away from the branch midline (Fig. 4C, D); frontal shield smooth or slightly nodular, imperforate except for scattered, rounded marginal areolar pores varying in size; sometimes a row of smaller rounded pores at the base of the peristome; on some zooids a small avicularium replacing an areolar pore (Fig. 4D). Peristome well developed, hiding the primary orifice and forming three tapered, cylindrical umbones (one median and two lateral) with nodular surface, and three inner avicularia placed at the base of each umbo (Fig. 4D, E): the median avicularium wider than long and larger than lateral ones, directed upwards and outwards (Fig. 4E). Primary orifice wider than long, with a wide and short lyrula at its proximal border (Fig. 4F). All avicularia (frontal, peristomial and distal) circular to oval, small, rostrum with serrate border and complete, thick crossbar (Fig. 4D, E, G). Ovicell slightly wider than long, globular, with a smooth surface and two, rounded to elliptical pores near the proximal border; becoming immersed with secondary calcification (Fig. 4G); opening inside the peristome but not closed by the zooidal operculum. On the abfrontal side: several scattered small and rounded pores, more frequently near the sutures; avicularia (similar in size and shape to those on the frontal side) placed near the sutures as well, directed randomly (Fig. 5A, B).

Remarks. Porella tenuis has been described by Calvet (1906) from Cape Spartel (Gulf of Cádiz), occurring at 717 meters depth as Palmicellaria tenuis. Subsequently, d’Hondt (1974) recorded this species from the Gulf of Gascogne (North Atlantic) (as Palmicellaria skenei subspecies tenuis). Souto et al. (2016), while describing a new species of Porella, P. biserialis, had the opportunity to examine the type-material of Palmicellaria tenuis (MNHN 419) and transferred P. tenuis to the genus Porella. These authors suggested that P. tenuis fits better in the family Bryocryptellidae instead of Celleporidae, indicating features that are distinctive between the two species: P. biserialis has branches with two longitudinal series of zooids but lacks a lyrula, peristomial umbo, and avicularia on the ovicellate zooids. Comparing the specimens of this study with the holotype of Porella tenuis some differences were detected: the holotype shows three or four latero-peristomial umbones (our specimens have three umbones, one suboral plus one on each side), and the avicularia inside the peristome are not constant (three avicularia always present in our specimens). Nevertheless, d’Hondt (1974) mentioned that these characters can be very variable within and between colonies. The material studied here consisted of fragmented colonies suggesting that variability may be lost. Thus, we maintain these specimens as Porella tenuis.

Habitat and associated species. This species occurs in fine sand and sandy mud with MDAC and coral rubble, L. phalangium, sponges [T. muricata and Pheronema carpenteri (Thomson)], M. oculata and some soft bottom octocorals (Isidella elongata (Esper), Radicipes cf. fragilis) and small gorgonians (Swiftia sp.).