Body size structure, biometric relationships and density of Chiton albolineatus (Mollusca: Polyplacophora) on the intertidal rocky zone of three islands of Mazatlan Bay, SE of the Gulf of California

Populations of the polyplacophoran mollusk Chiton albolineatus were studied at 6 sites with different wave exposure of the rocky shores of 3 islands of Mazatlan Bay (southeastern side of the Gulf of California). This chiton species is endemic to the Mexican Pacific coast. Chitons were sampled on wave-exposed and wave-protected sites in the intertidal zone of these islands from January to December 2008 to determine its demographic patterns based on density and body size. Length (L), breadth (B) and height (H) of chiton body ranged between 8.3 and 55.1 (X= 28.9 ± 6.2), 3.0-30.4 (X= 14.8 ± 3.4) and 0.9-19.1 (X= 5.0 ± 1.8) mm, respectively, and their wet weight (W) varied from 0.1 to 10.2 g (X= 1.7 ± 0.9). The biometric relationships were B= 0.478(L) + 1.212, H= 0.198(L) - 1.170 and H= 0.389(B) - 1.277, for size and W= 0.0002(L)^2.710, W= 0.0008(B)^2.743 and W= 0.0792(H)^1.908 for weight. The value of the slope of the length-weight relationship was significantly lower than 3, which indicates negative allometric growth. Average densities ranged from 0.1 to 2.9 org. m^-2 and were higher in the exposed sites than in the protected sites of each island, possibly because of a lower predation pressure in areas exposed to high wave energy than in sheltered areas, while its body size were larger in the exposed sites of two islands. This could be interpreted as a result of migration from sites protected from the wave action, where survival of smaller individuals is more likely than in exposed areas.