Effects of temperature and food availability on adult body length in natural and laboratory populations of Paracerceis sculpta (Holmes), a Gulf of California isopod

In the Gulf of California, Paracerceis sculpta (Holmes), a sphaeromatid isopod, feeds on coralline algae and breeds in the spongocoels of calcareous sponges (Leucetta losangelensis de Laubenfels). Near Puerto Penasco, Sonora, sea surface temperature and algal abundance fluctuate throughout the year. To investigate how these factors influence isopod growth, we plotted the body lengths of field-collected isopods against monthly sea surface temperatures, as well as against the relative abundance of coralline algae (Corallina; Amphiroa), over a 26-month period (19831985). We found that average body lengths of the four adult morphs in this species (α-, β-, γ-males and females), were larger in cool months and smaller in warm months. Our records of female reproductive condition at capture allowed identification of the approximate dates on which females matured, thus the effect of temperature on growth was most clearly seen in females. Monthly average body lengths correlated negatively with temperature for all adult morphotypes, with different slopes and intercepts; morphotypes undergoing more molts had steeper slopes. Coralline algae abundance showed no seasonal patterns, and we found no significant relationships between relative algal abundance and isopod body length for any adult morph. In the laboratory, animals reared at lower temperature (21°C) matured at larger body lengths than individuals reared at higher temperatures (27-30°C). However, well-fed individuals grew no more rapidly than poorly-fed individuals. Thus, variation in food availability had no recognizable effect on isopod growth in either the laboratory or in the field, whereas variation in temperature significantly influenced growth in both locations, with effects proportional to the number of pre-adult molts.