Italian Journal of Zoology The polyp and the medusa ...

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The polyp and the medusa of Zanclea costata Gegenbaur (Cnidaria, Hydrozoa) a

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Carlo Cerrano , Daniela Amoretti & Giorgio Bavestrello

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Istituto di Zoologia , Università di Genova , via Balbi 5, Genova, I‐16126, Italy Published online: 28 Jan 2009.

To cite this article: Carlo Cerrano , Daniela Amoretti & Giorgio Bavestrello (1997) The polyp and the medusa of Zanclea costata Gegenbaur (Cnidaria, Hydrozoa), Italian Journal of Zoology, 64:2, 177-179, DOI: 10.1080/11250009709356192 To link to this article: http://dx.doi.org/10.1080/11250009709356192

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Ital. J. Zool., 64: 177-180 (1997)

The polyp and the medusa of Zanclea costata Gegenbaur (Cnidaria, Hydrozoa) CARLO CERRANO DANIELA AMORETTI GIORGIO BAVESTRELLO

Downloaded by [200.110.243.134] at 04:36 24 March 2014

Istituto di Zoologia, Università di Genova, via Balbi 5, I-l6l26 Genova (Italy)

INTRODUCTION The chaotic status of the taxonomy of the genus Zanclea was recently discussed by Gravili et al. (1996) who, following an idea of Picard (1965) considered the cnidome as the most important character for species identification. On the basis of cnidome, Gravili etal. (1996) recognize three species of Zanclea for the Mediterranean Sea, namely: Z. sessils (Gosse, 1853), Z. costata Gegenbaur, 1857 and Zanclea sp. The first two correspond to Z. costata and Z. costata var. neapolitana described by Brinckmann-Voss (1970) from the Gulf of Naples and reported by her as living on different substrata: bryozoans and bivalve shells, respectively. The descriptions by Picard (1965) were short, without illustrations and stressed mainly the differences of cnidomes. No illustrations were provided either by Millard & Bouillon (1973) and Bouillon (1974), who agreed with Picard's idea about the distinction between Z. sessilis and Z. costata in the Mediterranean. Brinck^ mann-Voss (1970) figured the hydroid of Z. costata ( = Z. sessilis) and the hydroid and a 10-days-old medusa of Z. costata var. neapolitana ( = Z. costata) but did not describe the cnidome of either species. Finally Gravili et al. (1996) provided a detailed description with excellent pictures of the polyp and the medusa of Z. sessilis with their nematocysts but observed only a few fixed polyps coming from a sterile colony of Z, costata, which thus allowed the representation of undischarged nematocists only. Consequently, a complete description, comprising the cnidome of hydroid and medusa of this last species, is still lacking. \ The discovery of a rich population of this species, living on the shells of the bivalves Chamelea gallina and Spisula subtruncata from the bay of Laigueglia (Ligurian Sea, Italy) allowed us to observe the polyp and the newly liberated medusa of this species that are described below.

ABSTRACT The hydroid and the medusa of Zanclea costata, together with their cnidome, are described from a rich population living on the shells of the bivalves Chamelea galina and Spisula subtruncata from the Ligurian Sea. The high phenotypic plasticity of the hydranth of this species confirms the use of the cnidome as the main reliable feature for species identification within the genus Zanclea. KEY WORDS: Zanclea - Hydromedusae - Taxonomy - Life cycle Nematocysts - Mediterranean Sea. ACKNOWLEDGEMENTS The authors are indebted to Prof. F. Boero (Dipartimento di Biología, Lecce) for helpful comments and suggestions. This work is supported by the Italian Ministero dell'Universita e della Ricerca Scientifica e Tecnológica, MURST 60% and 40% funds. (Received 5 November 1996 - Accepted 31 January 1997)

MATERIAL AND METHODS Colonies were collected by diving from the sandy bottom of Laigueglia (Ligurian Sea) from April to August 1996. This habitat is characterized, from 3 m depth, by the presence of a Cymodocea nodosa bed with patches of Caulerpa taxifolia and, from about 9 to 15 m depth, by a non uniform Posidonia oceánica bed. The assemblage of filter-feeders is mainly composed by three species of bivalves: Spisula subtruncata present from 5 m depth with a density of about-1000 specimens/m2; Cardium edule from 5 m depth with about 3 specimens/m2 and Chamalea gallina, from 3 m depth with about 1 specimen/m2. The collected colonies, together with their bivalve hosts were reared in aquaria with areated, natural sea water, at 20° C both in calm and moving conditions. The aquarium bottom was covered by a 5 cm thick layer of sand collected from the original habitat, Under these conditions the colonies, fed every other day on Artemia salina naupli produced abundant medusae.

C. CERRANO, D. AMORETTI, G. BAVESTRELLO

178 TAXONOMIC ACCOUNTS Family ZANCLEIDAE Genus Zanclea Gegenbaur, 1857 Zanclea

COStata Gegenbaur, 1857

Description


Hydroid. Stolonal colonies composed by about 50 hydranths, living on the posterior side of the shells of Chamelea gallina and Spisula subtruncata. Hydranths (Fig. 1A) are 10-15 mm high, with 5-7 mm high unbranched stems of corrugated or ringed perisarc (Fig. IB). Perisarc tubes often covered by agglutinated sand grains.

Nematocysts: (i) Stenoteles of two sizes (17 and 29 \im) in tentacle capitations and in the hydrorhiza; (ii) holotrichous macrobasic euriteles (Fig. 2A) (32 (xm undischarged) with a coiled spiny shaft five-six times the length of the capsule, in the hydrorhiza and in the hydranth body; (iii) microbasic mastigophores (Fig. 2B) (18 um undischarged) with a coiled spiny shaft about as long as the capsule, rare in the hydranth body and in the hydrorhiza. If the colonies are reared in moving water their morphology profoundly changes. Hydranths shorten (0.7-1 mm) and the tentacle number decreases to 20-40 (Fig. 1C). The number of macrobasic euriteles drastically increases. These nematocysts became very abundant in hydroriza and hydranth body and constitute a continuous ring at the base of oral tentacles immediately under the hypostome (Fig. ID).

Fig. 1 - Zanclea costata. A) Hydranths with medusa buds, in different developmental stages. B) Corrugated perisarc tube of the hydranth. C) Hydranth from a colony living in a high water movement condition showing (D) the continuous ring of macrobasic euryteles under the hypostome. Scale bar, 100 urn.

Fig. 2 - Zanclea costata. A) Holothrichous macrobasic euritele of the polyp. B) Microbasic mastigophore of the polyps. C) Heterotrichous macrobasic eurytele of the medusa cnidophores. Scale bar, 10 urn.

Five-six oral tentacles and 40-60 aboral tentacles disposed on the distal third of the hydranth. Numerous medusa buds (up 20 per polyp), in different stages of differentiation in the middle and lower portion of the tentaculate part of the hydranth.

Medusa. In newly liberated medusae the umbrella is bell-shaped about as high as it is wide (1 mm) (Fig. 3A). The manubrium is tubular with a circular mouth surrounded by a circle of little stenoteles. Four" radial canals ending in tentacular bulbs, two small atentaculate and two

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POLYP AND MEDUSA OF ZANCLEA COST ATA


triangular and tentaculate. Very close to tentacular bulbs four exumbrellar, ovoid, nematocyst pouches are present containing four-five large stenoteles. Tentacles are covered by cnidophores containing two-three macrobasic euriteles. In one month old medusae (Fig. 3B) the umbrella diameter is about 2 mm, the manubrium is cilindrical with four interradial, elongate sperm masses. The tentacles does not increase in number and are very contractile; they may extend to 1 cm. The shape of the cnidophore differs according to their position: on the tentacular bulb they are sessile while their peduncles increase in length along the tentacles. In the distal portion we observed 1 mm long peduncles. Nematocysts: (i) stenoteles of two sizes (9 and 16 |xm) respectively disposed around the mouth and in the exumbrellar pouches at the base of the tentacles; (ii) apotrichous macrobasic euryteles (Fig. 2C) (15 urn, undischarged) with a shaft five times longer than the capsule, grouped in the tentacular cnidophores.

Ecology and behaviour

The colonies usually live on the posterior side of the shell of Chamelea gallina (about 85% of the specimens host the hydroid), more rarely have been noted on Spisula subtruncata specimens (frequence of about 0.2%). When the bivalve extrudes its siphons from the sandy bottom the hydranths form a circle around it. The constant position of the colony on the shell probably depends on two factors: the current produced by the pumping activity of the shell and the proximity of this portion of the shell to the sediment surface, allowing the polyps to live away from the bottom for long periods of time. In the aquarium several polyps detach themselves from the shell and live freely on the sandy bottom: it is probable that also in their habitat they may move from shell to shell surviving for some time directly on the bottom, agglutinating sand grains around their perisarc.

Fig. 3 - Zanclea costata. A) Newly liberated and (B) one month old medusae. Scale bar, 500 urn.

CONCLUSIVE REMARKS

These data afford the first complete description of the Zanclea costata cnidome. The shape of the shaft and the position of macrobasic euriteles allow a good distinction among the three mediterranean species. Moreover Z. costata is the only one presenting microbasic mastigophores. Gravili et al. (1996) observing only undischarged capsules dubiously interpreted this nematocysts as microbasic euryteles but observation of several discharged nematocysts of this kind never shown the typical inflate end of the eurytele shaft. Microbasic mastigophores are unknown in the family Zancleidae but forms of transition between euryteles and mastigophores are present in the genus Teissiera and Rosalinda (see Bouillon, 1974). The macrobasic euryteles of the medusa are very similar in their shape to those of the other two species but are about twice as large. Morphological variations in the same species of polyps living in different water movement intensities are well documented (Boero, 1983). In Z. costata the polyps exposed to high level of water movement overlap in hydranths size and tentacles number the features of Z. sessilis (Gravili et al., 1996). Also the nematocysts arrangement is influenced by this parameter. This clearly indicate that nematocysts morphology is still the only available character to identify these species. Gravili et al. (1996) hypothesized that some zancleid medusae with four tentacles collected from the Gulf of Marseille and the Ligurian Sea (Berhault, 1969; Goy, 1972) may be the medusae of Z. costata. Nevertheless our observations indicate that the medusae of this species mantain, in laboratory conditions, only two tentacles in all the phases of their life.

REFERENCES Berhaut J., 1972 - Etude qualitative, quantitative et écologique des Hydroméduses du Golfe de Marseille. Thethys, 1: 667-708. Boero F., 1983 - The ecology of marine hydroids and effects of environmental factors: a review. P.S.Z.N.I: Mar. Ecol., 5: 93-118. Bouillon J., 1974 - Description de Teissiera milleporoides, nouveau genre et nouvelle espèce de Zancleidae des Seychelles (Hydrozoaires; Athécates-Anthoméduses), avec une révision des Hydroides «Pteronematoidea». Cah. Biol. mar., 15: 113-154. Brinckmann-Voss A., 1970 - Anthomedusae/Athecatae (Hydrozoa, Cnidaria) of the Mediterranean. Part I, Capitata. Faune e Flora del Golfo di Napoli, 39: 1-96. Goy J., 1972 - Les hydroméduses de la mer Ligure. Bull. Mus. natl. Hist, nat., 3me ser. 83, Zool., 62: 965-1009Gravili C., Boero F., Bouillon J., 1996 - Zanclea species (Hydroidomedusae, Anthomedusae) from the Mediterranean. Sci. Mar., 60: 99-108. Millard N. A. H., Bouillon J., 1973 - Hydroids from the Seychelles (Coelenterata). Ann. Mus. r. Afr. Centr. (Zool.), 206: 1-106. Picard J., 1965 - Recherches qualitatives sur les biocénoses marines de substrats meubles dragables de la région marseillaise. Ph.D. thesis Universite Aix-Maréseille.