AUTHOR=de Silva Darren , Gobler Christopher J. 

TITLE=Effects of the harmful alga Margalefidinium (aka Cochlodinium) polykrikoides on clearance rates of the hard clam, Mercenaria mercenaria

JOURNAL=Frontiers in Marine Science

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1252540

DOI=10.3389/fmars.2023.1252540

ISSN=2296-7745

ABSTRACT=<p>Harmful algal blooms (HABs) such as those formed by the ichthyotoxic dinoflagellate<italic>, Margalefidinium</italic> (aka <italic>Cochlodinium</italic>) <italic>polykrikoides</italic> can have adverse effects on bivalves. While <italic>M. polykrikoides</italic> has caused significant die offs of bivalves and other marine organisms, the Northern quahog or hard clam, <italic>Mercenaria mercenaria</italic>, is comparatively more resistant to this HAB. This study quantified clearance rates of juvenile hard clams (10-20 mm) exposed to three different North American populations of <italic>M. polykrikoides</italic> (bloom, strain CP1, strain CPSB-1G) as well as the nonharmful cryptophyte, <italic>Rhodomonas salina</italic> and the nonharmful dinoflagellate, <italic>Gymnodinium aureolum</italic>, in single and mixed algal exposures. Multiple biovolume exposures with <italic>M. polykrikoides</italic> bloom water and <italic>R. salina</italic> (1,000, 1,500, 3,000 cells mL<sup>-1</sup><italic>M. polykrikoides</italic> biovolume equivalent) were completed to assess the effects of increasing biomass on hard clam clearance rates and selection. Hard clams opened and actively cleared algal mixtures at and below 1,000 <italic>M. polykrikoides</italic> cells mL<sup>-1</sup>. During single species exposures, strain CPSB-1G and <italic>R. salina</italic> were cleared significantly faster than wild <italic>M. polykrikoides</italic> populations and strain CP1. During mixed exposures, <italic>R. salina</italic> was cleared significantly faster than CPSB-1G but not other <italic>M. polykrikoides</italic> populations and there was no difference between hard clam clearance rates of <italic>G. aureolum</italic> and <italic>R. salina</italic>. Clearance rates of <italic>M. polykrikoides</italic> at ≥1,500 cells mL<sup>-1</sup><italic>M. polykrikoides/R. salina</italic> mixtures were not significantly different than zero unlike clearance of those at &lt;1,000 cells mL<sup>-1</sup> indicating a density dependent effect of blooms. Collectively, the results demonstrate that hard clams can actively clear <italic>M. polykrikoides</italic> cells at moderate (≤1,000 cells mL<sup>-1</sup>) but not elevated (&gt; 1,000 cells mL<sup>-1</sup>) bloom densities. Given this, and the documented survival of hard clams during blooms, <italic>M. mercenaria</italic> may be candidate for aquaculture and restoration in regions prone to HABs caused by <italic>M. polykrikoides</italic>.</p>