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  Reproduction
 

The echinopluteus larvaof sea urchins 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 few weeks and shortly thereafter the little juvenile is crawling about on the sea bottom. 
photograph of a 2-arm echinopluteus larva NOTE lit. “later form”  About 99% of all marine invertebrates produce a free-living larva.  Because the adaptations for a floating planktonic life are so different from those for a bottom-dwelling adult life, a complex metamorphosis must be passed through at the end of larval development


2-arm echinopluteus larva 50X

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  Larval feeding, growth, development, & life span
  This section deals with larval feeding, growth, & life span, while topics of GONAD GROWTH & SPAWNING, FERTILISATION, LARVAL SKELETON, and SETTLEMENT METAMORPHOSIS & RECRUITMENT are presented in other sections.
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Research study 1
 

graph showing effect of salinity on development of larvae of green urchins Strongylocentrotus droebachiensisphotograph of echinopluteus larva of sea urchinThe ffect of salinity on development and survival of larvae of sea urchins Strongylocentrotus droebachiensis, S. pallidus, and S. purpuratus is assessed in studies at Friday Harbor Laboratories, Washington.  Results show that echinopluteus larvae generally tend to develop more slowly in the lower salinities (of an experimental range: 30, 27.5, 25, 22.5, and 20‰) than in higher salinities, and survival times vary with the species in question.  For example, larvae of the more euryhaline species S. droebachiensis survive for the entire 32-d test period, while those of the more stenohaline species S. pallidus and S. purpuratus survive poorly in salinities <27.5‰.  Roller & Stickle 1985 Can J Zool, Lond 63: 1531.

NOTE   also tested are bipinnaria larvae of the sea star Pisaster ochraceus.  These survive well over the range of salinities tested, but development is abnormal at the low end of the range 

NOTE   lit. “broad/wide salt” G. referring to inhabiting seawater of broadly varying salinity; “stenohaline” refers to the inhabiting of seawater of a narrow range of salinity

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Research study 2
 

graph showing the relationship between ciliated band length and volume of egg in 4 species of west-coast echinodermsSea-urchin larvae feed on single-celled algae that they capture on ciliated bands surrounding the mouth. Feeding in an echinopluteus larva is enabled by ciliated bands that are borne up and extended into the seawater on 8 projecting arms. Phytoplankton is caught up in the cilia and directed into the mouth (located in the hollow between the arms).  Studies on several sea-graph showing the relationship of ciliated band length to body length in larvae of 2 west-coast sea urchinsurchin species in San Juan Islands, Washington show that during the period of arm formation (2- to 8-arm stages) the length of ciliated band scales with strong positive allometry with respect to body length (slopes = 2.7-3.4; see graph on Right). Ciliated band lengths, moreover, are positively correlated with egg size (see graph on Left).  Thus, species with larger eggs have a greater inherent capability for increased size of feeding structures relative to support structures during growth.  McEdward 1986 J Exp Mar Biol Ecol 96: 267.

NOTE the lengths of ciliated bands for purple and green urchins are expected to scale isometrically with body length (i.e., slope of 1 in the graph, indicated by a dashed line). A slope significantly greater than 1 indicates that the ciliated bands are growing disproportionately longer with increase in body length, thus creating a more efficient feeding structure

 

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Research study 3

 

Eggs of sea urchins Strongylocentrotus droebachiensis are larger in size than those of S. purpuratus.  Moreover, the hatched larvae of S. droebachiensis are correspondingly larger and develop faster. To answer the question whether these differences owe to the initial differences in egg size, scientists at Friday Harbor Laboratories, Washington employ an ingenious research protocol.  By separating embryos of both species at 2- and 4-cell stages, “eggs” of one-half and one-quarter volume are created.  This creates starting sizes ranging from an arbitrary value of 1 (full-sized eggs of S. droebachiensis) to 1/12 (isolated “eggs” from 2-cell embryos of S. purpuratus).  Subsequent rearing of these size-manipulated “eggs” results in the following: 1) smaller eggs yield smaller sized larvae with simpler body form, and 2) smaller eggs lead to slower development through the early larval stages.  In fact, larvae from size-reduced eggs of S. droebachiensis have similar developmental rates to those of larvae of the smaller-egged S. purpuratus.  The altered body form results in serious deficiencies in feeding capability of the larvae.  Not only do the small larvae have to grow more to reach metamorphic size, they must do so with reduced feeding abilities.  Slower growth leads to a longer time spent in the plankton with associated risk of mortality from predation.  The authors note that their study is the first definitive demonstration of a causal relationship between parental investment per offspring (egg size) and life-history characteristics in benthic marine invertebrates.  Sinervo & McEdward 1988 Evolution 42: 885.

NOTE  150 vs. 85µm diameter, respectively

NOTE  these “simpler” larvae have short arms relative to body size.  Thus, rather than yielding larvae that are smaller but with normal body shape, the experimental manipulations yield larvae that have disproportionately smaller arms

schematic comparing sizes of larvae of green and purple sea urchins, Strongylocentrotus droebachiensis and S. purpuratus, respectively
Size of eggs and larvae of green urchins Strongylocentrotus droebachiensis (top) are much larger than those of purple urchins S. purpuratus (bottom) at the same developmental stages
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Research study 4
 

Studies on the energetics of early development in sea urchins in California suggest that endogenous reserves (yolk) account for a maximum of only 38% of the total metabolic demand from fertilisation to 4d of age (4-arm pluteus) in Strongylocentrotus purpuratus and only 66% in Lytechinus pictus over the first 8d of development. Sea-urchin larvae are capable of transporting dissolved organic matter (DOM: e.g., amino acids) from seawater and the authors’ calculations show that energy deficit during early development could be met by these means. Shilling & Manahan 1990 Mar Biol 106: 119.

 

 

Significant decreases in contents of lipid and protein are measured
in larvae of purple urchins S. purpuratus from 2-4d (4-arm
pluteus) of age ,but not in content of carbohydrate

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Research study 5
 

A general observation regarding egg size and reproductive output in marine invertebrates such as sea urchins that produce planktotrophic larvae, is that large numbers of offspring are associated with small-sized eggs.  The evolutionary implication of this is that if small larvae from small eggs grow more slowly than large ones, they may incur greater risk in the plankton.  For obvious reasons, controlled experimental tests of this idea are usually not possible.  However, use of the foregoing research protocol at Friday Harbor Laboratories, Washington enables half- and full-sized larvae to be produced from eggs of green sea-urchins Strongylocentrotus droebachiensis.  The method involves experimentally dividing the developmentally “plastic” embryos at the 2-cell stage. 

The resulting larvae (see graph) feed normally, but are about half the size of normal larvae and possess ciliated bands that are not quite half the length of those of normal larvae (1.3 vs. 2.4mm in length, respectively).  Measurements of feeding show that the small larvae clear 10µm-diameter plastic microspheres from suspension at rates of about half that of normal larvae (0.75 vs. 1.8µl . min-1 for mean-sized larvae of each category). Interestingly, while being small has a significant effect on post-metamorphic test diameter (405µm for small larvae vs. 428µm for normal larvae), it has no significant effect on the time required from fertilisation to metamorphosis (39 and 37 days, respectively, at 8-13oC).  The author concludes that while larval feeding capability is widely thought to select against small egg size in species with planktotrophic larvae, other selective forces must also be involved.  Otherwise, what explains the lack of selection in a species like S. droebachiensis for even greater fecundity by producing ever more tiny eggs as long as they are still capable of development?  Hart 1995 The Amer Nat 146: 415.

NOTE  most marine invertebrates have determinate development where cell fates are fixed at an early developmental stage, usually after the first division.  In comparison, cell fates in echinoderms and chordates are not determined until much later in development (indeterminate).  A similar twinning experiment using a 2-cell stage polychaete embryo, for example, cannot be done because each cell is already fated to produce one side of the adult body.  The cells are separated using chemical methods

NOTE  one of several ideas relating to this question is that smaller eggs provide a smaller target for sperm; thus, lower fertilisation rates for smaller eggs would balance the increased fecundity. Other ideas are that smaller developmental stages would suffer greater losses to predators, and that smaller juveniles resulting from smaller eggs will incur greater risk to environmental vicissitudes including predation, desiccation, and food availability

graph showing rates of clearance of plastic microspheres from suspension whole and "half" larvae of green urchins Strongylocentrotus droebachiensis
Rates of clearance of plastic microspheres from suspension by dwarf larvae (produced from halved embryos: darker, small-sized symbols) and normal larvae (produced from whole eggs: lighter, large-sized symbols) of green urchins Strongylocentrotus droebachiensis. Drawings show the larvae at 7d of age. The light- and dark-dashed lines indicate mean clearance rates and mean ciliated-band lengths for normal and dwarf larvae, respectively
 

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Research study 6

 
drawings showing current flows past 4-arm echinopluteus larvae of sea urchins
Two 4-arm echinopluteus larvae in food-bearing currents, viewed from underneath (upper diagram) and from the side (lower diagram). The green spots represent phytoplankton cells deposited in the suboral pocket. The nearby mouth is the orange area.

Earlier studies1 on echinoids suggest that food capture involves periodic reversals of beating of the locomotory cilia.  However, frame-by-frame videotape analysis of feeding in larval sea urchins Lytechinus pictus2 reveals no ciliary reversal.  The 4-arm larvae are held on suction pipettes in flow-velocities that mimic those produced at natural swimming speeds (about 1 body length . sec-1), and dye3 is injected into the stream.  The shape of the echinopluteus is such that the 2 longer post-oral arms divert particle-bearing water into the suboral pocket, where the algae are then taken into the mouth and consumed. No beat-reversals are observed when the larva is feeding. The author notes that use of certain types of dyes does result in reversal of ciliary beat, which may explain the earlier observations.  Moreover, application of the calcium-blocker verapamil to inhibit ciliary reversal does not interfere with food capture.  The author concludes that ciliary reversal is of little or no significance in feeding by sea-urchin larvae and that food capture is by direct interception. Gilmour 1985 Can J Zool 63: 1354; Gilmour 1986 J Exp Mar Biol Ecol 95: 27.

NOTE1  see LEARN ABOUT SAND DOLLARS: REPRODUCTION: LARVAL FEEDING

NOTE2  a west-coast shallow- to deep-subtidal species found in southern California and Mexico

NOTE3  the dye is Evans Blue

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Research study 7
 

Effects of temperature, food ration, and other environmental conditions on size and developmental rate of sea-urchin larvae are becoming increasingly well known.  What has received less research attention, however, is effect of different nutrients in the adult diet on larval vitality.  Since nutritional requirements are not known for sea urchins, artificial and not natural diets must be used in such a study, and the formulation of such diets starts off as being somewhat “hit-or-miss”.  After tinkering with an artificial compounded diet known from earlier studies at West Vancouver Laboratory, DFO, British Columbia to promote good gonadal growth in green sea-urchins Strongylocentrotus droebachiensis, a diet is produced leading to production of extra-large eggs with higher energy content.  These eggs, in turn, hatch to extra-large larvae that have high percentage metamorphosis (+92%).

In addition to being absolutely larger than siblings reared on other dietary combinations, these larvae also have greater ciliated-band lengths - expected to provide more effective feeding. de Jong-Westman et al. 1995 Can J Zool 73: 1495; de Jong-Westman et al. 1995 Can J Zool 73: 2080.

drawing of an 8-arm larva of a green sea urchin Strongyloentrotus droebachiensis showing morphometric measurements used to determine larval sizes and ciliated-band lengths
Morphometric measurements used to determine larval size and ciliated-band lengths in 8-arm pluteus larva of Strongylocentrotus droebachiensis. The letter "G" designates the adult rudiment - the presence of which indicates the onset of metamorphic competency

NOTE  artificial diets are compounded from dry nutrients and pelleted using calcium ligno-sulfonate binder; a natural CONTROL diet is air-dried kelp Nereocystis luetkeana that is rehydrated prior to feeding.  1040 adult (approx. 6-7yr-old) urchins are collected in July when gonads are at their seasonally smallest size and reared on the diets for 9mos

NOTE  best artificial diets, of 7 tested, are ones with high protein content (20% dry mass vs. 10% for low protein diets) and additions of cholesterol and ß-carotene.  The best of these produce gonads that are 17% larger than on a kelp diet.  Moreover, spawned eggs are 10% greater in volume and 5% greater in energy content than ones produced by adults on a kelp diet

 

 

Graph on Right: ciliated-band lengths for pluteus larvae reared from adults raised for 9mo on several artificial diets. The best adult diets (defined as producing larvae that are largest in size with greatest ciliated-band lengths) are ones containing high protein content with addition of beta-carotene and cholesterol, and these produce larvae with longest relative cilitated-band lengths

graph showing ciliated band lengths in different larval stages of green sea urchins Strongylocentrotus droebachiensis on artificial diets differing in nutritional content
 
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Research study 8
 

graph showing relationshiop between gonad volume and egg size in sea urchins Strongylocentrotus droebachiensisgraph showing effects of different food-levels on size of pluteus larvae of sea urchins Strongylocentrotus droebachiensis in laboratory cultureA later study done at Friday Harbor Laboratories, Washington similiarly assesses the effect of maternal nutrition on larval fitness in green sea-urchins Strongylocentrotus droebachiensis, but additionally investigates effects of diet on larval growth and body form.  For the first part, the researchers do not regulate the adult diet but, rather, collect brood specimens from 2 depths, <6 and 100m, and infer nutritional history from relative sizes of gonads.  Thus, small ovaries and small eggs at greater depth are most likely indicative of poorer maternal nutrition (see graph upper Left).

drawing showing effect of larval diets on morhology of larvae of sea urchins Strongylocentrotus droebachiensisResults of laboratory rearing of larvae from each group reveal that while there are significant effects of maternal habitat on growth of the larvae, these are small in comparison with the direct effects of larval diet. For example, food limitation at depth does not produce larvae with larger relative feeding apparatus, nor does it cause development to be slowed.  Laboratory effects of rearing the larvae on high and low food rations, however, produce noticeable differences in larval growth rates and body proportions (see drawings lower Right). Note in the graph on upper Right that larvae in NO- or LOW-food treatments are significantly shorter than ones in HIGH-food treatment, and the authors report other differences (not shown here).   On the strength of their results the authors conclude that differences in larval form of S. droebachiensis found are most likely a reflection of planktonic conditions, most notably food availability, affecting the lavae.  Bertram & Strathmann 1998 Ecology 79: 315.

NOTE  the offspring of each maternal group are kept in stirred jars with 3 food levels: no additional food (NO), addition of 200 cells of Rhodomonas sp. . ml-1 (LOW) and addition of  5000 cells of Rhodomonas sp. . ml-1 (HIGH).  All treatments, however, are noted by the authors to be contaminated with residual microorganisms, so precise counts are not possible

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Research study 9
 

graph comparing feeding rates of pluteus-type larvae with non-pluteus-type larvae of echinodermsIn another interesting study on the functional morphology of suspension-feeding in echinoderm larvae, a researcher at Friday Harbor Laboratories, Washington compares, among other things1, the efficiency of feeding of pluteus-type larvae2 with non-pluteus-type larvae.  Clearance rates, corrected for the estimated number of ciliated3 cells present in the entire ciliated band of a larva, show that pluteus-type larvae are slightly more effective in gathering food than non-pluteus-type larvae (see graph).  Note in the graph that the ciliated bands of pluteus larvae comprise about 1000-10,000 cilia, while those of non-pluteus larvae comprise about 4,000-10,000 cilia. The author concludes that, given comparable energy expenditures by each type of cilium (this is not known), more efficient feeding by pluteus larvae may help explain the parallel evolution of of the pluteus larval form in the otherwise not closely related echioid and ophiuroid classes of echinoderms. The author also adds that the data presented should by no means be “over-interpreted”.  Hart 1996 Invert Biol 115: 30.

NOTE1 the author provides much more interesting comparative information on feeding in echinoderm larvae than can be considered here

photograph of ciliated band of sea urchin Strongylocentrotus droebachiensis showing individual ciliated cellsNOTE2  the pluteus larvae are from several sea-urchin species including Strongylocentrotus spp. (and Arbacia punctulata from the Atlantic coast), sand dollars Dendraster excentricus, and brittle stars Ophiopholis aculeata.  Non-pluteus larvae are from a variety of west-coast sea stars and the sea cucumber Parastichopus californicus.  All larvae are reared in the laboratory on mixtures of unicellular algae.  Clearance-rate tests are done with polystyrene beads of consistent size

NOTE3  the bands consist of rows of cells, each with a single cilium.  The ciliated band forms a closed loop over the larval body (see photograph). Density of cilia in the bands of  S. droebachiensis and S. purpuratus range from 1.1-1.2 . µm-1, as compared with 2.0-3.5 . µm-1  in the bands of 4 species of sea stars

Ciliated cells in the ciliated band of S. droebachiensis

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Research study 10
 

photos of larvae of sea urchins Strongylocentrotus purpuratus reared from single or 2 fused eggs showing size differences at different ages in laboratory cultureThe development of techniques to experimentally increase the size of echinoid eggs has allowed other developmental and evolutionary questions to be answered.  For example, what effect does egg volume have on development of the juvenile rudiment and how does this relate to evolutionary change in egg size witnessed in echinoids?  Experiments at Friday Harbor Laboratories, Washington with eggs of purple urchins Strongylocentrotus purpuratus show no accelerated development of the rudiment relative to the larval body even with a doubling of egg size.  However, larvae produced from such double-sized eggs are actually similar in form to those of S. droebachiensis, a species with eggs that are twice the size of those of S. purpuratus (about 155µm and 80µm, respectively). The authors note that such “allometric engineering” can provide a powerful means of experimentally teasing out possible consequences of evolutionary changes in egg size on other developmental traits. There is much more to this study than can be presented in this brief summary, and the paper should be required reading for anyone embarking on similar research.   Bertram et al. 2009 Evol & Development 11: 728.

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Research study 11
 
composite photo showing 3 common west-coast sea urchins Strongylocentrotus purpuratus, S. franciscanus, and S. droebachiensis

Larval life-spans for sea urchins, measured at Friday Harbor Laboratories, Washington at comparable temperatures to those in the field, are quite variable, both between and within species.  Strathmann 1978 J Exper Mar Biol Ecol 34: 23.

NOTE  in later publications the author discusses the selective value of pelagic development in marine invertebrates.  Although scientists generally believe that the primary benefit for larvae nearing competence for settlement and metamorphosis is in increased dispersal, there may be other possibilites.  Thus, a pelagic development may avoid the constraints of oxygen supply related to aggregation of embryos in egg masses or, for feeding larvae, may enhance growth and/or survival.  Both ideas are consistent with a dispersal-benefit hypothesis, and both are testable. This short summary does not do justice to this interesting paper, and anyone interested in the subject of larval dispersal should certainly read the full account.  Strathmann 2007 Bull Mar Sci 81: 167; Strathmann et al. 2002 Bull Mar Sci 70 (suppl.): 377; for an interesting discussion about trade-offs between adaptations for swimming and feeding in invertebrate larvae see Strathmann & Grunbaum 2006 Integr Comp Biol 46: 312.

Clockwise, upper Left: purple urchin Strongylocentrotus purpuratus: 9-12wk larval life span;
red urchin S. franciscanus: 9-19wk; and green urchin S. droebachiensis: 7-22wk

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