ABSTRACT1. Mass mortality events are becoming more common all over the world, both in tropical and temperate seas. An extensive mortality occurred in the Mediterranean Sea in 1999, affecting many benthic species, mainly sponges and gorgonians.2. The recovery of a population of the sea fan Paramuricea clavata, for a period of 3 yr, from 1999 to 2002, was studied by both line transects and fixed frames. The average size of the colonies decreased, indicating a size-dependent mortality episode, but their density, as a result of successful recruitment, was not altered after 3 yr.3. P. clavata showed three recovery patterns: (i) sexual reproduction, (ii) coenenchyme regeneration and (iii) fragmentation of affected branches. Moreover, the growth rates of small colonies varied in the different years. The sex ratio of the population was also altered, with females being more affected than males; the population studied showed a significant male bias (3.3:1, n ¼ 150), varying greatly from the typical sex ratio (1:1) previously recorded in the same population before the mass mortality event.

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Mineralogy of substratum plays several roles on benthic organisms, affecting both their distribution and abundance. Here we studied the long-term effect of biogenic limestone, travertine and quartz on the growth of the massive stage of the boring sponge Cliona nigricans. Experiments were performed in the field, on a detritic bottom 40 m depth, where a population of the massive C. nigricans lives. Experimental sets of sponges with the same dimensions were maintained for 500 days on the three different substrata and a comparison among relative growth was performed. At the end of the experiment all the sponges incorporated gravels but the specimens buried into calcareous particles, both biogenic and not, and increased in size while dimensions of the specimens buried into quartz sand decreased. Our results show a negative effect of quartz towards the growth of this species and highlight once again that mineral composition of substratum can affect morphogenetic processes.

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