In the pearl cultivation farms of the Ehime Prefecture, Japan, mass mortalities of the pearl oyster
Pinctada fucata have occurred since 1994. The occurrences of mass mortality roughly coincided with a shift of the dominant phytoplankton
from
Skeletonema and
Chaetoceros to
Chaetoceros and
Nitzschia all of which belong to Bacillariophyceae. Hence, we evaluated
Nitzschia, together with
Chaetoceros and
Isocrysis, as food for the oyster. Wet weights, lengths, widths, glycogen contents, and growth rates in terms of wet weight of the
oysters in all the feeding treatments were significantly higher than those in the non-feeding treatment. The highest glycogen
content (2.34%) and growth rate (2.21 g month
−1) were found in the
Chaetoceros treatment. Growth rate in the
Isocrysis treatment (1.63 g month
−1) was also high, although glycogen content in this treatment (0.41%) was low. In the
Nitzschia treatment, growth rate of the oyster (0.94 g month
−1) was the lowest and glycogen content (0.83%) was also low relative to that in the
Chaetoceros treatment. Chlorophyll
a concentration in fecal pellets was lowest in the
Nitzschia treatment (<2.7 μg mg
−1), suggesting more complete digestion of
Nitzschia by the oyster. Thus,
Nitzschia was edible and digestible but not assimilated by
P. fucata. We propose the following scenario for the relationship between
Nitzschia dominance and mass mortality. When
Nitzschia dominates in a culture area, the physiological condition of
P. fucata deteriorates due to low assimilation of
Nitzschia by the oyster, followed by susceptibility of the oyster to infection by agents lethal to the oyster.
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