Phorbas dives ( Topsent, 1891 )

Phorbas dives ( Topsent, 1891) View in CoL

Fig. 22 View Fig

Microciona dives Topsent, 1891: 543–544 View in CoL , pl. XXII figs 2–3.

Material examined (2 specimens)

SWEDEN • 1 spec.; Saltbacken ; 59.0832° N, 11.2242° E; 26 m depth; 28 Aug. 2018; Mats Larsson leg. [MM-180828-1]; SCUBA; LAR-180828-7014–7016; GenBank no.: OM436258 View Materials (coxI); voucher: GNM Porifera 1104 GoogleMaps • 1 spec.; Bergylteskär ; 58.8290° N, 11.0831° E; 26 m depth; 9 Dec. 2018; Mats Larsson leg. [MM-181209-1]; SCUBA; LAR-181209-8253, 8255, 8257; voucher: GNM Porifera 110 . GoogleMaps

Description

All the specimens examined presented a thick encrusting or cushion morphology. The surface is smooth with wide, clearly observable, subsurface channels terminating in the oscula ( Fig. 22A View Fig ). The colour of the specimens alive was light-beige turning to whitish-beige when preserved in ethanol.

Skeleton

The skeleton presents a plumose conformation with multispicular tracts of acanthostyles and tornotes disposed perpendicularly to the surface while sigmas and arcuated isochelae are mostly found near the surface. The acanthostyles ( Fig. 22B–C View Fig ) are divided in two categories with overlapping sizes but with different spinations: a) slightly curved and sparsely spined throughout the shaft to the tip, measuring 167.5– 200.1– 327.2 ±37.80 ×2.9– 7.0 –12.1 ± 2.4 µm (N= 25), and b) heavily spined, measuring 160.5– 194.6– 318.8 ±30.70 ×2.7– 5.9– 9.6± 1.60 µm (N=31). The tornotes measure 108.5– 164.3 – 187.5± 12.55 ×1.8– 3.7 –5.2 ± 0.68 µm (N =34). The arcuate isochelae ( Fig. 22C View Fig ) occur in two size classes: a) 11.4– 16.3– 21.2 ± 2.4 µm (N =36), and b) 26.2– 32.9– 1± 3.7 µm (N=17). Finally, the sigmas ( Fig. 22C View Fig ) measure 12.1– 26.9– 39.9 ± 8.1 µm (N =25).

Ecology and distribution

The type locality for this species is Roscoff in Brittany, France ( Topsent 1891). However, P. dives has been previously reported from Wales ( UK), Ireland to northern Spain (Descatoire 1969), the Canary Islands and the Azores ( Simó 2002), as well as from the Mediterranean, in Italy ( Sarà & Siribelli 1960) and Tunisia ( Mustapha et al. 2003). In spite of this large geographical range, this species had not been previously reported for Sweden or Norway. Our specimens were collected by SCUBA in Idefjorden and the Koster area, between 15 and 25 m depth, thereby extending the geographical range of this species.

Phorbas dives seems to prefer a hard substrate and vertical or overhanging sites, possibly to avoid sediment deposition ( Ackers et al. 2007). This combined with its encrusting morphology makes this species difficult to detect and collect.

Remarks

Two other species are similar to P. dives : Phorbas bihamigera (Waller, 1878) from Torbay near Plymouth, southern England, and Phorbas microchelifer ( Cabioch 1968) from Brittany, France. None of these species have been reported in Sweden or Norway. Phorbas bihamigera differs from P. dives by having a higher abundance of chelae and more prominent aquiferous channels while P. microchelifer has smaller chelae (20 µm vs 36 µm in P. dives ) ( Cabioch 1968). Cabioch (1968) has seen many specimens from the type locality of which only the holotype (MNHN-IP-2015-649) was deposited. Cabioch (1968) compared P. microchelifer with specimens of P. dives from Roscoff, identified by Topsent himself (according to Cabioch 1968), thus we can reasonably consider this species as valid even if awaiting confirmation from molecular data.

Our coxI phylogeny ( Fig. 2 View Fig ) places our specimens of P. dives within a Myxilla incrustans clade. This contradicts current classifications, as Phorbas belongs to Hymedesmiidae , while Myxilla belongs to Myxillidae . We are confident in our identifications since the spiculation of our specimens matches better P. dives , and differs from M. incrustans in its tornotes: our specimens’ tornotes are completely smooth (as reported for P. dives ), while M. incrustans has typical spines at the terminations of the tornotes. Additionally, the family Hymedesmiidae is often regarded as a taxonomic waste basket, and shown to be polyphyletic (e.g., Morrow et al. 2013; Redmond et al. 2013: this study) thus the presence of species from other clades, still classified as hymedesmids is unsurprising. A full revision of Hymedesmiidae and Myxillidae with molecular data is required to revise the precise allocation of P. dives .