Water movement activating fragmentation: a new dispersal strategy for hydractiniid hydroids Giorgio Bavestrello*, Stefania Puce*, Carlo Cerrano O , Laura Castellano P and Attilio Arillo O *Istituto di Scienze del Mare dell'Universita© di Ancona,Via Brecce Bianche, 60131Ancona, Italy. *E-mail: bavestrello@pocsi.unian.it. O Dipartimento per lo studio delTerritorio e delle sue Risorse dell'Universita© di Genova,Via Balbi 5, 16126 Genova, Italy. P Acquario di Genova, Area Porto Antico, 16100 Genova, Italy The fragmentation process in Hydractinia pruvoti Motz-Kossovska, 1905) which live on shells inhabited by hermit crabs is activated by the intensity of water movement that induces a rapid and abundant production of propagules by transversal ¢ssion. Being produced by all kinds of polyps in the polymorphic colony, fragments are very di¡erent from each other both in shape and size. After their liberation, fragments ¢rst settle on the bottom, then dedi¡erentiate and produce a network of hydrorhizae from which new polyps arise. Asexual reproduction is prevalent in hydroids and takes place by means of di¡erent strategies. Cloning can be produced accidentally due to injury of the colony, physiologically by budding, or by the autotomy of di¡erent portions of the colony. Budding is the most common mechanism of asexual reproduc- tion among hydroids. Buds generally arise from hydranths, branches or stolons producing both medusae and/or polyps. Stolonization is the formation of stolons from hydrorhizae or hydrocauli which grow and attach to another substratum near the parent colony and then segregate to form a new colony. In addition to the stolons, other fragments of colonies, such as frus- tules and free hydranths, may detach by autotomy giving rise to propagules with very di¡erent degrees of specialization for a review see Gravier-Bonnet,1992). In this work we report on the water activated fragmentation process of Hydractinia pruvoti with a description of the fate of their released fragments. Our original colony, living on the shells of Monodonta turbinata inhabited by the hermit crab Clibanarius erythropus, was collected by SCUBA divers at a depth of 10m along the coast of Alassio Ligurian Sea) in September 1998. They were reared in an aquarium ¢lled with ¢ltered natural seawater and fed daily with Artemia salina nauplii.Within a month, several colonies arose on the carbonatic fragments of the bottom. These colonies 50^55 polyps each), which were individually reared in cups of 250ml, were used for the water movement experiments. In our colonies polyps arose from a creeping hydrorhiza composed of a confused arrangement of coalescent stolons.Three main kinds of polyps were present: i) gastrozooids, 1^2 mm in height, with 5^10 ¢liform tentacles; ii) gonozooids, 1^2mm in height, characterized by a single, very extensible tentacle 0.5^2mm), arising laterally at the base of a vestigial hypostome. Each gonozooid bore 3^5 medusoids in the middle of its body; iii) dactylozooids, made up of a spherical body 0.5mm in diameter) which directly arises from the hydrorhiza and bears, at its apex, a long tentacle very similar to that of the gonozooids. Conical spines 0.2^0.3mm high were present too Figure 1A). The occurrence of anomalous gastrozooids with two tentacular rings and two hypostomes Figure1B) was also observed. When mature medusoids Figure 1C) only male specimens in our material) detach from their gonozooids, they vigorously swim for several days before the gamete release. They show four well-developed radial canals ending in the tentacular bulbs on the circular canal.The gonads are concentrated in a large masse around a non-functional manubrium. During the ¢rst phase of rearing, it was observed that increased water turbulence in the aquarium produced a dramatic increase in the number of detached fragments so it was decided to test colonies in water arti¢cially circulated.The water movement was obtained by means of a moving plane on which cups containing the hydroid colonies were placed. The rotary movement of the plane was adjusted in order to produce turbu- lence causing oscillation of the polyps without changing the position of the entire colony to avoid any mechanical injuries. Plotting the average number of fragments released vs the time of induced turbulence, the maximum production of ten propagules per day per 100 polyps was achieved. Colonies maintained in calm water produced fragments at a rate of about 0.5^1/d per 100 polyps. The main section of fragmentation were on the long tentacles of gonozooids and dactylozooids 48%) followed by gastrozooids 40%) and the body of gonozooids 12%). The ¢rst propagules settled four days after the start of fragmentation despite the continuous water movement, while, on the other hand, the fragmentation process continued for some days after the cessation of the water oscillation. Fifteen days after the start of the experiment seven days after the stop of water movement) the success of propagules settlement was very near 100%. The fragmentation process stems from a transversal ¢ssion acting at di¡erent levels both in gastro- Figure1D) gono- and dactylozooids which produce fragments of variable shape Figure 1E), while their basal portions remain adherent on the original substratum producing new hypostomes and tentacles Figure 1F). Fragments of gastrozooids were composed of almost entire polyps or mere tentacle rings which surrounded the hypostome.The gonozooids which had detached from the mother colony immediately released mature medusoids, and proceeded to undergo either body or long tentacle fragmentation, giving origin to planula-like propagules. The fate of the medusoids released during the fragmentation process di¡ers according to their developmental stage: mature medusoids actively swim and spawn, while immature medusoids undergo a regressive Journal of the Marine Biological Association of the United Kingdom 2000) J. Mar. Biol. Ass. U.K. 2000), 80,361^362 Printed inthe United Kingdom