Zanclea sessilis ( Gosse, 1853 )

Gravili, Cinzia, Vito, Doris De, Camillo, Cristina Gioia Di, Martell, Luis, Piraino, Stefano & Boero, Ferdinando, 2015, The non-Siphonophoran Hydrozoa (Cnidaria) of Salento, Italy with notes on their life-cycles: an illustrated guide, Zootaxa 3908 (1), pp. 1-187 : 72-74

Zanclea sessilis ( Gosse, 1853)

Fig. 49 A–J

See Schuchert (2010) for a complete synonymy.

Material examined. HCUS-S 0 54 and HCUS-S 0 55 (Hydrozoa Collection, University of Salento—fauna of the Salento Peninsula)—polyp and medusa stages.

Description (based on our own observations; Gravili et al. 1996; Boero et al. 2000; Schuchert 2010, 2012):

Hydroid. Hydrorhiza reticular, covered by the bryozoan skeleton in the central part of the colony and uncovered at its margin; colonies stolonal; hydrocaulus as hydranth pedicel, short, covered by thin perisarc, encrusted by sediment particles, not annulated; hydranth cylindrical; 4–6 oral capitate tentacles; 20–40 aboral capitate tentacles, scattered on the distal half or three quarters of hydranth column; medusa buds in clusters, growing below the tentacled part of the hydranth, fertile hydranths gradually decreasing in size and tentacle number. Colours: perisarc whitish; hypostome milk white, column transparent, with reddish gastric wall.

Habitat type. Only on cheilostomate Bryozoa (depth range: 1–60 m) ( Brinckmann-Voss 1970; Gravili et al. 1996)

Substrate. The bryozoa Cellepora pumicosa , Turbicellepora avicularis , Beania sp., and Schizobrachiella sanguinea .

Seasonality. Along the Mediterranean French coast in October ( Galea 2007); from February to April (Puce et al. 2009) in the Ligurian Sea; March ( Gravili et al. 1996), and from November to August (De Vito 2006; Piraino et al. 2013; this study) in Salento waters.

Reproductive period. September–October ( Gravili et al. 1996).

Medusa. Adult. Similar to young one but larger, with bell-shaped or almost spherical umbrella, lateral walls evenly thin, mesoglea slightly thicker at the apex; exumbrellar nematocyst pads elongated to a linear swelling reaching from bulbs halfway up the bell, containing 16–20 capsules mostly in a single row; manubrium reaching almost to velum level; mouth simple, circular; with 4 simple radial canals; male gonads in 4 interradial masses, female animals with few eggs (1–3 per interradius); bulbs unequally developed, with spots; 0, 2 or 4 marginal tentacles, 12–13 times as long as exumbrella, with numerous abaxial cnidophores; without ocelli. Colours: bell greenish, bulbs with red pigment spots.

Developmental stages. Newly released medusa almost spherical; 4 perradial, oval nematocyst buttons on exumbrella, containing stenoteles; manubrium simple, tubular, spanning about half of the subumbrellar cavity; mouth surrounded by some stenoteles; 4 radial canals, well visible, usually in middle region with somewhat irregular outline due to thickened walls; no gonads visible; radial canals end in 4 marginal bulbs, one opposite pair large, the other very small; each large bulb bearing one long, tapering, contractile tentacle, three times as long as exumbrella when fully extended, with up to 50 cnidophores, all on one side of tentacles, contractile, each usually with three macrobasic euryteles (2–5 range) and rarely a stenotele, distally with very long cilia.

Cnidome. Apotrichous macrobasic euryteles with shaft coiled in a horseshoe shape or the Greek letter alpha (α), disposed in circle in between oral tentacles and in hydrorhiza, rare in hydranth column; stenoteles of two sizes in tentacle capitations (polyp); two size-classes of stenoteles, telotrichous macrobasic euryteles in cnidophores (medusa).

Distribution. Atlantic, Mediterranean ( Gosse 1853; Russell & Rees 1936; Teissier 1965; Fey 1970; Brinckmann-Voss 1970; Gravili et al. 1996, 2008a; Medel & López-González 1996; Bouillon et al. 2004; Schuchert 2010).

Records in Salento. Rare at: La Strea, S.ta Caterina, Ionian Apulian coast ( Gravili et al. 1996); Miglietta et al. 2000); Otranto (De Vito 2006; Gravili 2006; Gravili et al. 2008a; Piraino et al. 2013; this study).

Remarks. The whole life cycle was examined in the present study. If disturbed, hydranths bend over their distal part; a double-layered perisarc is due to regeneration (see Russell & Rees 1936). Moreover, the studies of Gravili et al. (1996) and Boero et al. (2000) have clarified the status of Z. sessilis polyps and established it as a recognizable species.

References. Trégouboff & Rose (1957), Picard (1965), Brinckmann-Voss (1970) as Z. costata, Gravili et al. (1996) , Medel & López-González (1996), Boero et al. (2000), Miglietta et al. (2000), Bouillon et al. (2004), De Vito (2006), Gravili (2006), Galea (2007), Gravili et al. (2008a), Puce et al. (2009), Schuchert (2010, 2012), Piraino et al. (2013).

Figures

FIGURE 49. Zanclea sessilis: A, extended, living hydranth; B, the same, contracted; C, reduced living hydranth, bearing medusa buds (A, B, C same scale bar); D, hydroid, undischarged macrobasic euryteles in “ frontal ” and “ lateral ” view; E, the same, discharged; F, hydroid, undischarged stenoteles of two sizes (D, E, F, H, I, J same scale bar); G, adult medusa; H, medusa, undischarged macrobasic eurytele; I, the same, discharged; J, medusa, undischarged stenoteles of two sizes (modified after Gravili et al. 1996). Scale bars: A, B, C, 1.0 mm; D, E, F, H, I, J, 10 µm; G, 0.5 mm.