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 iso pod, feeds on coralline algae and breeds in the spongocoels of calcareous s ponges (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 b ody lengths of field-collected isopods against monthly sea surface temperat ures, as well as against the relative abundance of coralline algae (Coralli na; Amphiroa), over a 26-month period (1983-1985). We found that average bo dy lengths of the four adult morphs in this species (alpha-, beta-, gamma-m ales and females), were larger in cool months and smaller in warm months. O ur records of female reproductive condition at capture allowed identificati on of the approximate dates on which females matured, thus the effect of te mperature on growth was most clearly seen in females. Monthly average body lengths correlated negatively with temperature for all adult morphotypes, w ith 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 a t lower temperature (21 degrees C) matured at larger body lengths than indi viduals reared at higher temperatures (27-30 degrees C). However, well-fed individuals grew no more rapidly than poorly-fed individuals. Thus, variati on in food availability had no recognizable effect on isopod growth in eith er the laboratory or in the field, whereas variation in temperature signifi cantly influenced growth in both locations, with effects proportional to th e number of pre-adult molts. (C) 1999 Elsevier Science B.V. All rights rese rved.