Botrylloides leachii ( Savigny, 1816 )

Brunetti, Riccardo, 2009, Botryllid species (Tunicata, Ascidiacea) from the Mediterranean coast of Israel, with some considerations on the systematics of Botryllinae *, Zootaxa 2289, pp. 18-32 : 19-21

Botrylloides leachii ( Savigny, 1816)

Botryllus leachii Savigny, 1816: 199 , pl. IV, fig. 6; pl. XX, fig. 4, Great Britain.

Botrylloides diegense Ritter & Forsyth, 1917: 462 , pl. 43, figs. 46–49, San Diego Bay; Van Name, 1945, p. 229, synonymy, California.

Botrylloides nigrum: Kott, 1973:252 , Bare Island, Botany Bay, Australia.

? Botrylloides nigrum: Kott, 1952: 257 , figs. 73, 74, Australia

? Metrocarpa nigrum: Pérès, 1949: 204 , fig. 25, Dakar, Senegal; 1958: 150 (only record), Caesarea, Israel.

? Metrocarpa nigrum var giganteun Pérès, 1949: 205 , figs. 26, 27, Dakar, Senegal.

Material examined: 7 colonies from Bay of Àkko (May 1991), 2 colonies (? 2004) and 17 colonies (July, August, November 2007) from Mikhmoret.

Description. Colonies are about 1.5 mm thick and, in preservative, are dark brown in colour. The living animals were recorded by the collector to have been orange with yellow bands, brownish-red, brown with white bands, or violet with white bands. The zooids, up to 1.5 mm tall, lie inclined up to perpendicularly to the colony surface. They lack atrial siphons and are arranged in leachii type systems (see Discussion, 1). The distal portion of the dorsal lip of the atrial opening has two, very small, lateral pigmented spots, only visible when the zooid is observed by transparency at high magnification. There are usually 8 tentacles: 4 larger and more posterior, and 4 smaller alternating ones. Among the larger tentacles, two lateral and the ventral one have a vascular lacuna filled with pigmented blood cells at the base; exceptionally, some cells also occur at the base of the dorsal tentacle, which is usually much less developed. These cell masses are sometimes visible through the external surface. There are other smaller tentacles, usually 7 in the dorsal sector and 5 in the ventral one (Plate 1, c). However, the number and size of tentacles are not regular (i.e., all tentacles may be equal in size, or there may be only three large and three small ones). The dorsal tubercle has a small, more or less oval, longitudinal opening (Plate 1, c). The posterior edges of the pre-pharyngeal groove end, as two wing-shaped expansions, at the sides of the longitudinal dorsal vessel. The dorsal lamina rises on the latter, at the level of the third row of stigmata, as a relatively short double membrane (Plate 1, c). The branchial sac is cylindrical, with 9 rows of stigmata, the second row being dorsally incomplete (Plate 1, a, b). Around the middle of the branchial sac, the stigmata are arranged between the three inner longitudinal vessels, usually as follows: 4,3,3,5 DL 5,3,3,4. A mass of pigmented blood cells is usually found on each side of the endostyle, at the level of the stigmata rows (Plate 1, a). The stomach is bell-shaped with 8 folds, excluding the typhlosolis (Plate 1, d, e1, e2). The folds, extending the whole length of the stomach, are slightly spiral and not very deep, but with a large ovoid swelling at their cardiac end. The last posterior quarter of the glandular epithelium of the folds is drastically reduced, sometimes giving the impression that they are not posteriorly closed (Plate 1, e1, e2). Between the typhlosolis and the 8th fold (folds, observed from the cardiac end, are numbered clockwise from the typhlosolis) there is a more or less wide smooth rhomboid area. The typhlosolis does not extend over the pyloric end of the stomach. A pyloric caecum rises from its posterior half, as long as half the length of the stomach, slightly directed into the intestinal loop. The caecum ends with an ovoid swelling, as large as the cardiac ones of the stomach folds (Plate 1, d, e1, e2). The duct of the pyloric gland is connected to the base of the pyloric caecum (Plate 1, d). The anterior edge of the intestinal loop reaches the penultimate row of stigmata, and the smooth-edged anus is only slightly further forward. In buds, several oocytes per side are present posteriorly to the testis primordia. However, in adult (i.e., filtering) zooids, only one egg per side is found, located along the anterior edge of the secondary intestinal loop on the left side and at the level of the stomach on the right side. The testis, situated anteriorly and ventrally to the eggs, consists of several lobes, usually about 10, forming a rosette (Plate 1, a, b). Sometimes only one egg is present apparently without a preferential side. Unfertilised eggs are about 30 μm in diameter; embryos develop in an incubatory pouch, located posteriorly and dorsal-ventrally, growing outwards from the body wall (Plate 1, b). Developing embryos were found in a colony collected in July.

Remarks. The present species may be Metrocarpa nigrum (= Botrylloides nigrum ) recorded, but not described or illustrated, by Pérès (1958a, b) from Caesarea. B. nigrum , originally described from Bermuda by Herdman (1886, p. 52, pl. III, fig. 21), was re-described by Hartmeyer (1912) and Van Name (1945). According to the last author (p. 228) the most characteristic morphological aspect of the species are the conspicuous and prominent rounded bulbous projections of the cardiac ends of the stomach folds. However, cardiac swellings are also present in other species, particularly in Botrylloides leachii . These two species are so closely related that Michaelsen (in Hartmeyer & Michaelsen, 1928, pag. 323) believe it is difficult to consider them different. Botrylloides nigrum would differ from B. leachii in having a lower number of stigmata rows (10 versus 13–14) and a larger pyloric caecum, which in B. leachii is stated to be so small that it may appear to be absent.

As regards the number of stomach folds in B. nigrum, Herdman (1886) mentioned 10–12, Hartmeyer (1912) and Van Name (1945) counted 9–10 and Kott (1952) 10. In B. leachii , of which I have analysed many colonies from both the Adriatic and Tyrrhenian Seas, the number of stomach folds are 9, although the 9th is smaller and may be difficult to make out without staining (Plate 1, e3, e4). The pyloric caecum is small, but, not always as small as that drawn by Michaelsen (1921, text fig 1), finger-like or slightly curved, with a slightly swollen tip and it does not really differ from that described for B. nigrum by all authors ( Herdman, 1886; Hartmeyer, 1912; Van Name, 1945; Rodrigues, 1962; Vasseur, 1970; Monniot, 1983). As regards the branchial structure, the second row of stigmata in B. leachii is dorsally incomplete, but a comparison with B. nigrum is not possible, because the previous authors did not specify whether it is complete or not in B. nigrum . Van Name’s fig. 137 (1945, p. 228) shows it complete, but the author, who did not report the number of rows of stigmata, probably did not examine the branchial structure.

On the basis of this knowledge only, it is practically impossible to distinguish B. leachii from B. nigrum . However, I believe that the two species are different and may be recognised by the mode of incubation of the larvae.

In Bermuda animals, Van Name states that a single egg, on each side, develops in a more or less protruding brood pouch posterior to the testis (p. 228), but his fig. 137 shows an egg located anteriorly to the intestinal loop and no real brood pouch is present. The same condition is illustrated by Monniot (1983) in his fig 1. Instead, in B. leachii , the single egg develops in a brood pouch located in the posterior dorsal corner of the peribranchial cavity, below the intestinal loop, beside the stomach; and this structure is clearly present already in the last developmental stages of the bud, as already described by Ärnbäck (1923). Therefore the two species, nigrum and leachii must be considered distinct.

Botrylloides nigrum: Kott (1952) is doubtful: the author noted the cell masses at the base of the main tentacles, if I correctly interpret her sentence Four statocysts lie at the base of perradial tentacles. (p. 257). However, in describing the stomach, she did not mention the cardiac swellings, but a later description ( Kott, 1973) almost certainly refers to B. leachii: The s tomach is expanded at the cardiac end and narrows toward the pyloric ends …” (p. 252), The stomach caecum is only barely distinguishable. (p. 253), Eggs are present in a pouch evaginated from the posterior part of the body …” (p. 253).

In Israeli species the appearance of the stomach and caecum is the same as in B. leachii , although there is one stomach fold less and, in both species, the Israeli colonies and B. leachii , there is a smooth rhomboid area between the typhlosolis and the last fold (Plate 1, e1–e4). A similar morphological aspect has never been reported for B. nigrum by previous authors. It was described in B. diegense ( Ritter & Forsyth, 1917, pag. 463; Van Name, 1945, p. 230), a species which I believe must be considered a junior synonym of B. leachii . Lastly, in Israeli animals, the embryo develops in a true brood pouch like/identical to that in B. leachii .

In conclusion, I believe that the analysed colonies from Israel belong to the species leachii , and that the observed differences (fewer rows of stigmata and only 8 stomach folds) are due to the particular environmental parameters of the collecting site, especially temperature (29–30°C), which is at the limit of tolerance for the species ( Bancroft, 1903a; Brunetti, 1976; Burighel et al., 1976). A reduction in body size, due to high salinity and/or high temperature, is known to occur in several species of marine taxa ( Kinne, 1971), (although it should be noted that the reduction in stomach size does not regard the pyloric caecum). Sonin et al. (2007), comparing two fish populations living in the Strait of Sicily and along the coast of Israel respectively, recently evidenced a statistically significant reduction in the body size of the Israeli population. This result confirms our hypothesis that the Israeli Botrylloides leachii is a dwarf form.