Distaplia fortuita, Lagger, Cristian & Tatián, Marcos, 2013

Lagger, Cristian & Tatián, Marcos, 2013, Two new species of Distaplia (Tunicata: Ascidiacea) from the SW Atlantic, Argentina, Zootaxa 3620 (2), pp. 192-200 : 196-199

Distaplia fortuita sp. nov.

( Figures 2 D; 4A, B)

Material examined. Las Grutas (Río Negro), Argentina. Holotype: MZUC T00003, one colony ( Fig. 2 D); washed ashore on sand beach off Las Grutas; 18/XI/2005. Paratypes from type locality: MZUC T00004, one colony; 18/ XI/2005.

Etymology. From fortuita latin adjective meaning casual, by chance.

Description. Both colonies are almost spherical in shape. The holotype, the larger of the two available colonies, is up to 4 cm in length, with a small flat basal area. The cherry red colour of fresh colonies quickly faded when fixed in formalin. The tunic is soft and smooth without any incrustations. The pale zooids that may be seen through the tunic are organized in circular or elongated systems of up to 15 individuals.

The zooids reach a maximum of 2.8 mm but most are approximately 2 mm long. The elongated oral siphon is narrow but gets wider closer to the aperture, without lobes (smooth-edged) and with very few circular muscle bands. The atrial siphon exhibits two possible shapes: the most frequent is a tube-shaped siphon that projects upwards, with a slightly elongated upper margin. The other shape is a short siphon with smooth edge and situated close together with the oral siphon at the upper part of the thorax.

Each side of the thorax bears about 25 thin oblique muscles that extend from the ventral region to the dorsal line. There are about five to seven very small oral tentacles arranged in a circle. The pharynx has four rows of 15- 17 stigmata per side and are crossed by parastigmatic vessels.

The aperture of the oesophagus is broad and the oesophagus enters the stomach laterally. The obliquely oriented stomach is oval and has an almost smooth wall, with some longitudinal fine stretch marks on its internal wall (not always noticeable). There is a constriction between the duodenum and the mid-intestine, but it is not possible to see unless gonads are removed. A pyloric ampulla, clearly visible above the gonads, extends between the stomach and the intestine. The bi-lobed anus opens at the level of the third row of stigmata.

All zooids of the two colonies are males. Testis consist of a cluster of 13 to 22 circular or oval-shaped follicles that lie beside the intestinal loop. The straight sperm duct originates from the centre of the testis and it opens near the anus. Some zooids possess a very small curved brood pouch (it was never developed).

The mature larvae lie in the upper layer of the colony; they are of a rounded shape and may be up to 1.4 mm long but are generally shorter (about 1.2 mm). The larval tail wraps in a half circle around the trunk, ending at the level of the adhesive papillae. Larvae have three adhesive organs arranged in triangle and their sensory vesicles each contain an otolith and an ocellus.

Discussion

Both Distaplia fortuita and D. naufragii have similar cushion-shapes, although the smaller colonies of D. naufragii can also be mushroom-shaped. Despite the external similarity, the appearance of both species differs in colouration, size of the colonies and number of zooids surrounding the common cloacal opening. The zooids of Distaplia fortuita and D. naufragii are easily distinguished. The measurement of zooids revealed that those of D. fortuita are two or three times smaller than D. naufragii . In the former species, there are very few circular muscle bands around the never-lobed oral siphon, while in the latter the zooids show a strong circular musculature in the usually sixlobed oral siphon. The atrial siphon in D. fortuita is smaller and more closed than D. naufragii . Furthermore, Distaplia fortuita has fewer oral tentacles, thoracic muscles and stigmata than D. naufragii . Finally, D. fortuita contains more testis follicles than D. naufragii . Both species have internal longitudinal ridges, but these are more evident or marked in D. naufragii . While zooids of D. fortuita do not possess fully developed brood pouches, zooids of D. naufragii possess elongated brood pouches containing numerous developing larvae. The two colonies of D. fortuita were exclusively males. However, the presence of an undeveloped brood pouch suggest that there is a female phase at some time, probably after regression of the male phase. Although the larvae of both species are of similar size, they differ somewhat in the shape of their trunk, in that the larval thorax of D. naufragii is more elongated than that of Distaplia fortuita .

A total of ten species belonging to the genus Distaplia have been reported in the Atlantic shallow waters (Rocha et al. 2012). Distaplia bermudensis Van Name, 1902 commonly forms an incrusting sheet with a thickness that varies between 2–7 mm. D. bermudensis has a smooth stomach, few testis follicles and oocytes, all located inside the intestinal loop, and a smaller larval trunk (0.8–1 mm). The species, common and widely distributed in the Caribbean (Van Name 1945), was also found in South eastern and Southern Brazil (Rocha & Costa 2005) and was also considered a relatively recent immigrant in the Mediterranean Sea (Mastrototaro & Brunetti 2006).

Distaplia stylifera (Kowalewsky 1874) is distinguished from the other West Atlantic Distaplia species by its characteristic pear-shaped or oval sac-like postabdomen (containing the reproductive organs) which connects to the abdomen by a very narrow and elongated neck. The species, widespread in the Caribbean, was also found on artificial substrata in São Paulo State, Brazil (Rocha et al. 2011).

Other two Distaplia species were recorded in the Caribbean: Distaplia corolla Monniot, 1974 and Distaplia crassa Monniot, 1983 . Both species have a smooth walled stomach and neither have internal longitudinal ridges or fine marks, a clear diagnostic characteristic of the two new species presented in this paper. Furthermore, colonies of D. corolla are bright yellow or orange in colour and form cylindrical isolated lobes with zooids arranged in a ring, like a rosette. D. crassa have intestines without any morphological differentiation along their length and their ovaries are situated in the centre of the rosette of male follicles; thus, the gonads are entirely contained in the intestinal loop (Monniot 1983), differing from Distaplia fortuita and D. naufragii .

Two Distaplia species have been reported from the North Atlantic: Distaplia clavata (Sars, 1851) and Distaplia rosea Della Valle, 1881 . D. clavata consist of clavate or narrowly capitate colonies, sometimes with more than one head arising from a common expanded base, or occurring as flattened incrusting forms (Van Name 1945). According to Van Name (1945), Huntsman (1912) described living colonies as light yellow. The zooids of D. clavata share some characters in common with D. naufragii , including the size of zooids, the number of stigmata and the presence of ridges on the stomach wall (that are not very conspicuous, in contrast to D. naufragii ). However, the figure drawn by Sars (1851) depicts a smaller cloacal siphon opening, fewer muscle fibres in the oral siphon and the thorax, a very robust vas deferens and indicates that all gonads lie over the intestinal loop, between the stomach and the mid-intestine (for comparison see Fig. 69 in Van Name, 1945). Inconspicuous colonies of D. rosea may be recognized not only by their characteristic rose-pink colour, but also by the peculiar soft sticky consistency of their tunic (Berrill 1950) D. rosea has fewer stigmata per row than our colonies. These two species recorded from the North Atlantic have external morphological characteristics completely different from the current samples.

Within the South East Atlantic, the slightly stalked colonies of Distaplia capensis Michaelsen, 1934 have zooids with only 7–9 stigmata per half row in the branchial sac. In Distaplia skoogi Michaelsen, 1924 , like D. stylifera , the gonads are in a pedunculate pouch behind the abdomen, with the testis vesicles clustered around a central ovary (Monniot et al. 2001). The presence of two round ampullae at the base of each papilla is also a diagnostic feature in the small larvae of this species and is absent in the larvae of our species.

Only two Distaplia species have been recorded in the SW Atlantic to date. Distaplia cylindrica (Lesson 1830) is a circumpolar Antarctic species common not only in the Magellan area and the Patagonian shelf (Millar 1960; Sanamyan & Schories 2003), but also present up to 49º latitude south in the Pacific Ocean (Tatián & Lagger 2009). The externally observable cylindrical rods and the soft consistency of this species make it distinctive such that its identification could not be confused. The other Distaplia species recorded in the tip of South America (Strait of Magellan) and South Georgia Island is Distaplia colligans Sluiter, 1932 . Its low and flattened encrusted colonies form thin sheets and its tunic is glassy gray or yellow (Sanamyan & Schories 2003).

Distaplia arnbackae Sanamyan et al. 2010 is a new species from the Pacific Ocean (Central Chile). Colonies of D. arnbackae are red in colour, but different from the present colonies: younger colonies consist of several small button shaped cormidiums containing mostly a single zooid system, while larger colonies are always encrusting sheets or flat cushions that rarely reach more than 5 mm thickness.

There are no other Distaplia species similar to the material here described. The morphology of the colonies and of the zooids justifies the proposal that they represent two new species.

As was stated, the SW Atlantic ( Argentine Sea) is among the poorly known marine regions in terms of biodiversity (Orensanz et al. 2002). In the last ten years, several reports on the emergence of marine invasive, exotic and cryptogenic species in this vast region were published (Casas et al. 2004; Hidalgo et al. 2005; Penchaszadeh et al. 2005; Spivak et al. 2006; Calvo-Marcilese & Langer 2010; Tatián et al. 2010; Boltovskoy et al. 2011; Fiori et al. 2012). Today, non-native ascidians are increasingly being documented throughout world (Shenkar & Swalla 2011). However, even knowing that ascidians have been recently involved in many cases of bioinvasion (Lambert 2007, 2009), little is known about the diversity and distributions of ascidians in the Argentine Sea.

The current results emphasize the need to increase the sampling effort for this group, and reinforce the importance of new studies in this area, considered pristine, but actually susceptible to biological invasions. Ascidian richness within the San Matias Gulf, examined by sampling for the first time using SCUBA diving, will be presented in a future publication.

Figures

FIGURE 2. A, Distaplia naufragii sp. nov. at the shipwreck “ Don Félix ” (photo by R. Sahade). B, Large colony. C, Small colonies. D, Colony of Distaplia fortuita sp. nov. Scales: 2 cm.