Reproduction
 

sectional view of gonads in the perivisceral coelom of a sea urchin

Most sea-urchin species have separate sexes and release their gametes into the sea at breeding time in the spring. There are 5 gonads communicating with the outside via 5 gonopores visible, as shown here, as 5 holes in the test. The gonads are discrete and easily accessible, leading to many studies on seasonal cycling of growth, maturation, and spawning.

NOTE  hemaphroditism in sea urchins is so rare that as a phenomenon it has been termed “accidental” - one with no known cause.  One west-coast study cites an unpublished observation that out of 10,000 purple urchins Strongylocentrotus purpuratus examined, only 20 hermaphrodites are found (or 1 in 500).  Boolootian & Moore 1956 Biol Bull 111: 328

NOTE  for a guide to culturing the purple urchin Strongylocentrotus purpuratus see Leahy 1986 p.1 In, Methods in cell biology (Schroeder, ed), Academic Press, NY

 

 

Fertilisation occurs in the open ocean and hatching is to a 4-arm pluteus larva, known as an echinopluteus.  Successive 4-, 6-, and 8-arm stages feed for 5-8 weeks on phytoplankton.  After this time, the 8-armed pluteus larva begins to show adult features internally and, a few days later, spines and tubefeet are visible externally.  Metamorphosis is complete within a week and shortly thereafter the little juvenile is crawling about on the sea bottom. 

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

 
drawings of developmental stages of a sea urchin

Stages in development of a sea urchin from egg to metamorphosis at 17oC. By the time the adult rudiments appear the larva is beginning to settle to the sea bottom. Metamorphosis results in spines (red in the drawing) and tube feet (yellow) protuding from the side of the larva. Although labelled "post-metamorphosis" by the author, the stage shown is just part-way along in the process. Drawings from Noguchi 2000 In: The sea urchin: from basic biology to aquaculture (Yokota et al., eds.) AA Balkema, Rotterdam

 

 

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Gonad growth & spawning

  This section includes topics on gonal growth & spawning. Topics on FERTILISATION, LARVAL FEEDING GROWTH DEVELOPMENT & LIFE SPAN, LARVAL SKELETON, and SETTLEMENT METAMORPHOSIS & RECRUITMENT can be found elsewhere. 
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Research study 1
 

Measurements of gonadal indices in purple sea-urchins Strongylocentrotus purpuratus in the Yankee Point area of Califorrnia show that the major spawning occurs in January-March.  Holland & Giese 1965 Biol Bull 128: 241; see also Bennett & Giese 1955 Biol Bull 109: 226.

NOTE  calculated as live mass of gonads/live mass of sea urchin x 100. Data for the two sexes are combined

graph of gonadal indices in purple urchins Strongylocentrotus purpuratus at Yankee Point, California
 
 

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Research study 2

 

Off the coast of central Oregon purple-urchins Strongylocentrotus purpuratus grow their gonads from Jun-Dec and spawn in Dec-Mar.  The same pattern occurs in 3 nearby populations.  Note in the graph that gonadal indices do not correlate with sea temperature.  Gonor 1973 J Exp Mar Biol Ecol 12: 45; see also Gonor 1972 J Exp Mar Biol Ecol 10: 89 and Gonor 1973 J Exp Mar Biol Ecol 12: 65.

NOTE  gonad index is calculated as total dry mass of gonad divided by total dry mass of body + gonad x 100.  This is one of few studies that indicate clearly that the body mass denominator includes the gonad when calculating the index

graph of gonadal indices of purple urchins Strongylocentrotus purpuratus in relation to water temperature in central Oregon
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Research study 3
 
graph showing relationship of water temperature and % sea urchins Strongylocentrotus purpuratus oozing gametes in Palos Verdes, California Notwithstanding the above, it would be expected that temperature would have significant effects on timing of reproductive events in sea urchins.  This is suggested by data from studies of a subtidal population of purple urchins Strongylocentrotus purpuratus in Palos Verdes, California, where a seasonal rise in seawater temperature above 17oC (in June) coincides with cessation of gamete production and storage (red dots on graph). Sea urchins held in the laboratory at low temperature (13oC) retain gametes for nearly 2mo beyond the date when the field population has spawned out.  Cochran & Engelmann 1975 Biol Bull 148: 393.

NOTE  individuals are considered reproductively active if mature gametes ooze from gonads that are removed and cut open

 
Research study 4
 

map showing collection sites for study of reproductive cycles in green sea-urchins Strongylocentrotus droebachiensis in San Juan Island, Washingtongraph showing gonadal indices of green sea-urchins Strongylocentrotus droebachiensis over a 2-yr period, along with seawater temperaturesGreen sea-urchins Strongylocentrotus droebachiensis at 3 sites in San Juan Island, Washington (see map) grow their gonads in late spring/autumn, and spawn in late winter/spring (Mar-Apr).  Vadas 1977 Ecol Monogr 47: 337.

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Research study 5
 

Gonadal growth in purple sea-urchins Strongylocentrotus purpuratus appears to be under photoperiodic control.  This is shown for urchins of 4-8mm diameter collected in autumn near Ensenada, Mexico and reared in a laboratory in southern California for a period of 18mo on a diet of kelp Macrocystis pyrifera.  The researchers divide the sea urchins into 2 treatment groups, one kept in-phase with the natural photoperiod; the other, kept out-of-phase by 6mo.  At intervals, individuals are removed and measures taken of gonad ripeness and gonad index. 

After 4-6mo in culture (winter/spring) almost 100% of the in-phase animals contain numerous mature gametes, while only a few of the out-of-phase animals contain mature gametes (see graphs on Right). The pattern is repeated one year later when in-phase animals are 85-100% ripe, while the out-of-phase animals are only about 20% ripe.  Note that the out-of-phase treatment group is 100% ripe in September of the next year, approximately 6mo later than the in-phase group.  Simultaneous assessments show that gametogenesis in both groups parallels the “ripeness” scenario. 

Interestingly, gonadal indices are similar in both groups, and other measurements show that both feeding rates and growth rates do not differ (see bottom graph on Right).  Thus, photoperiod does not influence energy intake, but it seems to determine whether energy is allocated to gametogenesis and gamete maintenance, thus reflecting in state of “ripeness”. 

None of this or even gonad index is reflected in live mass because as the gonad grows it simply displaces coelomic fluid; thus, live mass stays about the same.  Pearse et al. 1986 J exp Zool 237: 107; see also Bay-Schmith & Pearse 1987 Int J Invert Repr Dev 11: 287.

NOTE  a “ripe” gonad is one that oozes mature gametes on removal from an animal.  Percentage ripeness is the percentage of 5-20 individuals sampled that show “ripeness”.  Gonad index is the ratio of gonadal material to total live mass of animal x 100%

graph showing gonadal index versus % ripeness in sea urchins Strongylocentrotus purpuratus in southern California
gonadal indices and % ripeness of sea urchins Strongylocentrotus purpuratus kept on a photoperiod schedule 6mo out of phase with the natural photoperiod in southern California
 
NOTE  sampling dates are too infrequent to be more precise graph showing live masses of sea urchins Strongylocentrotus purpuratus kept on a natural photoperiod versus ones kept on a 6-mo out of phase photoperiod
photograph of a purple urchin Strongylocentrotus purpuratus
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Research study 6
 

The spawning season of purple sea-urchins Strongylocentrotus purpuratus in the Yaquina Head area of Oregon is from Dec-April.  Chatlynne 1969 Biol Bull 136: 167.

 

NOTE  an extract of radial nerve from this species will induce spawning within a moment or two in conspecifics as well as other echinoids.  The nerves are easily stripped from the ambulacral grooves.  Cochran & Engelmann 1972 Science 178: 423.

photograph of purple urchins Strongylocentrotus purpuratus in a tidepool in their protective hollows
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Research study 7
 
Many species of west coast invertebrates spawn in springtime, coincidental with seasonal phytoplankton blooms.  This makes sense, for most species produce free-swimming larvae that feed on phytoplankton.  For example, green sea-urchins Strongylocentrotus droebachiensis in Howe Sound, British Columbia spawn in precise timing with the phytoplankton bloom.  At first this was thought to be simply correlative, with both events responding to other factors such as temperature and sunlight, but observations that spawnings occur at the same relative timings even if the phytoplankton bloom is offset in successive years, suggests that causative factors are involved.  Later studies show that a chemical bound to, or released by, the phytoplankton may induce the adults to spawn.  Himmelman 1975 J Exp Mar Biol Ecol 20: 199; Starr et al. 1990 Science 247: 1071.
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Research study 8
 

Researchers in California grow red urchins Strongylocentrotus franciscanus in the laboratory at 2 temperatures (13 and 16oC) and 2 diets, one artificial and the other, the kelp Nereocystis luetkeana.  Gonad indices calculated at the beginning and end of a 4mo period (July-October) reveal significant change (from 3% to 19%), but no significant effects of either temperature or diet.  The authors note no obvious effects of the artificial diet on gametogenesis, but do not follow up with with a study of possible effects on larval development or metamorphosis.  McBride et al. 1997 J World Aquaculture Soc 28 (4): 357.

NOTE  the composition of this diet is given but not explained

NOTE  calculated as live gonad mass/llive body mass x 100

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Research study 9
 

histogram showing effect ofsupplemental feeding on gonad indices in red urchinsSea-urchin barrens are areas where the consumer, in this case, red urchins Strongylocentrotus franciscanus, have removed most or all seaweed foods.  In such areas gonadal indices (GIs) would be expected to be lower than in more food-rich areas.  Researchers from the University of British Columbia find that this is indeed so, and also find that supplemental feeding over an 83-d period not only significantly raises GI levels, but also decreases the incidence of dark-brown coloration of the gonads – a condition associated with starvation and resorption of gonads.  Note in the histogram that mean GI of photograph of red urchins at depthfield animals from March-June ranges around 3.2, while that of individuals fed over the same 83-d period is 8.3.  These values can be compared with GIs of about 12-18 for S. franciscanus from non-barren areas from other studies. Mooney & Bunnell 2001 Northwest Sci 75: 327.

NOTE  test individuals are maintained in field cages in Clayoquot Sound, British Columbia and fed ad libitum on kelp Macrocystis integrifolia for 83d.  Gonad indices are calculated as live gonad mass/total live mass

 

Red urchins Strongylocentrotus
franciscanus
in a "barrens" area

 
Research study 10
 

graph comparing swimming speeds of blastulae of sea urchins Strongylocentrotus spp. and sand dollar Dendraster excentricusResearch focus on aspects of planktonic development in sea urchins is mainly on the larval stage, but what about earlier developmental stages, such as blastulae?  Are they passively buoyant, or do they swim?  If they swim, what is its function?  An investigation at Friday Harbor Laboratories, Washington on 4 echinoid species abundant in the San Juan Islands shows that the blastulae swim actively, and that swimming speeds scale negatively with their size (see graph on Left).  In contrast, sinking speeds of unhatched blastulae show no significant scaling relationship with size.  Temperature effects on sinking are consistent for all 4 species, with significantly greater speeds being recorded at 10oC than at 14oC (contrary to expectation based upon seawater densities).  Interestingly, active swimming by the blastulae creates localised disturbances that, in the author’s opinion, may actually decrease encounters with planktonic predators, rather than increase them as has been commonly hypothesised.  So, rather than just a depth-regulating behaviour, swimming may actually play a defensive role.  This idea should be testable.  McDonald 2004 Biol Bull 207: 93; for an account of evolution of embryonic swimming (34 species in 10 phyla) see Staver & Strathmann 2002 Biol Bull 203: 58.

NOTE  Strongylocentrotus franciscanus, S. droebachiensis, S. purpuratus, Dendraster excentricus

NOTE  an idea that the motion of swimming may enhance oxygen uptake is also tested with Dendraster embryos, with results tentatively indicating a tolerance for low-oxygen conditions

 
Research study 11
 

photograph of green sea urchins Strongylocentrotus droebachiensisgraph comparing gonadal indices in green sea-urchins Strongylocentrotus droebachiensis under different photoperiodsAs shown in previous Research Studies, spawning in west-coast sea urchins is a winter-spring event, meaning that maximal gonad size occurs during a relatively short period before this.  This restricts the timing of harvesting of gonads along the west coast for the profitable sushi trade and the question arises as to whether manipulation of photoperiod following the start of gametogenesis can be used to modify the timing of gonad development, as well as their pre-spawning “marketability” condition (the best time for harvesting for the sushi trade is while the gonads are still growing; that is, before they start to “leak” gametes).  This is investigated by researchers in British Columbia using green1 sea-urchins Strongylocentrotus droebachiensis feeding ad libitum on bull kelp Nereocystis luetkeana under 5 different photoperiod regimes2. The experiments start in autumn when gametogenesis  begins, and runs for 12wk.  Results show that photoperiod has significant effects on gonad indices, with highest values occurring in the 16D treatment and lowest in the 8D and 0D treatments, but with considerable statistical overlap between the means (see graph).  Interestingly, photoperiod has little or no effect on rates of food consumption3, absorption, or assimilation, so differences in final gonad size or condition cannot be ascribed to variation in these parameters.  The factor or factors that cause the differences in gonadal indices are therefore unclear.  The authors conclude that once gametogenesis commences, manipulation of photoperiod will not significantly affect the timing of final spawning.  Dumont et al. 2006 Mar Biol 149: 365.

NOTE1  the authors do not explain why they choose green urchins for their study over the commercially important red urchin S. franciscanus, but presumably the general principles are broadly applicable.  Although the study is primarily aquaculture-oriented, it provides good data on consumption, absorption, and assimilation in a west-coast sea-urchin species; hence, its inclusion in the ODYSSEY

NOTE2  these are 24h light: 0h dark(OD), 16h light: 8h dark (8D), 8h light: 16h (16D), 0h: 24h (14D), and ambient (10-15h D).  A sixth treatment consists of starved individuals in 0D conditions

NOTE3  results in this regard are similar to those obtained for purple sea-urchins S. purpuratus in Research Study 5 above

 
Research study 12
 

map showing collecting sites for study of latitudinal variation in reproductive output in purple sea-urchins Strongylocentrotus purpuratusTo what extent does reproductive output in sea urchins vary geographically?  This is investigated in 15 populations of purple sea-urchins Strongylocentrotus purpuratus at sites from central California to the species’ southernmost range of distribution in Baja California, Mexico, spanning 9o of latitude.  The authors measure gonad indices at 2-monthly intervals over about a year at each site.  Seasonal variation in reproductive output follows the pattern reported by other authors (see Research Study 2 above), with a peak in late autumn/early winter and lowest levels in spring/summer.  All sites cycle in phase with each other.  Somewhat surprisingly, based on known latitudinal differences in algal productivity along the west coast, greatest reproductive outputs are at  the southern range boundary.  However, a definitive latitudinal gradient in reproduction is lacking.  Instead, the researchers identify smaller-scale variability correlative with certain topographical features known to influence seawater circulation and thus nearshore nutrient flux.  Purple urchins obtain most of their nutrients from algal bits that they snag from the currents, so both upstream macroalgal abundance and along-shore current patterns will be strong determinants of food availability.  Lester et al. 2007 Ecology 88: 2229.    

NOTE  the indices are calculated as the ratio of live mass of gonads to live total body mass

NOTE  as a proportion of the species’ entire north-south distributional range (Alaska to mid-Baja California = 32o latitude), 9o is relatively small.  However, an earlier study using S. purpuratus collected from 3 sites from Vancouver Island to Baja California
arrives at the same conclusions regarding lack of latitudinal differences: see
POPULATION & COMMUNITY ECOLOGY: GENE FLOW

 
Research study 13
 

Another investigation on purple urchins Stongylocentrotus purpuratus by researchers at the Scripps Institution of Oceanography, La Jolla focuses on the effects of temperature, water motion, and food availability on gonadal indices over a 2-yr period.   The population selected for study lives in and around a large kelp bed at Point Loma, California, an area with considerable contrasts in depth, temperature, light, food, and water movement.  Food in this photograph of purple urchins Strongylocentrotus purpuratusregion is mostly drift kelp Macrocystis, in greatest volume in shallow water and sometimes intertidally, depending on season and storms.  Analyses show that nitrogen content is greater in live than drift kelp.  Water temperatures vary from 11-22oC, with lower temperatures being found in the deeper parts of the study area (18m depth).  Water movements are greatest at depth.  The authors present masses of data, that can only be briefly summarized here.  Expectedly, high GIs occur during spring-autumn, and low GIs during late autumn-early winter.  Highest GIs are at 8m depth, lowest at 18m, and intermediate GIs occur in the intertidal zone.  Temperatures higher than 17oC have a slowing effect on gametogenesis.  The authors conclude that despite higher population density, high temperatures, and sporadic occurrence of drift algae, intertidal populations exhibit overall greater gonadal production than subtidal populations.  Basch & Tegner 2007 Bull Mar Sci 81: 255.

NOTE  gonad index (GI) = live mass gonads/total live body mass X 100%

 

Large assemblage of purple urchins Strongylocentrotus
purpuratus
, both intertidal and in a tidepool. Most
individuals are in protective depressions in the sandstone rock

 
Research study 14
 

map showing collection sites used in study of "terrestrial" effects on egg quality in sea urchins Strongylocentrotus franciscanushistogram showing number of eggs produced by red sea-urchins Strongylocentrotus franciscanus from sits varying in degree of "oceanic" exposure"histogram comparing protein contents of eggs of red sea-urchins Strongylocentrotus franciscanus in several island populations differing in degree of oceanic vs. river run-off exposureIn open-coast habitats where rivers are absent, egg production in sea urchins is solely dependent upon oceanic sources for nutrients and energy. But what about these same sea urchins in closer-to-shore estuarine habitats, where rivers may represent a source of dissolved nutrients for egg production?  In other words, what effect does carbon source, whether marine or terrestrial, have on egg quality in sea urchins.  This interesting idea is tested with red sea-urchins Strongylocentrotus franciscanus, sampled from several locations around the Bamfield Marine Sciences Centre, British Columbia differing in degree of “terrestrial” input.  Thus, sites are selected on a gradient from high oceanic exposure to proximity to freshwater. Note on the map that the "Bordelais" site is the most oceanic, while the "Tzartus" site is the most estuarine. The authors use the ratio of stable carbon isotopes (13C: 12C) as a bioindicator of extent of assimilation of terrestrially derived carbon in the production of eggs. 

Results show that sea urchins in the lowest wave-energy environments produce relatively more eggs than ones at the highest wave-energy sites (see graph upper Right). However, egg volume and egg biochemistry (protein, carbohydrate, and lipid contents) are the same at all sites (see graph lower Right for example data for protein only).  Most importantly, no significant spatial pattern is revealed for 13C: 12C ratios, suggesting that input of terrestrial organic matter is well diluted, even at the site closest to river discharge at the Sarita estuary.  The idea is a good one and the results, while not supporting the authors’ initial premise, have interesting ecological and environmental implications. The authors note that the consistency in biochemical composition of eggs of S. franciscanus is suggestive of significant phenotypic plasticity with respect to allocation of resources to egg production.  Berger & Jelinski 2008 Mar Ecol Progr Ser 364: 119.

NOTE  the ratio of carbon isotopes (13C: 12C) can be used to infer whether an organic substance has a terrestrial origin or a marine origin.  The researchers also analyse for stable nitrogen-isotope ratios, but the results are not included here

 
Research study 15
 

Use of gonad indices as a measure of change in reproductive state in invertebrates has inherent problems relating to methodology and scaling.  An informative review by a research consortium based in Oregon lists methodologies used in the past for sea urchins1, brittle stars, sea stars, sea cucumbers, crustaceans, cnidarians, and cephalopods, and points out that an assumption of isometric scaling is theoretically only applicable when the measurements made are in the same units.  Thus, scaling relationships are predicted to be isometric for mass/mass or volume/volume, but allometric when units are mixed, such as volume/mass, volume/test diameter, or the like.  However, even when the units used are the same, an assumption of isometry may not hold throughout all stages of the reproductive cycle, and gonads must be further assumed2 to begin to develop at zero size.  To test these ideas the researchers compare live masses of gonads in purple urchins Strongylocentrotus purpuratus with total live masses and test diameters over 31 monthly collections from Gregory Point on the Oregon coast.  Results show that gonads begin to develop at a size of about 0.7cm test diameter, therefore requiring an additional parameter be included in regression analysis.  Further, the allometric exponent obtained for the collection is not the expected 1 for mass/mass comparisons, nor is it 3 for mass/diameter comparisons.  For this reason, the authors recommend3 that comparisons be made at a standard size of 5cm test diameter (60g total live mass) and, to enable this, that data be collected from as wide a  size-range of individuals as possible, rather than from, say,  just the larger individuals in a population.  This allows gonad indices to be calculated using an allometric growth model.   Ebert et al. 2011 Mar Biol 158: 47.

NOTE1  in their review the authors list 13 different methodologies that have been used for sea urchins from 1955-2007, the most recently popular being the ratio of live mass of gonads/total live mass

NOTE2  another statistical problem, noted by the authors, concerns the inclusion of the same variable in both numerator and denominator of a ratio

NOTE3  the authors also recommend that the summary statistics for any study be included in publication

 
Research study 16
 

graph showing effect of food ration on gonad growth in purple urchins Strongylocentrotus purpuratus Researchers at San Diego State University, California find that food limitation during laboratory culture of purple urchins Strongylocentrotus purpuratus may not just slow down gonad development, but may affect gamete viability depending on time of year.  In southern California gamete development begins in late summer/autumn, with spawning in springtime.  In a series of laboratory experiments the researchers monitor the effects of food ration on gonadal growth and gamete formation over 3mo periods in autumn (Sept-Dec), spring (Feb-May), and summer (July-Sept).  Results show that gonad growth, while significantly affected by ration, does not differ among seasons (see graph).  Note, however, that a trend of higher gonad indices in autumn is present, as would be expected based on previous studies (see Research Study 2 above), but significance is likely obscured by high variability in the data. The main finding in the study is that food limitation in autumn when gonads are just beginning to grow may lead to failure to produce viable gametes.  In comparison, food limitation later in gonad development does not inhibit production of viable gametes, but does reduce gamete output.  Dodge & Edwards 2012 Mar Biol 159: 427. 

NOTE  rations are set at 7 levels, in accordance with how many days per week the test animals are fed.  Food consists of fronds of giant kelp Macrocystis pyrifera

NOTE  gonad indices are calculated as the ratio of live gonad mass to total live mass expressed as percentages

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