Reproduction
  The echinopluteus larva spends many weeks in the plankton floating passively with the currents and feeding on phytoplankton.  The larvae have long projecting arms that bear the ciliated bands used in feeding. Metamorphosis is complete within a week and shortly thereafter the little juvenile is crawling about on the sea bottom. 
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  Larval skeleton
  This section includes larval skeleton, while GONAD GROWTH & SPAWNING, FERTILISATION, LARVAL FEEDING GROWTH DEVELOPMENT & LIFE SPAN, and SETTLEMENT METAMORPHOSIS & RECRUITMENT can be found elsewhere.
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Research study 1
 

Echinoplutei larvae of echinoids and ophioplutei larvae of brittle stars have slender calcite rods that support the long projecting arms.  In addition to supporting the ciliated feeding bands, the rods may also:

1) provide attachment points for muscles to spread the arms, as in defense;
2) add stiffness for defense;
3) support an epidermis that is under tension;
4) support the soft body parts against distortion from muscular contractions during ejection of unwanted particles from the mouth; and,
5) contribute to the vertical orientation of the larvae.  Pennington & Strathmann 1990 Biol Bull 179: 121; echinopluteus drawing modified from Sinervo & McEdward 1988 Evolution 42: 885.

NOTE calcium carbonate, with up to 16% of the calcium replaced by magnesium.  The extra magnesium increases the hardness and possibly stiffness of the calcite

An 8-arm echinopluteus larva of a sea urchin. The gut system is
shown in blue with the mouth being at the top, narrowing to an
esophagus, and terminating in an anus. The adult rudiment, shown in
red, signifies that the larva is nearing metamorphic competency

drawing of a sea-urchin larva showing details of skeletal rods, ciliated bands, and adult rudiment
 

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Research study 2

 

Tests of any of these hypothesised functions would almost require that the rods be removed – which is exactly what scientists at Friday Harbor Laboratories, Washington have done using using several species of Hawaiian and Washington echinoids to test aspects of passive orientation.   Dead larvae (killed with formalin) retain their body shapes quite well after decalcification, but live larvae collapse in on themselves.  photographs of sea-urchin larvae showing differences between control untreated larvae and experiemental decalcified onesLive larvae resume swimming after decalcification, may even regrow a skeleton (although imperfectly), and may survive for periods up to several weeks.  If a skeleton is not regrown the arms completely disappear within a few days.  Tests of normal and decalcified plutei show that most orientate passively with their arms upward.  Actively swimming plutei with skeletons mostly swim with their arms upward, but this orientation is less pronounced after decalcification.  100% of dead plutei with skeletons sink with arms upward as compared with a value of about 80% for dead plutei without skeletons.  Calculations based on reduced density of decalcified larvae and on known rates of oxygen consumption for normal larvae show that swimming costs to maintain neutral buoyancy account for less that 1% of total aerobic energy expenditure.  With respect to orientation, the authors conclude that while the skeletons are not essential for passive vertical orientation, they do enhance it.  All feeding echinoplutei swim with their anterior ends upward, so any mechanism that enhances this orientation will be of selective advantage.  The authors also note that the increased density and sinking rate imposed by the presence of a skeleton contribute little to the overall energetic cost of locomotion. Pennington & Strathmann 1990 Biol Bull 179: 121.

NOTE  skeletons of dead larvae are dissolved in a 1:1 mixture of seawater and EDTA-saturated seawater.  Skeletons of live larvae are dissolved in a mildly acidic artificial seawater solution. 

NOTE  most of this work is on tropical genera such as Tripneustes, Echinometra, and Colobocentrotus, but some observations on local species of Strongylocentrotus are included (as well as some observations on sand dollars Dendraster excentricus)

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Research study 3
 
photographs of calcite spicules that form the arm rods in a sea=urchin larva
Simple (left) and fenestrated (right) calcite spicules comprising the arm rods of an echinoid, showing the difference in construction
The calcite spicules comprising the arm rods in an echinopluteus larva have 2 forms: simple (solid), and fenestrated (porous).  Both are about 500-800µm in length in Strongylocentrotus spp. However, while simple spicules are 2-4µm in diameter, fenestrated ones are more than twice that size (5-10µm in diameter). Studies at Friday Harbor Laboratories, Washington show that the stiffness and buckling strength of fenestrated spicules of larval sea urchins Strongylocentrotus spp. and sand dollars Dendraster excentricus are about 3 times greater than those of simple spicules.  The author notes that if a solid spicule were constructed with the same dimensions as a fenestrated spicule its stiffness would be increased by an order of magnitude, but it would require about 6 times the amount of material.  The use of less material in a fenestrated construction must be important for a planktonic larva, which has to overcome gravity to stay afloat. Emlet 1982 Biol Bull 163: 264.
 

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Research study 4

 

Feeding efficiency in an echinopluteus is dependent upon total length of ciliated bands which, in turn, is dependent upon total arm length. Is there any plasticity of ciliated band length/arm length, photograph of green sea urchins Strongylocentrotus droebachiensissay, under conditions of warmer temperature (higher energy demands) and low food conditions?  This is tested in Strongylocentrotus droebachiensis in Nova Scotia, Canada using larvae in culture maintained at 3 food rations1 and 3 temperatures2 over an approximate 40d study period.  The results are as expected, that larvae maintained on a low food ration develop longer arms relative to body length, while ones kept on a high food ration develop shorter arms relative to body length.  The study is actually a test of an earlier prediction3 that echinoplutei would show adaptive shape changes in response to energy-limiting conditions of high temperature and low food.  Hart & Scheibling 1988 p. 277 In: Echinoderm Biology (Burke et al., eds.) Balkema, Rotterdam.

NOTE1   rations are a 1:1 mixture of flagellates Dunaliella tertiolecta and diatoms Chaetoceros gracilis fed at rations of 500, 1000, and 5000 cells . ml-1

NOTE2   temperatures are not dealt with here; rather, comparisons are made at a common temperature of 9oC

NOTE3   this idea is considered elsewhere in the ODYSSEY: LEARN ABOUT SAND DOLLARS: REPRODUCTION: LARVAL FEEDING

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