Utility of salps as a baseline proxy for food web studies

In recent years, pelagic tunicates (mostly salps, but potentially doliolids, appendicularians and pyrosomes as well) have been used in isotopic studies as a baseline consumer (trophic position 2) when recreating food web dynamics to overcome the challenges of using particulate organic matter (POM). While pelagic tunicates are continuous filter feeders, recent evidence has shown that they have selective feeding behaviors, and preferentially assimilate certain particles. In this review, we combine available stable isotope data for POM and pelagic tunicates and identify that trophic enrichment in 13C and 15N relative to POM is highly variable, and suggests tunicates prefer to consume smaller, heterotrophic organisms. Here we propose that it is not appropriate to consider pelagic tunicates as representative first level consumers in the classical pelagic food web in stable isotope analyses. Rather it needs acknowledgment that they are members of the microbial food web, and thus reflect an alternate food chain.