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

Acantharians are 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 preservatives. As a result, 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 high-throughput sequencing, to investigate acantharian abundance, vertical distribution, and life-history in the western North Pacific. Observed concentrations of acantharian cells correlated well with sequence abundances from acantharians with known, recognizable morphologies, but not to sequences from those without known morphology (basal environmental clades). These results suggest basal clades may lack characteristic star-shaped skeletons or are much smaller than known acantharians. The decreased size-range of acantharians imaged at depth supports current hypotheses regarding asymbiotic acantharian life cycles: cysts or vegetative cells release reproductive swarmer cells at depth and juvenile cells grow as they ascend towards the surface. Moreover, sequencing data present the possibility that photosymbiotic acantharians also reproduce at depth, like their asymbiotic, 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.