Sponge assemblages were examined simultaneously in 2 shallow habitats (consolidated rocks and boulder tops) in May and November 2002 in Mazatlán Bay. Important differences were found between the assemblages in the 2 habitats, despite them being only 40–50 m apart. Total (35) and exclusive (25) species richness were higher on boulders than on consolidated rocks (22 and 12, respectively), and only 10 species occurred in both habitats simultaneously. Non-metric multidimensional scaling (nMDS) in combination with analysis cluster revealed a disjunct pattern in the distribution of sponges between habitats. These analyses also showed that the structure of these assemblages varied less over time in consolidated rocks (60% similarity between times) than in the boulders (40% similarity). In fact, the number of species was less variable on consolidated rocks (from 15 to 16 species from May to November) than on boulders (from 12 to 32). The patterns presented here show that the sponge assemblages were not homogeneous in rocky habitats but varied over small spatial scales. The assemblages were more stable on consolidated rocks (the most stable habitat), but diversity was higher in the unstable habitat than on consolidated rocks. However, although the habitat type played an important role in explaining these differences, variables covarying with depth, such as water movement and habitat stability, constituted major environmental factors responsible for the differences observed in sponge assemblage structure between habitats.

Nomenclature: Hooper & van Soest, 2002.