Pycnoclavella stolonialis, Pérez-Portela & Goodwin & Picton & Turon, 2010

Pérez-Portela, R., Goodwin, C. E., Picton, B. E. & Turon, X., 2010, Pycnoclavella stolonialis n. sp. (Tunicata: Ascidiacea), with phylogenetic and distributional remarks on the genus in Europe, Zootaxa 2407 (1), pp. 51-66 : 53-58

Pycnoclavella stolonialis sp. nov.

Material examined. Six colonies from different localities were examined (see details in Table 1).

Holotype: ( Mv 716) Kinrea, Rathlin Island, Northern Ireland 55 18.798’N, 06 14.317’W. Depth 28.8 m. The holotype has been deposited in the Ulster Museum (National Museums of Northern Ireland).

Description. The colonies are variable in size, comprising from a few to dozens of zooids. The pigment in living zooids is found all over the thorax and abdomen, showing a diffuse distribution but concentrated between stigmata rows, and in the endostyle and the siphonal area. The zooids are distinctively different from other Pycnoclavella species in having a white-yellowish patch of pigment surrounding the oral and atrial siphons and the inter-siphonal area but absent over the neural zone, making the neural ganglion clearly visible as a rounded unpigmented dot ( Fig 2a ). The tunic is soft and transparent over the thorax, and becomes translucent-whitish and thicker in the abdominal region, where debris and mud adhere. Colony structure is very variable; zooids are usually clustered in groups (2–6 zooids) united by the fused posterior part of the abdominal tunic. These groups of zooids are basally connected to each other by thick tunic strings which, in some cases, fuse to form a kind of basal plate. Additional zooids or groups of zooids are connected to the thick tunic strings or the basal plate by means of numerous, slender, and elongated stolons ( Fig 2b ). A combination of thick and fine stolons is therefore often found in colonies of this species, with a variable degree of development of the basal tunic. Although budding could not be observed, a few small zooids were found growing up from the basal part of the adult zooids and connected to them by fine and transparent tunic strands.

The zooids are small, up to 12 mm in size, of which the thorax is 2–2.2 mm ( Fig. 2c ). The siphons are smooth-rimmed. The buccal siphon has about 12–14 simple tentacles (plus many smaller ones). The tentacles do not form defined circles, instead they originate at several levels, the more posterior the position the larger the tentacles. The largest tentacle originates at the dorsal-most part of the tentacular ring, just anterior to the neural gland, which is a transverse oval.

Thoracic muscle bands are oriented longitudinally. There are up to 6–7 bands originating at the oral siphon or the upper part of the endostyle ( Fig 2d ). In some individuals muscle bands bifurcate in the anterior part of the thorax just below the base of the oral siphon. The musculature continues down the abdomen to the posterior part of the zooid. Fine circular bands line the siphons. Stigmata are in 6–8 rows on the ventral side, the first one dorsally divided into two. Dorsal languets are small. The number of stigmata per half row, counted in the middle rows, are from 10 to 12 in the individuals examined.

The internal structures in the abdomen are concealed by abundant pigment-laden hemocoelic cells. The abdomen may be twisted but not always, and sometimes we found intermediate forms. The oesophageal neck is long, and the posterior part of the abdomen is bulbous. The stomach is quadrangular but, except for the dorsal typhlosole, true folds are not present. A short duodenal region continues posteriorly, and there is a clearcut constriction that marks the junction between it and the mid-intestine at the posterior end of the gut loop ( Figure 2c ). The gonads are below the stomach on the left hand side of the gut loop; the duodenum as well as the mid-intestine are displaced by the gonads ( Fig 2c2 ). Gonads consist of numerous round or pearshaped male follicles (up to 13) and a few oocytes (a maximum of 3 oocytes were observed) ( Fig 2c 1 , 2c2 and 2c 3 ). The sperm duct passes between the stomach and rectum.

Up to 9 small embryos and larvae, in a developmental sequence, are incubated in the posterior part of the peribranchial cavity, within the expanded end of the oviduct ( Fig 2e ). The larval trunk is globular (trunk length up to 0.5 mm) bearing three adhesive organs of the tubular invaginated type arranged in a triangle and projecting back into the trunk from the anterior end. The tail surrounds the anterior part of the larva passing to the left of the anterior process. There is an ocellus, but no otolith, in the sensory vesicle ( Fig 2f ). Internal structures are very hard to discern due to the abundance of reserve cells and they could not be described.

Etymology. The species is named as Pycnoclavella stolonialis because it has a number of both thick and fine elongated stolons.

Distribution. The distribution of this species is imperfectly known. It was first noted during the Northern Ireland Sublittoral Survey (1982–1986) where it was recorded as an undescribed Clavelina species ( Erwin et al. 1986). It is easily recognisable in situ because of the whitish-yellowish pigment patch in the anterior region which surrounds both siphons ( Fig. 2a ). As a result, a number of records of the species have been made under its common name “pin head” sea squirt, and it is expected that once a scientific name and description is available a better impression of the distribution will be obtained. The species is widespread in the Irish Sea with the majority of records from Northern Ireland and Wales. It also occurs on the west coast of Ireland and in Scotland. There is one record from Norfolk and two from Northumberland which implies that it is likely widespread. around the rest of the British Isles. Available records from Bunker et al. 1992, Rostron and Bunker 1997, Picton and Costello 1998, Bunker and Holt 2003, Bunker et al. 2004, Goodwin et al. 2008, 2009 have been collated here (Appendix I). P. stolonialis usually occurs on tideswept bedrock and boulder habitats but it has also been found on two wrecks. It has been recorded at depths between 7 and 36 m. Parts of its distribution overlap with that of Pycnoclavella aurilucens ( Figure 1 ), and colonies of both species may be found adjacent to each other in some locations. Differences in body size, colour patterns and the presence of granular bodies on the tunic of P. aurilucens allow for easy distinction of the two species underwater (see Appendix II).

Remarks. This colonial species, with small zooids linked to each other by basal tunic or fine stolons, short thorax, stomach and gonads at the posterior end of the long abdomen, reduced ovary and testis size, and a larva with three invaginated and elongated adhesive organs, presents all the diagnostic characters of the genus Pycnoclavella . Pigment distribution pattern and the presence of both fine and thick stolons are the most useful characters to identify and to recognize colonies of Pycnoclavella stolonialis .

The new species, with small zooids, is easily distinguishable from the larger species of the genus, such as P. filamentosa and P. inflorescens ( Kott 2005) . It does not have the turned thoraces characteristic of P.detorta , P. kottae , P. auracea , P. aurantia and P. narcissus ( Millar 1960; Monniot 1997; Kott 1990; Kott 2005). P. tabella , P. producta , P. martae , P. diminuta and P. belizeana mainly differ from the new species in having only three stigmata rows and larvae with ocellus and otolith ( Goodbody 1996; Kott 1990; Pérez-Portela & Turon 2008), P. arenosa ( Kott 1990) also differs in having a larva holding ocellus and otolith. Likewise, P. minuta differs from the present species in having 4 rows of stigmata and abdominal incubation ( Millar 1953). The mode of incubation also differentiates the present species from P. stanleyi (Berrill & Abbott 1949) . Zooid organization and incubation mode of P. stolonialis resemble that of P. atlantica ( Pérez-Portela et al. 2007a) , but the later species has a very characteristic larva with a dorsal furrow separating the anterior process and posesses an ocellus and otolith. Among the smaller pycnoclavellids displaying similar body size and number of stigmata rows (between 5 and 8), both P. aurilucens and P. taureanensis (Brunetti 991; Pérez-Portela et al. 2007a) have an abdominal incubation mode and zooids of the colony form a basal mass of test. The number of muscular bands in the thorax (up to 9 and 30 for P. aurilucens and P. taureanensis , respectively) is also higher than in P. stolonialis . In addition, P. aurilucens has spherical granular bodies on its test ( Pérez-Portela et al. 2007a), a character not observed in P. stolonialis . The same type of spherical bodies have been observed in colonies of P. flava ( Monniot 1988) ; this species differs also from P. stolonialis in having an expanded lip on the dorsal rim of the oral siphon and a higher number of stigmata rows. Morphologically, the closest species to P.stolonialis are P. brava , P. communis and P. nana ( Pérez-Portela et al. 2007a) . The four species have similar zooid body size (around 10–15 mm length), number of stigmata rows (6–8), larval morphology and incubation in the peribranchial chamber, but they differ in the pattern of thoracic pigment distribution and colony structure ( Pérez-Portela et al. 2007a). In addition, P. stolonialis has less muscular bands and stigmata than P. communis , P. brava and P. nana . Moreover, a higher number of testis follicles were found in P. communis ; and P. brava has been observed incubating up to 17 larvae, many more than those found in P. stolonialis (up to 9). The main diagnostic characters allowing distinction between Pycnoclavella species are summarized in Table 2.

Figures

FIGURE 1. Maps showing sampling sites and known distribution of Pycnoclavella spp in Europe. Distribution of Pycnoclavella stolonialis is also detailed.

FIGURE 2. Pycnoclavella stolonialis. a) Colonies, a1) and a2) Live colonies close up, neural ganglion visible as a rounded colourless dot; b) colony structure; c) Zooid morphology, c1) zooid removed from the tunic showing testes and ovary, c2) dorsal view of the gut loop and gonads, and c3) right view of the gut loop; d) thorax in lateral view showing stigmata rows, oral tentacles and muscular bands; e) Thorax incubating embryos; f) larva

FIGURE 3. Bayesian Inference (BI) and Maximum Parsimony (MP) combined trees generated from a fragment of the COI gene. Numbers on the nodes indicate the respective support (when P>0.5 for posterior probabilities and 50% for bootstrap). Scale bars indicate number of substitutions per site.