No abstract

Organisms develop through a series of stages leading to sexually mature adults. In a few cases ontogeny reversal is possible, but it does not occur typically after the onset of sexual reproduction. All stages of the medusa Turritopsis nutricula, from newly liberated to fully mature individuals, can transform back into colonial hydroids, either directly or through a resting period, thus escaping death and achieving potential immortality. This is the first metazoan known to revert to a colonial, juvenile morph after having achieved sexual maturity in a solitary stage. Selective excision experiments show that the transformation of medusae into polyps occurs only if differentiated cells of the exumbrellar epidermis and part of the gastrovascular system are present, revealing a transformation potential unparalleled in the animal kingdom.

Quantitative information about spatial patterns in subtidal hard substrate assemblages is scant. Such information is necessary to understand the responses to anthropogenic disturbances in these habitats. Along the coast of Apulia (Southern Italy), the collection of the European date mussel Lithophaga lithophaga is a strong source of disturbance: harvesting is carried out by demolition of the rocky substrate and causes epibiota disappearance. A hierarchical sampling design was used to quantify the spatial variability of subtidal epibenthic assemblages and the extent of rock damage due to L. lithophaga harvesting along 360 km of rocky coasts in Apulia. The surveyed coast was divided into 8 adjacent sectors, and replicate samples were taken by visual inspection at each of the 3 sites nested in each sector. Multivariate analyses indicated that assemblages differed consistently with spatial scale, variability being higher at the largest scale. However, variability among sites within each sector was also detected. Patchiness (i.e., average similarity among quadrats) was consistent among sectors. Some species were identified as 'important' in characterising and/or differentiating sectors. The pattern of distribution of these species as well as total cover and number of species were analysed by analysis of variance. Results recorded a considerable source of variation at site level. Damage by L. lithophaga fishing was shown to be extremely widespread. A humped relationship between patchiness and disturbances by L. lithophaga fisheries was obtained. In particular, patchiness at a small scale was highest at 'intermediate' levels of damage, because disturbance produces patches of different size and/or age, leading to 'mosaic' landscapes of epibenthic assemblages.

The date mussel Lithophaga lithophaga is a Mediterranean boring mollusc living in calcareous rocks. Its populations are intensely exploited by SCUBA divers, especially in southern Italy. Collection is carned out by demolition of the rocky substratum, so that human predation on date mussels causes the disappearance of the whole benthic community. The impact of this activity along the Apulian coast was evaluated by 2 surveys carried out by SCUBA diving inspection of the Salento peninsula. The Ionian coast of Apulia, from Taranto to Torre dell'orso (Otranto), was surveyed in 1990 and in 1992 by 2 series of transects (from 0 to 10 m depth, 2 km from each other), covering 210 km. Observations were transformed into an index of damage, ranging from 0 (no damage) to 1 (complete desertification). 159 km of the inspected coast are rocky. The first survey (1990) allowed us to estimate that a total of 44 km was heavily affected by this human activity (the index of damage ranging between 0.5 and l ] , whereas the second survey showed heavy damage along a total of 59 km. This Increase in length was accompanied by a high increase in the index of damage along parts of coast that were less intensely exploited in 1990 than in 1992. The second survey (1992) was extended to 90 km of the Adriatic coast of Apulia (from Otranto to Torre S. Sabina, Brindisi), with 47 km of rocky coast and a total of 10 knl heavily affected by date mussel fishery. Date mussel collection is one of the most destructive human activities along the Apulian coast and it has long-term effects because sea urchin grazing probably prevents recolonization of barren areas. Rarefaction of the resource due to overexploitation and habitat destruction caused rapid 'evolution' of the techniques of predation, with an lncrease in searching efficiency achieved by the use of underwater vehicles. Similar impacts of date mussel collection have been recorded in other parts of the Italian coast, especially Carnpania and Sardinia, and it is possible that such situations are common, but still undetected, in other areas of the Mediterranean.

The distribution of the 346 hydromedusan species hitherto recorded from the Mediterranean is considered, dividing the species into zoogeographical groups. The consequences for dispersal due to possession or lack of a medusa stage in the life cycle are discussed, and related to actual known distributions. There is contradictory evidence for an influence of life cycle patterns on species distribution. The Mediterranean hydromedusan fauna is composed of 19.5% endemic species. Their origin is debatable. The majority of the remaining Mediterranean species is present in the Atlantic, with various world distributions, and could have entered the Mediterranean from Gibraltar after the Messinian crisis. Only 8.0% of the fauna is classified as Indo‐Pacific, the species being mainly restricted to the eastern basin, some of which have presumably migrated from the Red Sea via the Suez Canal, being then classifiable as Lessepsian migrants. The importance of historical and climatic factors in determining the composition of the Mediterranean fauna of hydromedusae is discussed.