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Bianchi, C.N.; Morri, C.; Pronzato, R. (2014). The other side of rarity: recent habitat expansion and increased abundance of the horny sponge Ircinia retidermata (Demospongiae: Dictyoceratida) in the southeast Aegean. Italian Journal of Zoology. 81 (4): 564-570.
Bianchi, C.N.; Morri, C.; Pronzato, R.
The other side of rarity: recent habitat expansion and increased abundance of the horny sponge <i>Ircinia retidermata</i> (Demospongiae: Dictyoceratida) in the southeast Aegean
Italian Journal of Zoology
81 (4): 564-570
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Environmental change is commonly considered as a driver for the extinction of rare species. This belief, long established on land, may not apply to marine species. Dramatic environmental change in the shallow marine ecosystems of Kos, an island in the Aegean (east Mediterranean Sea) caused algal reefs to shift to sponge reefs. Among the sponge species that gained supremacy on Kos reefs, the Mediterranean endemic Ircinia retidermata was previously a rare species. Comparing surveys carried out in 1981 and 2013 by the same method (time-based visual census along random paths by scuba diving), in the same sites, by the same people, showed that I. retidermata increased its overall abundance by one order of magnitude, and expanded its occurrence to all the habitats examined. This outcome contradicts the current common belief that rare species in semi-enclosed seas are prone to extinction. Besides being the state preceding final extinction, rarity could represent the source of variation that marine ecosystems need in order to face environmental change. However, for many marine invertebrates, and especially sponges, inferred rarity may simply be the result of insufficient investigation. This study represents an attempt to assess change with time in a rare sponge species’ abundance using visual census by scuba diving.