The Light-Dependent Behaviour of Planula Larvae of Eunicella Singularis and Corallium Rubrum and its Implication for Octocorallian Ecology

Abstract

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.

Quantification of larval traits driving connectivity: the case of Corallium rubrum (L. 1758)

“…According to our results, light conditions do not affect activity frequency or swimming speed, confirming the lack of phototaxis of C. rubrum larvae observed by Weinberg (1979). In contrast, this study indicates that activity frequency increases after kinesthetic stimulation, as observed by Guizien et al (2006) in the polychaete Owenia fusiformis larvae.…”

“…Our results show an opposite trend that could be explained by a different composition of stored lipid in C. rubrum planulae, such as triacylglycerols, which are denser than seawater (Lee et al 2006;Harii et al 2007). The results of previous experiments showing an upward movement of C. rubrum larvae (Weinberg 1979) should then be interpreted as a consequence of active upward swimming behavior. Although surprising, high frequencies of activity are not uncommon in species with aposymbiotic lecithotrophic planula larvae (Dendronephthya hemprichi, Dahan and Benayahu 1998; Acropora latistella, Montastraea magnistellata, Favia pallida, Goniastrea aspera and Pectinia paeonia, Graham et al 2013).…”

Population dynamics and conservation biology of the over-exploited Mediterranean red coral

“…Each colony is a modular animal composed by several genetically identical modules (polyps). Each female and fertile polyp can brood and release lecitothrophic larvae (planulae) which settle near their parent colonies producing a new individual (recruit) (Vighi, 1972;Weinberg, 1979). Thus, to calculate the average number of planulae produced by a colony (b) in a certain size-age class, the following parameters have been estimated (Santangelo et al, 2004) q is the fecundity (number of larvae produced by each colony), F the fertility (percentage of fertile female colonies), sr the sex ratio, P the number of polyps in each colony so that b is calculated as the product of all parameters above…”

“…Red coral is a long-lived, gonochoric, brooder species, whose lecithotrophic larvae (planulae) do not travel very far from their parent colonies (Vighi, 1972;Weinberg, 1979). This species is structured in several genetically differentiated, self-seeding units (populations) (Abbiati et al, 1993) that are composed by several overlapping generations.…”

Phylogeography of the red coral (Corallium rubrum): inferences on the evolutionary history of a temperate gorgonian

“…C. rubrum has also been affected by mortality events linked with positive thermal anomalies in 1999 and 2003 (Garrabou et al 2009) which may have impacted the population dynamics of this species (Santangelo et al 2007). C. rubrum is a sessile species with relatively short larval phase (4-12 days in aquaria ;Vighi 1972;Weinberg 1979) and therefore, presumably low dispersal abilities. C. rubrum is an appropriate model to study the genetic consequences of past and current environmental fluctuations.…”