Dr. Cancino-Cancino, Juan

The mussels Perumytilus purpuratus and Semimytilus algosus are two dominant species of intertidal rocky shores of central Chile. These species have marked differences in their distribution patterns with P. purpuratus dominating the mid-intertidal zone and S. algosus dominating the lower intertidal zone usually in habitats influenced by sand. Although it has been suggested that differences in tolerance to environmental conditions, such as air exposure and presence of sand, can explain the distribution of these species, there are currently no experimental studies to support such hypotheses. Here, we evaluated the growth and survival rate in the field of both mussel species at four different tidal heights: 25, 75, 135, and 175 cm above the zero tide. In addition, filtration rates were estimated for both species in the presence and absence of sand in laboratory conditions. The results showed that shell and wet weight growth rates of P. purpuratus were highest in mid- and medium-high tidal heights, whereas the growth rate of S. algosus was highest in the medium-low level. Similar pattern was observed for survival percentage. Furthermore, small S. algosus individuals cultivated with sand in suspension had significantly higher filtration rates than P. purpuratus. In large individuals, no differences were observed between the two species nor between treatments (presence and absence of sand). These results indicate that the differences in the distribution patterns of P. purpuratus and S. algosus in the intertidal can be explained by differences in physiological tolerances to both air exposure and to the presence of sand.

Epifaunal polychaetes inhabit a range of habitat structures built by other organisms, such as ascidians. Here, we examine: i) the polychaete fauna inhabiting aggregations of the ascidian Pyura chilensis in central Chile; ii) the relationship between sample volume (aggregation size) and polychaete assemblage variables; and iii) the effect of a tube-building chaetopterid on the polychaete assemblage structure. The chaetopterid tube load on aggregations determines two ascidian morphotypes, those with a high load of chaetopterid tubes (HT morphotype) and those with a low load of chaetopterid tubes (LT morphotype). From a total of 38 aggregations studied, we found 5,524 specimens belonging to 35 species of polychaetes. Three species were the most abundant in the aggregations (Phyllochaetopterus socialis, Nicolea lobulata, and Typosyllis magdalena), reaching 22% of total abundance. The number of species and individuals increased with sample volume, but only the number of species number varied between morphotypes. Sample volume and the chaetopterid tubes influenced the polychaete assemblage structure, evidencing differences between morphotypes. We suggest that both sample volume and the habitat structuring capacity of the chaetopterid tubes change the habitat complexity of the ascidian aggregations and, hence, produce differences between morphotypes related to the polychaete assemblage structure.