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Marine invertebrates use a range of
physical, biological and chemical signals to influence their
metamorphosis and larval settlement. In the abalone hatchery
environment, these could include food sources and appropriate
substrates.
To determine what combination of substrate
and food source is best for abalone larvae, Rolando Gapasin and Bernice
Polohan of SEAFDEC/AQD subjected Haliotis asinina postlarvae to
different “substrate-diatom” complexes, together with gamma-aminobutyric
acid or GABA. Substrates made of plexiglass, rubberized canvas,
fibrocement board and corrugated plastic were placed on the floor bottom
of four plexiglass aquaria. Each of these were then inoculated with the
following diatom species: Amphora sp., Nitzchia cf. frustulum,
a 1:1 combination of Amphora and Nitzchia, and diatom
“slurry” composed of Amphora, Coscinodiscus, Coconeis,
Nitzchia, Diploneis and Mastolia.
Based on the results of two trials, abalone
larvae apparently preferred their diatom slurry served over roughened
plexiglass in combination with GABA; around 27% and 20%, respectively,
of abalone larvae metamorphosed using this combination of
substrate-diatom complex. The others did not fare as well: among the
diatoms, Nitzchia cf. frustulum gave the lowest percentage of
metamorphosed larvae, while fibrocement was the lowest-testing of the
four substrates.
What accounted for the difference? The
proponents speculate that the amount of extracellular substances
produced by Nitzchia cf. frustulum may not have been enough to
sustain the abalone postlarvae. This species may have exhibited low
digestion efficiency, or that sub-optimal culture conditions may have
contributed to poor growth of the diatom, which led to poor settlement
and metamorphosis of the abalone. Also, since it was found that
postlarvae preferred crustose coraline algae together with its
associated benthic diatoms in their natural habitat, the roughened
plexiglass was probably the next best thing in a culture setting.
The results given should serve as cues for
abalone postlarvae growers to make modifications if necessary to ensure
optimum growth of their stocks.
Read more of this research from
the journal Hydrobiologica (2005) 548:301-306.
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