Responses of Crassostrea virginica (Gmelin) and C. ariakensis (Fujita) to bloom-forming phytoplankton including ichthyotoxic Karlodinium veneficum (Ballantine).

Abstract

In the last several decades, the Eastern oyster (Crassostrea virginica) fishery in Chesapeake Bay has collapsed, largely because of overfishing and disease. As a consequence, there is increasing interest in the introduction of the Suminoe oyster Crassostrea ariakensis, which initial experiments suggest grows more rapidly and is more disease tolerant than the native oyster. In the present study, growth and clearance rates of the two oyster species at two different ages were compared for bloom-forming algae typical of the Chesapeake Bay, the ichthyotoxic Karlodinium veneficum and the spring blooming Prorocentrum minimum. Growth rates of 3–14 day-old spat of both species fed K. veneficum were severely depressed compared with growth rates for spat fed P. minimum or a hatchery phytoplankton mixture (Reed Mariculture Formula). Growth in C. ariakensis was more negatively affected by the toxic dinoflagellate than the native oyster. The ichthyotoxic dinoflagellate also appeared to reduce organ development in the developing spat even though clearance rates (pg C μm−1 h−1) for spat from both oyster species were similar for all food sources. When older juveniles (1–2 cm) were placed in bloom densities of Karlodinium (∼3 × 104 cells mL−1) for 6 h each day for 5 days, clearance rates were severely depressed for both oysters compared with rates noted for Tetraselmis, with clearance rates being reduced almost a hundred-fold for C. ariakensis. A nontoxic strain of K. veneficum was cleared at similar rates to Tetraselmis. These results suggest that the increasingly more common blooms of the ichthyotoxic dinoflagellate Karlodinium veneficum may preferentially impact the nonnative oyster more than the native after first set as well as later as it matures. If feeding on this Chesapeake Bay toxic dinoflagellate extends the period of time that new settled individuals remain small in size, the population structure of oyster reefs (e.g., through elevated losses to predation or siltation) may be ultimately altered.