Selective feeding and linkages to the microbial food web by the doliolidDolioletta gegenbauri

Abstract

Gelatinous zooplankton play a crucial role in pelagic marine food webs, however, due to methodological challenges and persistent misconceptions of their importance, the trophic role of gelatinous zooplankton remains poorly investigated. This is particularly true for small gelatinous zooplankton including the marine pelagic tunicate, Dolioletta gegenbauri. D. gegenbauri and other doliolid species occur persistently on wide subtropical shelves where they often produce massive blooms in association with shelf upwelling conditions. As efficient filter feeders and prodigious producers of relatively low-density organic-rich aggregates, doliolids are understood to contribute significantly to shelf production, pelagic ecology, and pelagic-benthic coupling. Utilizing molecular gut content analysis and stable isotope analysis approaches, the trophic interactions of doliolids were explored during bloom and nonbloom conditions on the South Atlantic Bight continental shelf in the Western North Atlantic. Based on molecular gut content analysis, relative ingestion selectivity varied with D. gegenbauri life stage. At all life stages, doliolids ingested a wide range of prey types and sizes, but exhibited selectivity for larger prey types including diatoms, ciliates, and metazoans. Experimental growth studies confirmed that metazoan prey were ingested, but indicated that they were not digested and assimilated. Stable isotopic composition (δ 13 C and δ 15 N) of wild-caught doliolids, during bloom and non-bloom conditions, were most consistent with a detrital-supplemented diet. These observations suggest that the feeding ecology of D. gegenbauri is more complex than previously reported, and have strong and unusual linkages to the microbial food web.

Pelagic tunicate grazing on marine microbes revealed by integrative approaches

“…Degraded DNA can lead to underestimates of prey abundances (Nejstgaard et al 2008), thus should be quantified. For example, Frischer et al (2014) tested the degree to which doliolids degrade diatom DNA using a differential length amplification qPCR (dla-qPCR) assay and found little DNA degradation, which supported additional studies that compared the doliolid diatom concentration factor across many life stages (Walters et al 2019) and numerous other phytoplankton taxa (Frischer et al 2021). Such analysis of DNA degradation levels would be a precursor to studies in other pelagic tunicate taxa and microorganisms.…”

“…Sequencing-based approaches can be applied to grazing questions in two ways. First, direct sequencing of dissected tissue from feeding organs of pelagic tunicates can reveal the taxonomic range of grazed prey and any feeding selectivity (Walters et al 2019;Frischer et al 2021;Thompson et al 2021). Second, coupling of sequencing to in situ incubation experiments can reveal diversity in retained prey, clearance rates, and enable comparative analysis between coexisting tunicate taxa (Dadon-Pilosof et al 2017).…”

“…For example, qPCR applied to appendicularian feeding revealed the highest ingestion efficiencies for the smallest phytoplankton species and showed how small prey species inhibit ingestion of larger algae when both prey are present (Troedsson et al 2007). Most recently, qPCR assays for Bacillariophyta (diatoms), Haptophyta, and Cryptophyta were developed and applied to doliolids across their life stages (Frischer et al 2021), which revealed grazing preference for diatoms but the ingestion of diverse microbial prey.…”

“…Small actively growing gonozooids concentrated diatoms most, suggesting that diatoms fuel the growth of doliolid blooms. A follow-up study addressed selective feeding across doliolid life stages during bloom and non-bloom conditions through multiple techniques (Frischer et al 2021). Molecular sequencing and qPCR from wild-caught doliolids showed a range of prey types and sizes, but larger prey (including large diatoms, ciliates, and metazoans) comprised most of the recovered gut sequences.…”

Selective feeding in Southern Ocean key grazers—diet composition of krill and salps

“…This shift may favor salps over krill, as krill are less effective than salps at feeding on small cell sizes 31 . Recent studies found indications for selective feeding of doliolids, a group closely related to salps, in the Atlantic Ocean 32 , and suggested that the diet of Mediterranean salps is determined by prey taxonomy, rather than size 33 . Yet, there are no recent studies on the diet of salps in the Southern Ocean and studies that simultaneously compare the diet composition of krill and salps from the same region are lacking.…”