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 juvenile is crawling about on the sea bottom. 
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  Settlement, metamorphosis, & recruitment
  This section includes settlement, metamorphosis, & recruitment (in 2 parts), while GONAD GROWTH & SPAWNING, FERTILISATION, LARVAL FEEDING GROWTH DEVELOPMENT & LIFE SPAN, and LARVAL SKELETON can be found elsewhere.
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Settlement & metamorphosis

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Research study 1
 

At about the time that adult features appear in the echinopluteus, the larva almost seems too heavy to swim with its load of spines and rudimentary test, and it drops to the sea bottom.  There, it ceases swimming and uses its 5 primary tube feet to crawl on, and to test the “chemistry” of, the substratum.  If things are suitable, it completes its metamorphosis, adopts an adult style of life, and initially begins feeding on bacterial and algal scums coating the surface of rocks. If things are not to its liking, the larva may continue to swim and investigate new areas.  Burke 1980 J Exper Mar Biol Ecol 47: 223.

NOTE  this is known as the “adult rudiment”.  During metamorphosis the larval tissues are resorbed into the rudiment, which eventually forms the juvenile sea urchin

Close view of a primary tube foot of a green sea urchin Strongylocentrotus
droebachiensis
. Note the secretory and sensory cells (only a few are shown)
in the epithelium of the tube-foot sucker for attaching to and investigating
surface chemistry of the substratum. Drawing of metamorphosing
pluteus larva from Noguchi 2000 In: The sea urchin: from basic biology to
aquaculture
(Yokota et al., eds.) AA Balkema, Rotterdam

drawings of a primary tube foot of a green sea-urchin larva, close-up vies
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Research study 2
 

graph of settlement of larvae of purple urchins Strongylocentrotus purpuratus onto various substrataSettlement and/or metamorphosis-inducers for sea-urchin species such as Strongylocentrotus droebachiensis and S. purpuratus, mostly determined using laboratory-reared larvae, are various coralline algae  (e.g., Corallina officinalis), some types of brown macroalgae and, for the first species, at least, filmed substrata from the field.  The first type of stimulus,corallines, may assure clean-water conditions for the juveniles; the second and third, a source of food after metamorphosis.  An example of laboratory settling of purple urchins Strongylocentrotus purpuratus onto different substrates is shown in the graph on the Right. The data show that the larvae are responding to cues other than just bacterial films, possibly algal exudates.  Rowley 1989 Mar Biol 100: 485; Pearce & Scheibling 1990 Biol Bull 179: 304.photograph of a coralline alga Corallina sp.

NOTE  laboratory studies on S. droebachiensis settlement in Nova Scotia show that the presence of adults, adult-conditioned seawater, and juveniles does not enhance metamorphosis success, suggesting that conspecific cues are not important.  These findings suggest that settlement of S. droebachiensis larvae in the field may not be
very selective. 
Pearce & Scheibling 1991 J Exp Mar Biol Ecol 147: 147.

NOTE treatments connected by horizontal lines do not differ significantly


Coralline alga Corallina sp.

 

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Research study 3

 

Similarly, field data collected on settlement of red sea-urchins Strongylocentrotus franciscanus (upper graph) and purple sea-urchins S. purpuratus (lower graph) in the Santa Barbara region indicate a preference for coralline algae and algal turf, especially by purple sea urchins.  At the time of collection the newly metamorphosed juveniles are about 500µm diameter. Rowley 1989 Mar Biol 100: 485.

 

 

A "barrens" (see far Right on graph) is an area completely
or mostly denuded of algae through the feeding activities
of sea urchins, often in herds. The author reports
densities of recruits in some areas as high as 2000 . m-2

graphs showing settlement of red urchins Strongylocentrotus franciscanus and purple urchins S. purpuratus onto various natural substrata in the field (Santa Barbara, California)
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esearch study 4
 
Not unexpectedly, mortality of newly metamorphosed sea urchins is high.  For example, purple sea-urchins Strongylocentrotus purpuratus in the Santa Barbara region suffer 75-90% mortality within 45d after settlement in barrens/crustose coralline-algae and kelp bed/algal turf areas (see graph on Right).  At this age the urchins are slightly less than 1mm in diameter. graph showing densities of settled purple urchins Strongylocentrotus purpuratus onto barren areas and kelp-bed areas around Santa Barbara, Callfornia
Based on laboratory data, the newly settled juveniles at first eat the surface cell layers of coralline algae. By 50d of age, at a size of 0.8-1.2mm diameter, they are capable of eating larger algae. Despite the initial higher mortality in kelp bed/algal turf regions (see upper graph on Right), those that survive there grow faster and reach size-refuge faster in comparison with ones inhabiting barrens/crustose coralline-algae regions (see graph lower Right). Rowley 1990 Mar Ecol Progr Ser 62: 229. growth of juvenile purple urchins Strongylocentrotus purpuratus in kelp-bed and barrens areas around Santa Barbara, California
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Research study 5
 

photograph of Yaquina-Head area of OregonThe larvae of purple sea sea-urchins Strongylocentrotus purpuratus spend about 9-12wk in the plankton before becoming competent to settle.  During this time they may be moved offshore, brought back onshore, or prevented from settling at suitable sites by oceanographic processes.  One of these processes is upwelling, which is prevalent in many areas along the west coast during spring and summer coincidental with the time the larvae are ready to settle.  Upwelling would tend to advect larvae away from the coast, which would affect subsequent size-frequency structure of populations of purple urchins.  A study of size-frequency distributions at 13 sites from Cape Blanco, Oregon south to Point Conception, California suggests that this is indeed what happens.  For example, poor recruitment at Cape Blanco, Oregon is correlated with seasonal upwelling, while good recruitment at Yaquina Head, Oregon is correlated with less histograms of size-frequencies of purple urchins Strongylocentrotus purpuratus at Cape Blanco and Yaquina Head, Oregonintense upwelling, and the pattern is repeated at several of the other sites. Based on knowledge of growth rates, the single mode of size-distribution at Cape Blanco may represent a recruitment pulse from one, or perhaps two, good years in succession, and one that is not repeated in the following 7yr.  These results differ somewhat from data obtained in earlier studies in areas of southern California, where upwelling is common but occurs at lower intensity than recorded at the more northerly sites.  The authors suggest that the differences may be explained by the fact that low levels of upwelling may enhance recruitment, perhaps through localised entrainment, while intense upwelling is more likely to inhibit settlement.  Ebert & Russell 1988 Limnol Oceanogr 33: 286.


Note the lack of juveniles at Cape Blanco,
indicating little or no recruitment for
about 7yr

The photo above is of Yaquina Head, Oregon in summer

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Research study 6
 
Later studies on larvae of sea urchins Strongylocentrotus franciscanus and S. purpuratus in southern Oregon show also that settlement is governed by the interaction of timing of spawning (Jan-Mar) and variability in seawater flow-regimes.  When alongshore winds are predominately northward, water is moved onshore and this contributes to settlement of larvae.  During periods of southward winds, however, strong upwelling leads to offshore transport of water, and settlement is absent.  If winds abate, the near-shore water column may become stratified, surface waters are warm, onshore flow is disrupted, and settlement again is negatively affected.  Miller & Emlet 1997 Mar Ecol Progr Ser 148: 83. photograph of purple urchins Strongylocentrotus purpuratus and a single red urchin S. franciscanus in a tidepool
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Research study 7
 

Different types of “collectors” are used to stimulate settlement, attachment, and metamorphosis marine-invertebrate larvae, but in a repeatedly quantitative way.  One such device used to monitor settlement of purple urchins Strongylocentrotus purpuratus is a high surface-area plastic matrix suspended 1m off the sea bottom (see drawing on near Right). Metamorphosis percentage on this type of sampler compares favourably with that on known coralline-algae inducers.  Many other species of larvae are caught on the samplers, including some crabs that actually create a nuisance by preying on the newly metamorphosed urchins. Harrold et al. 1991 J Exp Mar Biol Ecol 147: 81.

NOTE  for example, articulated corallines Calliarthron sp.and Bossiella sp., and rocks covered in encrusting corallines Lithothamnion sp. and/or Lithophyllum sp.

diagram of a larval sea-urchin collector graph showing settlement of larvae of purple urchins Strongylocentrotus purpuratus in Monterey, Californiapuratus
Deployment of samplers in the Monterey, California area indicates two main settlement peaks: November/December and May/June.  The strong settlement response on the samplers indicates their usefulness in providing a reliable index of settlement of sea urchins
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Research study 8
 
map of collecting areas in California for study of settlement of red and purple sea urchins, Strongylocentrotus franciscanus and S. purpuratus, respectively

A large-scale study of settlement of red and purple sea-urchins, Strongylocentrotus franciscanus and S. purpuratus, in California over a 4-yr period shows that the main settlement is between February-July each year.  The pattern is more regularly annual in southern California than in northern areas, and settlement densities are higher in the south. The authors speculate that this pattern may result from greater retention of water in the Southern California Bight (Santa Barbara to San Diego) and greater offshore movement of water in northern California. North of Point Conception strong southward winds generate intense upwelling and movement of water (and larvae) is predominately offshore.  In contrast, in the Southern California Bight, winds are weaker, upwelling is less, and there is less mixing of the water masses.  Ebert et al. 1994 Mar Ecol Progr Ser 111: 41.

NOTE  study sites are shown as A-J on the graph

NOTE  the newly settled urchins are distinguished by colour and by the presence of 1-3 dorsal pedicellariae on S. franciscanus, and the absence of same on S. purpuratus

  drawing of brush structure used to capture settling larvae of sea urchinsSettling larvae are captured on wooden-handled scrub brushes (with the handles cut off) attached to cables and suspended from piers or overhanging ledges.  A sonicator is used to help free the newly settled urchins
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Recruitment

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Research study 1
 

histograms showing size-frequency distributions in red urchins, courtesy Tegner & Dayton 1981 Mar Ecol Progr Ser 5: 255model of "pulsed-recruitment" kind of size-frequency distributionAnnual recruitment in red urchins Strongylocentrotus franciscanus should theoretically lead to a multi-modal size distribution, with peaks representing successive years of settlement and with the dominant peak being the most recent (see model upper Right).  In fact, owing to differential rates of growth and mortality, and general "long-livedness, this expectation is never realised in urchins, and a collection usually has a bimodal size distribution (see upper histogram on Left).  The degree of bimodality in populations of S. franciscanus, however, also relates to habitat depth.  At Point Loma near San Diego, for example, collections from depths of 15-18m are bimodal, while collections from 12m are not (see lower histogram on Left).  Why this should be is part of a of a study by researchers at the University of California, Davis.  They explain that the mode at larger sizes, which occurs in all habitats, results from the factors mentioned above: variable growth and mortality rates.  However, why bimodality is lacking in the shallow-water population at Point Loma is not so clear.  The authors develop a model to test different population parameters (such as pulsed recruitment, spine-canopy protection, and growth and mortality estimates) on size distributions, and the reader is directed to this later account.  Botsford et al. 1994 Ecological Applications 4: 42; in a later paper the same research group provides a statistical method for estimating growth and mortality parameters in S. franciscanus from size-frequency distributions lacking clearly defined modes: Smith et al. 1998 Can J Fish Aquat Sci 55: 1236.

 

NOTE  the data analysed here are from another study: Tegner & Dayton 1981 Mar Ecol Progr Ser 5: 255 at LEARN ABOUT SEA URCHINS: PREDATORS & DEFENSES: SHELTERING BY JUVENILES

NOTE  the authors place more emphasis on the protective role of spine-canopy protection in the species than it probably deserves.  In Barkley Sound, British Columbia, for example, the incidence of the behaviour is quite low (e.g., it is hard to find examples on any given SCUBA dive), and recent estimates of its occurrence elsewhere along the B.C. coast give values for incidence of sheltering of generally less than 15%: see Research Study 13 in the "sheltering by juveniles" section noted above

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Research study 2
 

Juveniles of some species, most notably red urchins Strongylocentrotus franciscanus, shelter under the spine canopies of adults for protection1 (see photograph). If sheltering under the spines of adult sea urchins is important for survival of juveniles, would we then expect enhanced recruitment in areas of high adult densities?  This is investigated as part1 of a much larger study of recruitment of red urchins Strongylocentrotus franciscanus along the northern California coast.  Researchers from the University of California, Davis sample size distributions at 12 locations between Cape Mendocino adult red urchin shelters juvenilesand Point Arena over the period 1994-1997.  Larvae of this species in northern California tend to settle during May-September with peaks around July.  Results of the censuses show that the presence of a potentially protective spine canopy is much less important to recruitment success than is the pattern of coastal circulation.  Thus, larvae are kept offshore during upwelling conditions that occur April-July, but during later relaxation of upwelling the larvae are moved onshore, with settlement and recruitment being especially favoured in areas north3 of promontories.  Morgan et al. 2000 Fisheries Oceanography 9: 83.

NOTE1  more on this subject can be found at LEARN ABOUT URCHINS: PREDATORS & DEFENSES: SHELTERING BY JUVENILES

NOTE2  the aim of the study is primarily to determine effects of water circulation on spatial patterns of settlement and recruitment, but post-settlement processes such as protection of newly settled individuals beneath the spine canopy of adults is an interesting and relevant (to the ODYSSEY) component of the study

NOTE3  the authors note that larvae tend to be retained in areas to the south of coastal promontories, termed “upwelling shadows”, and later may be moved onshore during reversal of upwelling winds when along-shore currents move northwards


Juvenile red urchins Strongylocentrotus franciscanus
shelter under the spine canopy of an adult 0.5X 

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