Botryllus gaiae, 2020

BOTRYLLUS GAIAE BRUNETTI View in CoL SP. NOV.

LSID: urn:lsid:zoobank.org:pub:DB18D5A7-0E96-458E-B3C0-5E93327359AC

E t y m o l o g y: Fr o m G a i a, t h e n a m e o f t h e f i r s t granddaughter of the author of this species, R. Brunetti.

Material examined: Syntypes: MSNVE 25086 , BT2, colony fragment in 10% formalin ; MSNVE 2508 , BT4, colony fragment in 10% formalin ; MSNVE 25088 , BT2A, another fragment of the BT2 colony preserved in 99% ethanol ; MSNVE 25089 , BT4A, another fragment of the BT4 colony preserved in 99% ethanol. All collected by F. Mastrotorato during clam sampling .

Type locality: Barletta ( Italy), 41°22’55.646”N, 16°10’06.586”E, 2.5–4.0 m depth on Cymodocea nodosa (Ucria) Asch. Seagrass meadow GoogleMaps .

Description: In both colonies from Barletta, labelled BT2 and BT4, the test surface is hard but, once it is torn, zooids are easily pulled out. Sample BT4 consists of two pieces of colony, each of them with 2–3 systems at the beginning of the generation change (developmental stage 9–11/8–9/6–7/1) ( Berrill, 1941). Therefore, the older generation is at the beginning of regression (stage 9–11) and the new generation presents syphons still closed and all internal organs completely developed (stage 8–9). The regressing zooids are clearly arranged in Botryllus schlosseri - type systems. Sample BT2 contains a piece of colony of about 1 cm 2 of surface with two systems consisting of 15 zooids. As shown in Figure 5 A-B, in life these two colonies are green (BT4) and green-yellow (BT2), whereas in preservative there are whitish.

The following description is chiefly based on the observation of the first order buds of sample BT2, which have virtually finalized their development (only the syphons have to be completed) ( Fig. 5C–E). The adult regressing zooids are about 1.5 mm high and the same height is recorded in first order buds. Therefore, it is justifiable to deduce a height up to 2 mm for the filtering zooid, because an increase in size is observed when the bud opens the syphons. The body wall presents fine muscle fibres running from the anterior-dorsal side to the ventral one. In the buds there are eight tentacles: four large and four intercalated short. The atrial syphons, still visible in some regressing zooids, is conic with a dorsal tongue. The branchial sac has nine rows of stigmata on the right and eight on the left side; the second row is dorsally incomplete ( Fig. 5D–E). The zooid has a cylindrical shape having about 20 stigmata in the first half row and about 15 in the seventh one. The branchial formula at the level of fourth row of stigmata is usually E 5.5.5.5 DL. Along the transversal branchial vessels, no muscle fibres are detected. A simple edged dorsal lamina rises from the dorsal vessel at level of fourth to fifth row of stigmata and reaches a height of three times the diameter of the dorsal vessel ( Fig. 5D–E) at the oesophageal opening which is at level of the last row of stigmata. The intestinal loop is horizontally arranged ( Fig. 5D). The stomach is almost totally below the branchial sac. It is slightly cylindrical, with nine folds slightly spirally arranged and all posteriorly closed. These folds have a smooth surface ( Fig. 5F, I–L), without the longitudinal groove observed in Botryllus schlosseri (Brunetti et al., 2017: fig. 1). They are longer on the mesial side (folds 5–8 in Fig. 5L) and without swellings at their cardiac end. The pyloric caecum is long, about half of the stomach, and has a rounded dilated tip; it rises from about the posterior third part of the typhlosole, the anterior part of which is much larger than the posterior one ( Fig. 5F, I). The intestine has a peculiar shape, with a hairpin bend (indicated with an arrow head in Fig. 5C–D) at level of the first intestinal loop. This aspect is not due to pressure of the gonads, as it is present also in zooids without developed gonads. The rectum is strongly stretched by a connection with the branchial sac through a trabecula at the level of the transversal vessel between the seventh and eighth rows of stigmata ( Fig. 5D–E, H). The anus opens at level of the seventh row of stigmata, that is one or two rows above the oesophageal opening. The anal opening has four lobes, each of them splits into three minor ones ( Fig. 5G). On the rectum surface, there are four grooves going from the anus to the pyloric gland. In sample BT4, well-developed gonads are present only in buds of the first order, which are ready to substitute the parents. There are up to four eggs (diameter of about 140 μm) on each side, anterior and dorsal to the testis follicles, which partially embrace them as a cup ( Fig. 5E). Gonadal primordia are present also in buds of the second order. No larvae and embryos have been found.

Remarks: Based on the structure of the cloaca, this species clearly belongs to the genus Botryllus , as defined by Brunetti (2009). The main morphological features distinguishing Botryllus gaiae from Botryllus schlosseri are reported in Table 4 and include several traits currently considered valid diagnostic features to discriminate species in the genus Botryllus ( Brunetti, 2009; Brunetti & Mastrototaro, 2017). These traits, highlighted in bold in Table 4, concern peculiarities of the stomach and the intestine, such as the shape of the first and second intestinal curve, the shape and position of the anus opening, the number and surface appearance of the stomach folds and the shape of the typhlosole. All these morphological differences are highly significant and support the hypothesis that clade E is a new species, distinct from Botryllus schlosseri sensu Brunetti et al. (2017) and therefore from clade A.