Paired high‐throughput, in situ imaging and high‐throughput sequencing illuminate acantharian abundance and vertical distribution

Abstract

Acantharians (supergroup Rhizaria) can be important contributors to surface primary production and to carbon flux to the deep sea, but are often underestimated because their delicate structures are destroyed by plankton nets or dissolved by chemical fixatives. As they are also uncultured, relatively little is known about acantharian biology, especially regarding their life cycles. Here, we take a paired approach, bringing together high‐throughput, in situ imaging and metabarcode sequencing, to investigate acantharian abundance, vertical distribution, and life history in the western North Pacific. Concentrations of imaged acantharian cells correlated well with relative abundances of 18S rRNA gene sequences from acantharians with known, recognizable morphologies, but not to sequences corresponding to acantharians with unknown morphology. These results suggest that morphologically undescribed clades may lack the characteristic star‐shaped acantharian skeleton or are much smaller than described acantharians. The smaller size of acantharians imaged at depth supports current hypotheses regarding nonsymbiotic acantharian life cycles: cysts or vegetative cells release reproductive swarmer cells in deep water and juvenile cells grow as they ascend toward the surface. Moreover, sequencing data present the possibility that some photosymbiotic acantharians may also reproduce at depth, like their nonsymbiotic, encysting relatives, which is counter to previous hypotheses. Finally, in situ imaging captured a new acantharian behavior that may be a previously undescribed predation strategy.

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