The life cycle of the gorgonian Eunicella singularis has been studied with emphasis on larval behaviour, metamorphosis and annual growth. Planulae are found to have a mobile phase lasting from several hours to several days. Once settled, they metamorphose into a complete primary polyp in approximately four days. In the first year, budding will yield colonies of a height between 10 and 30 mm. Subsequently, average growth rates range from 14 to 33 mm year-1. Death may be due to several causes. Predators may partly denude the gorgonian branches, thus facilitating the settlement of epibionts, which in turn may invade the entire skeleton, slowly pushing back the living tissue of the gorgonian. Colonies may also be torn off their substratum by wave or current action, this process sometimes being speeded up when tall epibionts such as fast growing bryozoans enhance resistance to water movement. Once toppled, the gorgonians die by necrosis of their living tissues, or by being buried under sediment. Colonies of E. singularis are estimated to reach an age of approximately 25 to 30 years. Some data have been obtained on growth rates and life spans of two other Mediterranean gorgonians, Lophogorgia ceratophyta and Paramuricea clavata.
The autecology of eleven mediterranean octocoral species (3 Stolonifera, 4 Alcyonacea, 4 Gorgonacea) was studied near Banyuls-sur-Mer (southern France). Field observations were carried out by means of SCUBA-diving in forty underwater stations. The ecological amplitude of each species was determined for a number of abiotic factors, viz. water temperature, submarine irradiance, water movement, slope of substratum and sediment. In this way the ecological niche for the different species was defined. The niches of several species partly overlap: the survival “strategy” of these species was defined as “competitivity”. Other species display “specialization”, a strategy by which they avoid competitive situations. A third survival mechanism is “opportunism”, encountered in species which are extremely tolerant with respect to their environment. Although experimental work is needed to determine how niche selection occurs, larval and juvenile stages are considered to be critical in this respect.
The behaviour of Mediterranean octocoral planulae was studied in light-dark situations and in a light gradient. Larvae of Eunicella singularis (Esper, 1794) reacted photopositively but it is uncertain which mechanism (klinotaxis or klinokinesis) determines this property. The blind larvae probably possess a dermal light sense, but it cannot be excluded that the yolk contains photosensitive carotenoids while the symbiotic zooxanthellae may also play a role. The photopositive behaviour of planulae of this species explains some aspects of the distributional ecology of adult colonies. It was also found that for the induction of settlement and metamorphosis the chemical properties of a given substratum seem to be far more important than its roughness. Larvae of Corallium rubrum (Linnaeus, 1758) are geonegative and indifferent to light. This latter fact is surprising, since in nature the colonies are exclusively found in dark places. It is supposed, therefore, that tolerance of the colonies rather than larval choice determines light-dependent zonation of this species in nature.
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