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Settlement & metamorphosis

  Setlement & metamorphosis are considered in this section, while GONAD GROWTH, SPAWNING, & FERTILISATION, LARVAL FEEDING & GROWTH, and LARVAL DEVELOPMENT & BEHAVIOUR are considered in other sections.
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Research study 1

histogram showing larval abundances of sand dollars Dendraster excentricus at different times of the year in Monterey Bay, Californiaphotograph of bed of sand dollars Dendraster excentricus taken in a commercial aquariumLarvae of sand dollars Dendraster excentricus in Monterey Bay, California are plentiful in the plankton in summer, when phytoplankton food is abundant, and water currents are slow and variable. In such years the summer current patterns tend to keep sand-dollar larvae close to the adult populations that spawned them, and in these years recruitment may be huge.  However, in years when currents do not favour onshore movement of 8-armed larvae (the settlement stage) then settlement is desultory. The authors estimate an 88% mortality of juveniles during their first year of life.  The authors note too that, up to the time of their work, little research has been done on the role of offshore larval abundance or influence of settlement and post-settlement mortality on recruitment and size structure in sand-dollar populations. Cameron & Rumrill 1982 Mar Biol 71: 197.

NOTE  mean density of Dendraster in July is 166 individuals . m-2

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Research study 2

graph of size of juveniles at different times after metamorphosis from sand dollars Dendraster excentricus metamorphosing at 2 and 4 weeks after the onset of metamorphic competency
As in other marine invertebrate larvae, echinoplutei of sand dollars develop to a competent metamorphic state, and may remain in this state for some time until they encounter stimuli that induce metamorphosis.  Studies at Friday Harbor Laboratories, Washington on the sand dollar Dendraster excentricus show that while metamorphosis can be delayed for several weeks, a cost is incurred in that growth rates of the resulting juveniles are less than in “normally” metamorphosing individuals.   Highsmith & Emlet 1986 Bull Mar Sci 39: 347.

NOTE a 2wk delay in settling after becoming competent in itself might be considered longer than "normal", but the data clearly indicate that a 4wk delay incurs metabolic costs that are later reflected in growth of the juvenile

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Research study 3

drawing of excised portion of the pre-oral hood of a larval sand dollar Dendraster excentricus used in metamorphosis studiesIt is not known what physiological mechanisms are involved in holding a larva in metamorphic abeyance, but hormonal, neural, and neurosecretory controls are all possible.  Studies in Victoria, British Columbia, on larval sand dollars Dendraster drawing of a pluteus larva of a sand dollar Dendraster excentricus showing elements of the nervous systemexcentricus using electrical stimulation of nerve centres and excision experiments to isolate portions of the nervous system suggest that metamorphosis is under nervous control. drawing of excised portion of the preoral hood of a larval sand dollar Dendraster excentricus used in metamorphosis studies
Interestingly, even excised parts such as the pre-oral hood and oral hood, if treated with a chemical “metamorphosing cue”, will show initial signs of metamorphosis.  The author's experiments point specifically to 2 nerve centres: the apical neuropile between the anterior-lateral arms and the oral ganglion located on the lower lip of the larval mouth. Either centre alone is sufficient to initiate metamorphosis.  The nerve centres appear to respond to water-borne chemical cues and initiate metamorphosis.  Burke 1983 Biol Bull 164: 176.

NOTE  this is prepared by stirring sand from an aquarium containing adult sand dollars with an equal volume of clean seawater, filtering out the bits, immersing test larvae in it for periods of up to 5min, and then monitoring % metamorphosis

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Research study 4

A later, more detailed, investigation of the metamorphosing cues in sand dollars Dendraster excentricus in Victoria, British Columbia indicates that not only will aqueous extracts of sand from a sand-dollar bed induce significant metamorphosis, but also aqueous extracts from gonads.  Extracts from whole body and gut preparations are also stimulatory, but much less so than either gonad or adult-sand extracts.  A purified preparation of the extract, identified as a peptide with a molecular size of 980 daltons, will induce metamorphosis within 3-5min at a concentration of 10-6-10-5 M.  Sand dollars can therefore be added to a list of other marine invertebrates, including barnacles, oysters, clams, ascidians, and others, whose larvae metamorphose preferentially in habitats suitable for the juvenile or adult.  Burke 1984 Science 225: 442.

NOTE  although the results as they are presented here are convincing, the author probably should have also included extracts of sand from an area where sand dollars are absent as a control treatment

NOTE  named after an English chemist-physicist who formulated atomic theory and the law of partial pressures; a dalton is a unit of molecular mass approximately equal to the mass of an hydrogen atom

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Research study 5

In San Juan Island, Washington and elsewhere, competent larvae of sand dollars Dendraster excentricus preferentially settle near to adults.  The pheromonal cue for this appears to be a small-molecular-weight chemical, possibly a peptide, released from the adults and stable for at least 7wk. It is not known whether this is the same or related chemical to the one considered in Research Study 4 above. Highsmith 1982 Ecology 63: 329.

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Research study 6

map of San Juan Islands, Washington showing locations for study of sand dollar Dendraster excentricus A study of distribution of larval sand dollars Dendraster excentricus in a small, shallow inlet in Orcas Island, Washington shows that larvae do not perist in the inlet longer than about 2wk after the population spawns in May-June. What happens is that the larvae are advected out in wind-driven surface currents. Advanced-stage larvae tend to frequent surface waters, thus enhancing their chances of finding suitable settlement sites, although the mechanism by which they regulate their depth is unclear.  The author notes that many of these advanced larvae have shorter arms, indicative of delayed metamorphosis.  Emlet 1986 Mar Ecol Prog Ser 31: 245.

NOTE  larvae of D. excentricus are readily identifiable in plankton samples by their characteristic body transparency and fenestrated arm-rod structure

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