Botryllus eilatensis, Shenkar, Noa & Monniot, Françoise, 2006

Shenkar, Noa & Monniot, Françoise, 2006, A new species of the genus Botryllus (Ascidiacea) from the Red Sea, Zootaxa 1256, pp. 11-19 : 13-17

Botryllus eilatensis n. sp.

Material examined. Israel, Eilat, Gulf of Aqaba (Eilat). Holotype: AS 25226, Zoological Museum Tel Aviv University, Israel. Paratype: MNHN S1 BOT B 226, Muséum national d’Histoire naturelle, Paris.

External appearance. Living specimens are a striking opaque white or orange color ( Fig. 2 ). Intermediate colors have not been found. Colonies are soft and vary from being small discs of a few millimeters diameter to large sheets, up to 30cm wide and 0.5cm thick. The colony surface is generally free of epibionts and foreign matter and is raised on low lobes. Ridges separate adjacent double rows of zooids. The small colonies are cushion–shaped with a central excurrent aperture; the largest colonies appear as thick encrusting sheets. The edges of the colonies are fringed with a thick band of parallel, elongated terminal ampullae of the tunic vessels ( Fig.2 ). The large common cloacal apertures are up to 0.5cm in diameter in life. The oral apertures are wide, in regular double rows, separated by wide spaces. After fixation, the colony becomes light brown and the tunic transparent.

The zooids are perpendicular to the colony surface, in a single layer. The largest ones measure three millimeters in length. The body wall contains sparse, round, brown and white pigmented cells in fixed specimens. Musculature is weak. The oral siphon is short and wide ( Figs 3 A, B; 4A) with a plain edge or six very shallow lobes. Three orange pigment spots can be seen at the base of the siphon, one at the top of the endostyle, and one on each side. The oral sphincter is weak. The atrial aperture is wide and exposes the whole branchial sac in fully developed zooids. The dorsal lip forms a hood over the opening ( Figs 3 A; 6). Its size varies, sometimes being as long as the zooids. The posterior edge of the atrial opening is also produced out forming concavity in the postero­dorsal part of the atrial cavity. Fine muscles are present around the margin of the atrial aperture when the muscles on the oral siphon and the thorax are not conspicuous. The 12 oral tentacles are short and well spaced ( Figs 4 A, 5A). The peripharyngeal band is bent in a deep V shape. The button­like dorsal tubercle opens in a vertical slit. The branchial sac is particularly large with 15 to 18 rows of stigmata and, halfway down the thorax ( Fig. 3 ; 4A), about 20 stigmata are in each half row. The second row is incomplete, and does not reach the dorsal line (fig. 4A). The three longitudinal vessels are equidistant, running the whole length of the branchial sac ( Figs 3 ; 5A).

The gut is small, mostly located behind the branchial sac ( Figs 3 A,B; 4A; 5A). The first loop is narrow, the secondary loop is variable, the plain anus opens at the third row of stigmata counted from the base of the thorax; The stomach has nine folds with the proximal part flaring out into protruding ampullae ( Figs 4 B,C; 5C). A very small buttonlike caecum is hardly visible without staining. The pyloric gland forms a ring of vesicles around the intestine at stomach level ( Fig. 5 C). The stomach and a segment of the intestine are orange in life.

In the few colonies that contained gonads, the male gonads appeared first. In these specimens (except in a single colony), the nourishing zooids have only testes ( Figs 3 B, 5A), and the ovaries appear only in buds ( Figs 4 D; 5B). The testis consists of rounded, branched follicles crowded into a spherical mass on each side of the body, just anterior to the gut lop ( Figs 3 B; 4B; 5A). One bud on each side is rapidly isolated from the parent zooid and develops in the deepest part of the colony. These buds acquire gonads at a small size, no more than 0.6mm. The testes appear first. The ovaries are small with one or two oocytes in the most developed buds which protrude from the body wall above the gut ( Fig. 4 D; 5B). A secondary bud grows above the gonads in the first­order buds, whose atrial opening is not yet differentiated but whose branchial sac and longitudinal vessels are similar to those of the adults. The stomach is not fully developed at this stage ( Fig. 4 d). Ovaries were found in only one colony collected in August. An ovary with a single oocyte (0.25 to 0.3 mm diameter) is on each side of the zooid, lateral to the testis and slightly extruding from the body wall. In this colony almost all zooids had ovaries and testes, with very small buds located deep close to the substrate. No embryos and no larvae were found in any of the various colonies collected in different seasons.

FIRURE 6. Botryllus eilatensis n. sp. Zooid with ovaries. Scale bar: 0.5 mm.

The Botryllinae are common worldwide but have not been well described. It is difficult to determine whether the new species is native to the Gulf of Aqaba or an invader as it has colonized harbor structures as well as natural substrates. Further investigations and collections in more southern parts of the Red Sea may answer this question.

B. eilatensis appears to be a very successful competitor for space in the northern Gulf of Aqaba (Eilat), overgrowing dead coral skeletons and rapidly colonizing artificial substrates. This may be partly the result of increased anthropogenic activity, particularly eutrophication ( Loya 2004) occurring in this area. This environment creates favorable conditions for filter­feeding organisms such as ascidians, providing an advantage in competition for space with reef­building corals. It is therefore imperative to continue monitoring its populations and further study ecological aspects of this species.

Remarks. This new species has some characters in common with Botrylloides lentus Saito & Watanabe, 1985 , from Japan: numerous stigmata rows, large atrial apertures, and similar number of stomach folds. However, B. eilatensis has a smaller gastric caecum, the ovary is lateral to the testis rather than beneath it; and; the opacity and color of the tunic are distinctive. Botrylloides perspicuum Herdman, 1886 has also numerous rows of stigmata, but the ovary is anterior to the testis. Botrylloides violaceus Oka, 1927 has much less stigmata rows.

In Botrylloides lentus the larva is incubated in a brood pouch hanging beneath the testis, as is characteristic of the genus Botrylloides Milne Edwards, 1841 (see Hartmeyer & Michaelsen 1928 and other authors). Saito and Okuyama (2003) compared Botrylloides and Botryllus Pallas, 1766 species. They concluded that morphological and life history characteristics are not good reasons for distinguishing these genera. We have adopted this interpretation and assigned the new species to the senior synonym Botryllus (see also Monniot and Monniot 1987).

Figures

FIGURE 2. Both color morph of Botryllus eilatensis n. sp.

FIGURE 3. Botryllus eilatensis n. sp. Two nourishing zooids stained with hemalum: A, without gonads; B, with testes. Scale bar: 1 mm

FIGURE 4. Botryllus eilatensis n. sp. A, young zooid with fully developed branchial sac; B, internal side of the gut and testes; C, external side of the stomach; D, first order bud with gonads and second order bud. Scale bar: A, B, C: 0.5 mm; D, 0.2 mm.

FIGURE 5. Botryllus eilatensis n. sp. stained with hemalum. A, zooid opened along the ventral line; B, first order bud; C, digestive tract. Scale bars: A, 1 mm; B, C, 0.2 mm.