Velum of Encapsulated Veligers of Petaloconchus (Gastropoda), and the Problem of Re-evolution of Planktotrophic Larvae
Abstract:Encapsulated veliger-stage embryos of the sessile vermetid gastropod Petaloconchus montereyensis feed on a massive quantity of nurse yolk contained within their monoembryonic capsules and metamorphose within the capsules before hatching as crawling juvenile snails. To determine if intracapsular metamorphosis is size dependent and if the veliger-stage embryos can swim and feed if freed from their capsular envestment, veliger-stage embryos were removed from their capsules, deprived of their nurse yolk, and maintained in aseptic conditions until they either metamorphosed or died. We found that embryos with shell diameters greater than 0.60 mm, extending up to a maximum of about 1.55 mm, frequently underwent partial or complete metamorphosis within 1–2 weeks of removal from their capsules; smaller embryos died after 1–14 days without metamorphosing. Apparently, embryos become competent to metamorphose when they are about 0.6 mm in diameter, but metamorphosis itself is triggered by the disappearance of the external food source. None of the liberated embryos showed any ability to swim or otherwise move. Ultrastructural examination of the velar lobes of the embryos showed that they lack the structures utilized by planktotrophic veligers for swimming and feeding (pre- and post-oral ciliary bands and food groove), and that a continuous, densely ciliated field covers the under surfaces of the velar lobes, the lower lip, and the circumference of the mouth. The velum of this encapsulated veliger stage is thus highly modified for the ingestion of nurse yolk. We conclude that there is little likelihood that a species like P. montereyensis could give rise to a form with a planktotrophic larva of the typical gastropod veliger type. Having metamorphosis triggered by the exhaustion of the intracapsular yolk supply, instead of by the achievement of a particular size or moment in development, greatly enhances the probability that all embryos will successfully hatch and be able to assume the adult life style.
Document Type: Research Article
Publication date: 1989-09-01
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