Differences in gonad somatic indices with depth in Pacific Northwest invertebrates

Abstract

Drift algal material exported from kelp beds plays a significant role in nearby ecosystems. Phenotypic traits such as gonad index and jaw diameter can be indicators of diet consistency and nutrient uptake and thus can be used to assess the possible role of exported kelp. In this study, individuals of an herbivorous (Strongylocentrotus droebachiensis) and a filter feeding (Balanus nubilus) species were sampled from near the kelp beds in the shallow subtidal photic environment (SSPE) and far from the kelp beds in the deep subtidal environment (DSE). Urchins in the DSE had significantly larger gonads and jaws in comparison to individuals in the SSPE. I conclude that urchins in the DSE were getting better fed, whether through quantity or quality of food, suggesting that drift algae was being exported from nearby kelp beds. There was no significant difference between barnacle gonad indices with depth suggesting that their particulate organic matter food was equal at both depths. Future studies could examine gut contents to determine if there are diet differences between the two depths.