Guillen C., Marquez-Farias J.F, Avila-Poveda O.H
The Bergmann’s Rule (described first for endothermic organisms, later for ectothermic taxa) established that intraspecifically the organisms of populations which live at colder climate (high latitudes) tend to have larger body sizes than those which live at warmer climate (low latitudes); and the Temperature-Size Rule (which described the relationship between growth of ectotherms and the environmental temperature) established that larger organisms can be found in colder environments and they are going to grow more slowly. For ectotherms inhabiting the intertidal zone such as chitons, temperature and latitude can influence life-history traits as growth. The Polyplacophora mollusk Chiton articulatus, is distributed along 8° latitudinal at the Mexican Tropical Pacific, between Sinaloa 23°N (high latitude at 25°C, in mean annual sea surface temperature, SST) and Oaxaca 15°N (low latitude at 28°C SST). Chitons were collected monthly for one year in seven sites along its whole geographic distribution, after scleritome length (mm) was measured and size frequency distributions were generated for all locations (Sinaloa, Nayarit, Jalisco, Colima, Michoacan, Guerrero, and Oaxaca). The chiton population body size was greater at high latitude and decreases towards low latitude, exhibiting a fragmented latitudinal pattern in five areas according to the Kruskal-Wallis test, those were a northern group of SIN that belong to the southern border of the subtropical zone; subsequently, a second northern group of NAY that belong to the northern border of the tropical zone; a middle group conform by JAL, COL, and MIC, which belongs to the Mexican Tropical Pacific ecoregion; the fourth group of GRO; and a southern group of OAX exclusively, belonging to the northern border of the Chiapas-Nicaragua tropical ecoregion. These results follow and support both the Bergmann’s Rule and the Temperature-Size Rule, besides the size groups observed of this edible Chiton articulatus coincide with each regional SST.
Acknowledgments: This research took place within the framework of the Project quitón del Pacífico tropical mexicano (see, DOI: 10.3897/rio.6.e60446); Specimen collection was done under a research permit (No. PPF/DGOPA-130/15) granted by SADER and CONAPESCA.