Physiology & functional morphology
  Aspects of physiology of sea urchins relating to temperature & thermal tolerance, gas exchange metabolism & Q10, salinity, pH & ocean acidification, and water currents are presented in separate subsections below.
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Temperature & thermal tolerance

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Research study 1
 

photograph of purple sea-urchins Strongylocentrotus purpuratus in protective hollowsA study on thermal tolerance and acclimation to temperature in purple sea-urchins Strongylocentrotus purpuratus at Hopkins Marine Station, Pacific Grove, California shows that they tolerate temperatures from 5-23oC.  If sea urchins are acclimated to low temperatures for periods of up to 35d, their respiratory rates gradually increase, and their behaviour, such as righting ability, becomes nearly normal.  In contrast, exposure to high temperature, such as at 20oC for 10d does not lead to acclimation, and exposure of individuals to 25oC is invariably lethal.  The authors provide evidence that S. purpuratus is acclimated to temperature conditions in different parts of its geographical range.  The means by which acclimation occurs is not known, but is likely related to quantitative and qualitative changes in metabolic enzymes.  Farmanfarmaian & Giese 1963 Physiol Zool 36: 237.

NOTE  in this area annual temperature records (10yr period) show a range of 9-17oC

 

 

 

 

 

Large aggregation of purple sea-urchins Strongylocentrotus
purpuratus
in protective hollows in sandstone. For individuals
exposed to air, harmful temperature and humidity fluctuations
in such habitats are additionally ameliorated by overhanging seaweeds
and by coverings of shells and stones. A single red urchins S.
franciscanus
has tried to fit itself into one of the too-small hollows

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Research study 2
 

map of collecting sites for purple urchins Strongylocentrotus purpuratusWhat is the thermal tolerance of sea-urchin embryos and larvae?  This is tested by researchers at the University of California, Santa Barbara by collecting adult purple urchins Strongylocentrotus purpuratus from 4 locations along the west coast (see map), rearing eggs to 4-arm pluteus stage, then measuring survival and levels of the stress-induced gene (hsp70) in gastrulae and larvae following 1h exposure to a range of temperatures (10-32oC). Results show no significant difference between the developmental stages at all sites in survival or levels of hsp70 expression.  Surprisingly, maximum hsp70 expression is exhibited at 25oC at all sites, but a somewhat purple urchins Strongylocentrotus purpuratus in tidepoolnarrower range in southern populations suggests that they are closer to their upper temperature limit than are northern populations.  The lack of a clear latitudinal trend suggests that the developmental stages of S. purpuratus have similar temperature tolerances throughout the portion of their distributional range studied.  Hammond & Hofmann 2010 Mar Biol 157: 2677.

NOTE  a family of genes that codes for heat-shock proteins.  These are molecular chaperones that aid in refolding or translocation of proteins damaged by temperature or other stresses, thus allowing the cell to maintain proper functions.  Heat-shock proteins are considered for adults of many marine invertebrates throughout the ODYSSEY, but this is the only report thus far for developmental stages

NOTE  with regard to this, one wonders why the authors did not extend their collections over the full range of the species’ distribution, i.e., from Baja California to Alaska?

 


Purple urchins Strongylocentrotus purpuratus in
protenctive hollows in a tidepool 0.25X

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Gas exchange, metabolism, and Q10

 

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Research study 1

 

Gas exchange in sea urchins occurs by diffusion through the thin-walled epithelia of the tube feet and, to a lesser extent, through the peristomial gills on the oral surface.  There is no circulatory system, and gases move to and from the organs via the coelomic fluid in the main body cavity.  During reproduction this cavity becomes largely filled with gonads, and the question arises as to whether oxygen consumption will rise correlative oxygen consumption of purple sea-urchins Strongylocentrotus purpuratus in relation to gonad indexwith the extra mass of gonads, or fall because of reduced diffusion.  This issue is addressed in a study at Hopkins Marine Station, Pacific Grove, California on purple urchins  Strongylocentrotus purpuratus.  Results show that oxygen consumption actually decreases with the extent of reproductive maturation (see graph). Measurements of oxygen consumption of different bodily components show that the body wall accounts for about 72% of the respiration in non-gravid sea urchins, with gonads accounting for only about 2%.  In gravid animals, however, gonads should theoretically increase respiratory needs by about 30-50% for females and males, respectively.  That these increased respiratory needs are not reflected in overall oxygen-consumption values in gravid animals is thought by the authors to owe to reduced diffusion of gases.  Giese et al. 1966 Biol Bull 130: 192.

NOTE  diffusion rates will be impeded both by the bulk of the gonads and by relatively less coelomic fluid being present

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Research study 2
  graph showing the rate of decline in oxygen tension within the coelomic fluid of a purple sea-urchin held in air for several hoursAnother problem with living in a calcium-carbonate box is that gas exchange by diffusion is limited.  This is no problem when the sea urchin is immersed because the many tube feet can be extended to provide a large surface area for diffusion of oxygen and carbon dioxide.  In air, however, it is a different story.  Studies at Friday Harbor Laboratories, Washington show that if a purple urchin Strongylocentrotus purpuratus is exposed to air the oxygen tension in its coelomic fluid decreases steadily to a level about 25% of the level when immersed and stays at this level for up to 15h.  The volume of coelomic fluid in sea urchins therefore represents a store of oxygen that can be drawn on in times of need. The gas-exchange capability in air is effective for the duration of normal tidal exposure.  Johansen & Vadas 1967 Biol Bull 132: 16.

NOTE  although 2 other species are included in the study (S. franciscanus and S. droebachiensis), because they are not common intertidally and because they behave in a similar pattern to the one shown for S. purpuratus, their data are not included here.  Water temperature in the study is about 10oC and air temperature, about 15oC
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Research study 3
 

graph comparing oxygen-consumption rates in 3 species of sea urchins Strongylocentrotus purpuratus, S. franciscanus, and S. fragilisThree species of sea urchins occur in shallow waters off the coast of Oregon.  One of these, Strongylocentrotus purpuratus, occurs intertidally as well as subtidally; a second species, S. franciscanus, occupies a vertical range extending from the low intertidal area to subtidal regions; and a third species, S. fragilis1, lives only subtidally.  The intertidal species would, of course, be exposed to greater thermal variation than the subtidal species.  Predictions with respect to acclimatisation of metabolic rate to temperature would then be greater temperature indepence in the intertidal species and greater conformity in the subtidal ones.  This is tested at the Marine Science Center, Newport, Oregon, with results as shown in the graph2.  Although the experimenters do not analyse their data statistically for differences in slope, they do provide a “running” estimate of Q10 values for each species for 3oC temperature increments.  For S. purpuratus Q10’s start at about 2.6 and end at about 1.8 suggesting a degree of temperature compensation.  If the 9oC "outlier" datum point is omitted as likely being aberrant3 for S. franciscanus, the comparable values are 2.8 and 1.5, again suggesting temperature compensation.  Finally, for S. fragilis, the Q10 values are 1.1 photograph of deep-water sea-urchin Strongylocentrotus fragilis, courtesy NOAA, Goverment of U.S.and 2.2, indicating little or no temperature compensation over the range tested.  Ulbricht & Pritchard 1972 Biol Bull 142: 178. Photograph courtesy NOAA, Federal Government, USA.

NOTE1  formerly in the genus Allocentrus; now Strongylocentrotus

NOTE2   sample sizes are 12 for S. purpuratus and S. franciscanus, and 6 for A. fragilis.  The authors, however, use some individuals multiple times, adding a component of pseudoreplication to their data

NOTE3  however, if the 9o value is accepted as being valid, as the authors rightly do, then S. franciscanus at least for the time being can be considered to be temperature-dependent

 

Aggregation of sea-urchins Strongylocentrotus fragilis

 

 

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Salinity

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Research study 1
 

Most species of echinoderms are intolerant of low salinities and west-coast sea urchins are no exception.  A study on purple urchins Strongylocentrotus purpuratus collected from Moss Beach, California and nearby areas shows that exposure to 70% seawater for as little as 3h can cause deleterious effects, both on adults and on developmental stages.  Giese & Farmanfarmaian 1963 Biol Bull 124: 182. 

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pH & ocean acidification

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Research study 1
 

histogram showing pH effects on developiing sea-urchin larvaeThere is growing interest in effects of increased ocean acidification accompanying climate change, but what is not generally known is that predicted century-end pH values are presently manifested in certain west-coast upwelling systems.  For example, even in Santa Barbara Channel, an area not known for intense upwelling, researchers from University of California, Santa Barbara have measured day-to-day pH fluctuations of up to -0.7 units.  The extent of effects of low pH on development of purple urchins Strongylocentrotus purpuratus is examined by rearing 4-arm pluteus larvae at 3 pH levels down to pH 7.7 and measuring body at Days 3 and 6 of development.  Results show no disruption of normal developmental pattern, but larvae in the lowest pH treatment are significantly smaller than ones raised under control conditions (see sample graph).  The size reduction is small, only 7-13% but, as noted by the authors, may have implications for swimming and feeding.  Yu et al. 2011 J Exper Mar Biol Ecol 400: 288.

NOTE  the differing pHs are obtained by equilibrating seawater with C02 of 370, 1000, and 1450ppm.  The first is a control, and the others give pHs of 7.7 and 7.5, respectively  

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Research study 2
 

photographs of larvae of mussel Mytilus trossulus and sea urchin Strongylocentrotus franciscanusA similar investigation but with broader goals is undertaken by researchers at the Bamfield Marine Sciences Centre, British Columbia who attempt to quantify the potential for  evolutionary response to ocean acidification in sea urchins1 Strongylocentrotus franciscanus and mussels Mytilus trossulus.  The measures chosen to assess this are levels of phenotypic and genetic variation in larval2 size (arm and shell lengths, respectively) for predicted future atmospheric carbon-dioxide levels.  Estimates of maternal and paternal sources of variation provide indices of potential heritability, and are used in simulations of multi-generational selection to the year 2100 to assess the potential of each species for adaptive evolution.  This is a tall order, and the authors should be commended for their ambitious research aims. Larvae of each species are cultured from hatching to start of feeding (65h for M. trossulus and 5d for S. franciscanus) at pH levels of 8.3 (ambient) and 7.9 (predicted for year 2100).  Initial growth results are presented here only for sea urchins S. franciscanus, but levels of phenotypic and genetic variationin this species are found to be much greater than in mussels M. trossulus.  Note in the graph that effects of increased acidity on growth are about -2%, and hereditary effects come mostly from maternal contributions. Estimates of heritability over 45 overlapping generations (to year 2100) are up to 140 times greater for sea urchins than mussels. The presumptions3 and analyses in the study are complex, and readers are invited to assess for themselves the level of success achieved.  In using this approach, the authors have attempted to sidestep the criticisms leveled at other similar studies that simply assess acute responses to pH without incorporating an estimated time-scale for adaptation.  Sunday et al. 2011 PLoS ONE 6 (8): e22881.

NOTE1  the selection of such disparate species appears to be based on their mutual economic importance to the west-coast mariculture industry

NOTE2  larvae are cultured in a large factorial-design cross with 9 females and 10 males for S. strongylocentrotus and 10 males and 4 females for M. trossulus.  Culture temperature is 12oC

NOTE3  for example, larval fitness is estimated from literature reports on survival in relation to larval duration in the plankton (mortality from predation, advection, starvation, and disease).  On these bases, longer developmental time is equated with decreased fitness

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Research study 3
 

graph showing effect of 6d exposure to different pHs on growth of larvae of purple sea-urchins Strongylocentrotus purpuratusgraphs showing effect of pH on lipid utilisation during early development of purple urchins Strongylocentrotus purpuratusWhat may not be commonly known with respect to ocean acidification and global warming is that in regions of the California coast natural upwelling events regularly bathe inshore areas with water of pHs1 equivalent to, or higher than, those predicted by Climate-Change experts to occur 100yr in the future.  A question raised by scientists from the University of California, Santa Barbara and University of Aukland with respect to this relates to the extent of plasticity of development in sea urchins Strongylocentrotus purpuratus, given their past evolutionary contact with lower pH waters.  The researchers rear larvae for 6d to the 4-arm pluteus stage in culture2 pHs of 8.1 (control), 7.7, and 7.5, and monitor rates of growth and lipid/protein/phospholipid utilisation at intervals over that period.  The larvae are not fed in culture in order to force them to use their endogenous lipid stores.  Results show a significant reduction in growth at elevated pCO2 levels (see sample graph for a single female on Left) but with no associated change in lipid/protein utilization rates (sample graphs on Right for the same female).  Initial phospholipid contents of the eggs are observed later to affect larval size, but only at control pCO2 levels, not elevated ones.  The authors suggest that in upwelling regions with relatively high natural levels of pCO2, purple urchins may have an inherent physiological plasticity to resist exposure to moderately low pHs (at least during early development).  The study provides a useful introduction to the growing subject of ways in which impending ocean acidification may affect the physiology3 of marine invertebrates.  Matson et al. 2012 Biol Bull 223: 312.

NOTE1  as low as 7.6 have been recorded during such upwelling events

NOTE2   pHs are obtained by bubbling seawater with CO2 at concentrations of 365uatm (pH 8.1), 1039uatm (pH 7.7, and 1444uatm (pH 7.5)

NOTE3  an interesting sidebar comment by the authors is that meta-analyses of data on responses of a variety of marine species to ocean acidification can be positive or negative but are, on average, negative

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Research study 4
 

In a related study to the preceding one (Research Study 3), members of the same research group at University of California, Santa Barbara investigate the capacity of purple urchins Strongylocentrotus purpuratus to adapt to increased levels of ocean acidification expected from future climate change.  Larvae are bred from adults collected from 2 locations that differ markedly in their natural pH levels, and larval size is again used as an expression of fitness.  In the present study, male and female urchins are cross-bred from the 2 sites.  Since males contribute only genetic material, this enables the genetic variation for size of larvae to be teased out and compared for males from low- and high-pH sites.  Thus, if there is local adaptation to pH, offspring of males from the low-pH site would be expected to produce larger larvae under low-pH experimental conditions than offspring from males from the high-pH site.  Larvae are reared for 5d (just to the end of the nonfeeding stage) under 2 pH levels: 8.0 for present ocean conditions and 7.6 for expected year 2050 conditions.   Results show that larvae, contrary to prediction, are actually about 10% smaller under future conditions than under present-day conditions, but that genetic variability is higher, indicating greater scope for heritability of size (i.e., evolution).  Comparison of growth performance of larvae cultured from adults from the 2 locations also suggests a slightly greater scope for growth of larvae cultured from adults from lower pH natural conditions (pH~7.9) than from normal pH conditions (pH~8.1).  There thus appears then to be local adaptation to different pH conditions in the field.  The concept and design of the experiments are admirable, and the authors are to be complimented on their fine work.  Kelly et al. 2013 Global Change Biol 19: 2536.

NOTE  for whatever reason the authors do not cite the previous study in their paper

NOTE  upwelling in some California locations brings colder, lower pH waters to the surface, in some cases with pHs as low as projected for the end of this century

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Research study 5
 

graphic illustration of pH effects on transcriptomic responses in purple sea-urchins Strongylocentrotus purpuratusWhat can transcriptomics1 tell us about the level of metabolic activity in an organism, such as a sea-urchin embryo, being challenged by exposure to low pH seawater?  Will levels in offspring of parents from upwelling areas that are regularly exposed to fluctuating2 low-pH conditions differ significantly from ones from more steady higher pH conditions?  These questions and others are addressed by University of California Santa Barbara researchers using developmental stages of purple urchins Strongylocentrotus purpuratus cultured in 3 pCO2 levels to reflect present-day pH ("high"=8.0 and "mid"=7.8) and predicted future ocean conditions ("low"=pH 7.6).  Results show an ability of gastrulae (44h post-fertilisation) and early plutei (92h) to maintain calcification3 activity under "mid" pH conditions, but this ability falters in simulated future "low" pH conditions.  Thus, transcriptomic response is actually greater under present-day pH regimes than under simulated future conditions.  The authors identify 4 cellular processes in gastrulae most affected by elevated pCO2 level: calcium homeostasis, ion transport, cell signaling, and transcription.  Of 153 genes identified as being differentially expressed in gastrulae in the mid-pH culture (7.8), 48 are down-regulated and 105 up-regulated relative to the high pH culture of 8.0.  In comparison, only 10 genes are identified as being differentially expressed in the low-pH future-culture conditions (7.6), and all are down-regulated (see “heatmap” illustration).  Three of these 10 genes have previously been identified as transport proteins involved in pH regulation.  Given that present-day upwelling-induced reductions in pH are moderate and temporary, the gastrulae could recover, but on the basis of the transcriptomic evidence the authors doubt the ability of the gastrulae to cope with more drastic future CO2-driven decreases in pH.  In the plutei only a handful of genes  are differentially expressed across all pH levels, indicating a generally less robust transcriptomic response to decreased pH levels.  The researchers consider these results as only temporary, and are now on a quest to find purple-urchin habitats with even more variable pH regimes.  The present research makes an excellent contribution to the topic of ocean acidification and the researchers are once-again to be congratulated.  Evans et al. 2013 Mol Ecol 22: 1609.

NOTE1  this is the measure of all RNA molecules being produced in a cell and is thus an indication of gene expression at any given time.  The processes of RNA extraction and labeling,  and quantification and statistical analyses of gene expression are complex and labour intensive

NOTE2  natural fluctuations in pH in the area of the Oregon coast from which the broodstock urchins are collected range from 8.4 to 7.5.  Some of the fluctuation owes to daily respiration/photosynthetic cycles, but extreme low pHs are mostly credited to upwelling

NOTE3  the larval skeleton begins building in the gastrula stage in the form of calcareous spicules, and culminates in the pluteus stage in the form of calcareous body framework and arm rods

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  Water currents
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Research study 1
 

graph showing effect of current speed on spine angle in red sea-urchins Strongylocentrotus franciscanusSCUBA-diving observations by researchers at Friday Harbor Laboratories, Washington reveal that red sea-urchins Strongylocentrotus franciscanus adopt more streamlined spine-angle shapes in faster current flows.  The streamlining changes the shape of an urchin from typical pincussion-shape to more of a limpet-shape. The authors use experimental flumes to quantify decrease in spine angle in flow speeds of 0-65cm . sec-1 (see graph and photo series below).  Measurements of drag and lift in photograph of red sea-urchin Strongylocentrotus franciscanus with green alga Ulvarealistic models show that streamlined models have less drag but more lift than “normal” models in current speeds between 10-40cm . sec-1, rates that are commonly encountered by sea urchins in the field.  The authors remark that the behaviour, although doubtless contributing to better attachment in high current flows, may lead to reduced capture of drift algae, especially in deeper water.  The algae, which ar trapped on upright spines and then eaten, pass by in quantities directly related to current speed.  In shallower water, the sea urchins tend to feed more directly on attached algae such as kelps.  Stewart & Britton-Simmons 2011 J Exp Biol 214: 2655. Photo series below courtesy the authors.


Red sea-urchin S. franciscanus with
entrapped green algae Ulva sp. 1X

 
Effect of current speed on spine angle
in red sea-urchins Strongylocentrotus franciscanus. Spine angles range from about 40o up to about 35o down at
the 2 extremes of current speed
photo series showing effect of current speed on streamlining of spines in red sea-urchins Stroongylocentrotus franciscanus    
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