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One of the major problems that have
perplexed abalone hatchery operators is the poor settlement or the
attachment and metamorphosis of abalone larvae. To increase the
production of seeds needed for stock enhancement, suitable inducers for
the settlement of abalone larvae have to be provided.
Since tests using different abalone species showed that
responses to settlement cues vary depending on the species, Wenresti
Gallardo and Shelah Mae Buen-Ursua of SEAFDEC/AQD decided to test the
effect of larval settlement inducers on the tropical abalone
Haliotis asinina.
Inducers tested included abalone mucus,
Navicula sp., Navicula + mucus, mixed diatoms, and mixed
diatoms + mucus. Variables requiring mucus were produced by allowing a
juvenile abalone to crawl for one hour into treated Petri dishes, while
the other samples that did not require the mucus were prepared using
certain laboratory procedures. Four Petri dishes of each treatment and
control were randomly placed on the bottom of each of the five 60-liter
fiberglass tanks, which contained 30 liters of treated seawater. Larvae,
which were stocked at 200 per liter, were counted and monitored during
the experiment.
The three trials conducted had a common
ground, wherein the type of settlement substrate had a significant
effect on larval attachment. Of the five treatments used, abalone had
greater attachment on mucus, Navicula, Navicula + mucus,
and mixed diatoms + mucus on Day 1 of the study. By Days 3 and 7, the
Navicula and Navicula + mucus treatments resulted in higher
live abalone postlarvae compared to the other treatments. By Day 10, the
Navicula + mucus treatment registered the highest number of fully
metamorphosed postlarvae.
The authors think that the higher larval
attachment to the samples mentioned above could be due to the abalone
mucus, which acts as an attractant for larval attachments. Since mucus
consists primarily of proteins and polysaccharides, this material, also
produced by other marine gastropods like sea slugs, could have positive
effects on abalone metamorphosis. It is also possible that the mucus
secreted by Navicula has had a hand in the good results for Day
1. Furthermore, factors such as the uniform cells and prostate type of
cell growth of Navicula make it a suitable attachment. Although
Navicula was the dominant species in the mixed-diatom treatment,
the presence of three-dimensional species like Melosira,
Thalassiothrix and Fragilaria may have prevented the
attachment of the abalones, which suggests that being one-dimensional
isn’t so bad if you’re a diatom. In addition, other researches have
noted the high nutritional value of Navicula.
Given these results, the authors suggest
that techniques for the mass culture of Navicula on settlement
plates be developed, and that these be grazed by abalone juveniles to
produce the best possible outcome.
Read more from the journal Aquaculture (2003)
221: 357-364
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