Stable isotopic analysis of Barnacle larvae and their faecal pellets to evaluate the ingested food

Stable isotopes of carbon and nitrogen are useful in the evaluation of food web dynamics and have been used extensively in the feeding studies. Barnacles are dominant component of the intertidal and biofouling communities. Life cycle of barnacles include planktonic naupliar stages followed by a nonfeeding pre-settling cyprid stage. Development success of the nauplii depends on the food available for them in the natural environment. However, the food preference of the nauplii in the natural environment is still not well understood. In this study experiments were carried out initially to assess the C and N fractionation between the food and the Balanus amphitrite nauplii that were fed with Chaetoceros calcitrans. Laboratory reared larvae fed with Chaetoceros calcitrans diet pointed out an enrichment in C by 0.95‰ and N by 1.42‰. The enrichment level in the faecal pellets was comparatively higher than that obtained from the larvae either fed or starved indicating the role of heterotrophic organisms. Expanding on these observations, efforts were made to evaluate the food of the acorn barnacle larvae collected from the wild during different seasons. The δC ratios of the larvae and their faecal pellets obtained during the post-monsoon season were heavier compared to pre-monsoon season indicating the feeding of larvae on isotopically heavier food sources during the post-monsoon season. However, the δC and δN ratios of the larvae and their faecal pellets did not differ significantly between the seasons and this could possibly be due to the broad range of isoptopic signatures observed at the base of the food web.