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  Metamorphosis & symmetry
  Topics of metamorphosis & symmetry are considered in this section, while GONAD GROWTH, SPAWNING & FERTILISATION, and LARVAL LIFE & BROODING are considered in other sections.
Research study 1

drawings showing generalised life cycle of a sea cucumberIn laboratory culture of eggs of Parastichopus californicus a blastula stage is reached after 24h, a hatched gastrula by 64h, and a feeding auricularia larva after 13d at 10-12oC).  Metamorphosis is of a gradual kind, first to a doliolaria stage occurring at 65-125d, and then to a pentacula stage occurring 1-2d later.  The early pentacula remains pelagic at first, but then, as more adult features develop, settles to the sea bottom. The young photograph of juvenile sea cucumber Parastichopus californicuspentacula can crawl about using its tube feet and tentacles. Sexual maturity is attained after about 4yr in field populations.  Cameron & Fankboner 1989 J Exp Mar Biol Ecol 127: 43.

NOTE lit. “jar” L.

A juvenile Parastichopus californicus.
At this early age the tubercules retain features of the tube feet from which they originally derived 1X

  black dotResearch study 2

Isolation and purification of an oocyte-maturation hormone in Parastichopus californicus has permitted controlled induction of meiosis and fertilisation in the resulting ova, and has enabled detailed analysis of metamorphic events.  The researcher also describes a gradual kind of metamorphosis, that commences 3-5wk after fertilisation, the precise time depending upon seawater temperature, feeding regime, and source of eggs.  At this time the doliolaria1 larva is 1mm long and about 0.3mm in diameter.  Four hours after commencement of metamorphosis the feeding auricularia larva changes to a non-feeding doliolaria.  At about 24h after commencement of metamorphosis, other changes lead to the formation of a pelagic stage2, wiith the oral photograph of the doliolaria larval stage in a sea cucumber Parastichopus californicus opening now at the anterior end. Shortly thereafter the juvenile3 settles to the sea bottom and assumes the adult crawling mode of life.  Smiley 1986 Biol Bull 171: 611.

photograph of an auricularia larval stage of the sea cucumber Parastichopus californicusNOTE1 referred to by the author as a "mid-metamorphic" larva

NOTE2  considered by other authors to be a doliolaria

NOTE3 note that this is equivalent to the pentacula stage in Research Study 1 above

Mid-metamorphic doliolaria stage of Parastich-
opus californicus
showing mouth at anterior end

Auricularia larva of Parastichopus californicus showing
features of the gut system. The mouth is shown here at
the opening to the oral cavity, but in other reports is shown
as being within the oral cavity at the anterior end of the
esophagus. The larva is about 400um in length

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Research study 3

During metamorphosis, as in other echinoderms, the larva of an holothuroid changes from bilateral to pentaradiate symmetry.  This is seen mainly in the radial structure of the water vascular system, including tube feet and associated tentacles.  Superimposed on this adult radial symmetry, however, is a bilateral symmetry, evident externally in the division of the body into a ventral trivium and dorsal bivium, and internally by the bilateral nature of the complex hemal system and associated gut tube.  Traditional thinking is that phylogenetically, holothurians derived from ancestors with a primary radial symmetry and later adopted a crawling habit of life with accompanying secondary bilaterality. The later imposition of radial symmetry is therefore generally considered a tertiary event in their evolution.  However, based on the observation that during metamorphosis the bilateral features of a juvenile Parastichopus californicus derive directly from the bilaterality of the auricularia larva, the author of this research study argues that the bilaterality is not a secondary feature; rather, that the pentaradiate symmetry of the adult should be considered a secondary feature.  Smiley 1986 Biol Bull 171: 611.

NOTE consideration of symmetry is always interesting but, from an objective viewpoint, it seems that the author's argument is one of semantics, revolving around whether the primary symmetry is the radiality of the ancestor or the bilaterality of the larva. It is not known how extensively the author's proposal has filtered down into current-day thinking, but perusal of 3 of the most popular invertebrate texts currently in use shows just cursory references to the general topic of symmetry, and no references at all to this specific idea

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