The medusae are sharply individual; they can be gathered by winds and currents to form extensive swarms, but it is not known if they have any kind of social interaction while being

Stinging hydroids (Lytocarpus philippinus). (Photo by A. Flowers & L. Newman. Reproduced by permission.)
The species Turritopsis nutricula in its hydroid stage. (Photo by S. Pi-raino [University of Lecce]. Reproduced by permission.)

in close contact. The colonies of Hydrozoa, especially polymorphic ones, have been compared to superorganisms because of the complexity of the functions that they perform through the different types of zooids. The zooids of a colony usually derive from a single planula, thus being interconnected members of a single clone. It is possible, however, that different colonies merge their tissues or that different planulae aggregate to form coalescent colonies. In these cases, different individuals can be in such close connection that they become a single individual, possibly one of the extreme forms of social organization.

Most Hydrozoa have separate sexes. Fertilization is usually internal, with no copulation. Males spawn in the water and the sperm actively swim toward the eggs that are still on the maternal body (either a medusa or a polyp colony). The Hydrozoa are the first animals in which sperm attractants have been demonstrated, with species-specific attraction of sperms by the eggs. For many medusan species, both males and females spawn in the water, where fertilization occurs. Also in this case, however, sperm attractants facilitate gamete encounter.

The members of the same colony perform coordinated behaviors that surely involve communication. In Thecocodium brieni, for instance, the dactylozooids catch the prey with their tentacles, while the gastrozooids perceive that a prey is available and stretch towards the dactylozooids, detaching the prey from their tentacles and ingesting it. This division of labor, involving great coordination, is frequent in polymorphic colonies.

Planktonic animals do not show particular territoriality, being, by definition, transported by currents. It is probable, however, that medusae actively prevent being in too close contact to avoid competitive interactions while foraging. Terri-toriality is very strong in sessile organisms, where competition for the occupation of space is very evident. The arrangement of dactylozooids on the edge of many colonies is related to the defense of the territory from overgrowth by nearby animals. The feeding polyps can eat the settling larvae of potentially competing species, thus preventing competition for space.

When unfed, both polyps and medusae are always in search for food, with species-specific activity patterns. When the coe-lenteron is full of food, tentacles are usually contracted and do not catch prey, showing some control of cnidocyst discharge. Many medusae perform daily migrations through the water column.

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