Ventromma velella

Velella velella (Linnaeus, 1758)

Fig. 45 A, B

See Schuchert (1996) for a complete synonymy.

Material examined. HCUS-S 0 50 (Hydrozoa Collection, University of Salento—fauna of the Salento Peninsula)—polyp stage.

Description (based on our own observations; Vanhoeffen 1906; Brinckmann-Voss 1964, 1970; Larson 1980; Calder 1988; Pagès et al. 1992; Schuchert 1996, 2012):

Hydroid. Floating polymorphic colony, flattened, oval or elliptical, with a triangular sail, up to 40 mm long and 20 mm wide, higher in the centre than at the edges, float and sail kept rigid by a chitin support covered by mantle tissue: margin of float soft and flexible, chitin float oval to slightly S-shaped with concentric air chambers, mantle tissue with network of endoderm canals, with a single large gastrozooid or "siphon" in centre of underside encircled by a ring of medusa producing gastro-gonozooids and a peripheral band of dactylozooids; gastrozooid broadly oval with an elongated hypostome, central, without tentacles or medusa buds; dactylozooids long and tapering, oval in cross section, with cnidocysts concentrated in two lateral bands on the narrow sides, mouth lacking; gastro-gonozooids fusiform with a swollen mouth region, lacking tentacles but with warts of cnidocyst clusters concentrated in distal half and numerous medusa buds growing in groups from short blastostyles on proximal half. Colours: float deeply blue when alive, medusa buds yellow-olive from symbiotic algae.

Habitat type. Colony floating on water surface, often forming swarms (Edwards 1966b).

Seasonality. In the western Mediterranean Sea, Velella velella occurs from February to June, October–December (Woltereck 1904; Brinckmann-Voss 1970), and April (Galea 2007); April–May (this study).

Reproductive period. Along the Mediterranean French coast, fertile colonies occur in April (Galea 2007).

Medusa. Adult. Up to 2.8 mm high and 2 mm wide, bell cylindrical, flat top; four exumbrellar cnidocyst rows; manubrium conical with quadrate base, mouth tubular; gonads irregularly arranged perradially and interradially; four radial canals; two pairs of opposite, perradial tentacles, a short adaxial one and a long abaxial one, each with a large terminal cnidocyst cluster; two perradial marginal bulbs without tentacles; marginal sense organs absent. Colour: females have red eggs.

Developmental stages. Newly released medusae bell-shaped, up to 0.8 mm in diameter and 1 mm high; 4 perradial rows of stenoteles on exumbrella originating from the marginal bulbs; manubrium conical, short; broad radial canals with many zooxanthellae in groups; 4 broad marginal bulbs connected by circular canal; no tentacles or rudiments of tentacles. Colour: manubrium red.

Cnidome. Stenoteles, microbasic euryteles, isorhizas (polyp); stenoteles, macrobasic euryteles (medusa).

Distribution. Atlantic, Indo-Pacific. In the Mediterranean it is mostly present in the spring, when it can be extremely abundant (Metschnikoff 1886a; Brinckmann-Voss 1964, 1970; Edwards 1966; Bouillon 1978a; Larson 1980; Calder 1988; Pagès et al. 1992; Schuchert 1996; Bouillon et al. 2004; Gravili et al. 2008a).

Records in Salento. Moderately frequent at Porto Cesareo (F. Boero unpublished observations).

Remarks. Two mirror images of the animal (left and right sailing) exist, the prevalence of one form or the other in one region may be due to sorting by prevailing winds (Edwards, 1966). The polyps of V. velella are generally found in huge swarms and despite that each of them bear thousands of medusa buds the adult medusa of V. velella has very seldom been found in nature. In the Mediterranean Sea, adult medusae have been found in the field only by Metschnikoff (1886a) in Messina. Male medusae were raised in the Laboratory at Naples by Brinckmann-Voss (1964, 1970). Elsewhere, mature medusae where described from nature by Bouillon (1978a) in Papua New-Guinea and by Larson (1980) in the North Atlantic. Woltereck (1904) described colony development through a series of “larval” stages. Gegenbaur (1856) described a small medusa from Messina having 16 radial canals and one tentacle called Chrysometra striata that he identified as the medusa of Velella velella but this is doubtful (see Brinckmann 1964; Brinckmann-Voss 1970). Gonothecae and medusae not seen in the present study.

References. Metschnikoff (1886a, 1874), Lo Bianco (1909), Moser (1925); Leloup (1929), Totton (1954), Trégouboff & Rose (1957), Picard (1958a), Mackie (1959, 1960), Edwards (1963, 1966b), Brinckmann (1964), Brinckmann-Voss (1970, 1987), Daniel (1976), Larson (1980), Castello i Tortella (1986), Gili (1986), Riera et al. (1986), Gili et al. (1988), Calder (1988), Pagès et al. (1992), Medel & López-González (1996), Schuchert (1996, 2012), Bouillon & Boero (2000), Bouillon et al. (2004), Touzri et al. (2004), Gravili (2006), Galea (2007), Gravili et al. (2008a).