Physiology & functional morphology
  Aspects of physiology of sea urchins relating to temperature, salinity, and pH are presented first, followed by a section on FUNCTIONAL MORPHOLOGY.
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  Physiology
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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
 

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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Research study 3

 

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 4
  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 5
 

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 can be considered to be temperature-dependent

 

Aggregation of sea-urchins Strongylocentrotus fragilis

 

 

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Research study 6
 

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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Research study 7
 

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 pHs down to pH 7.7 and measuring lengths 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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  Functional morphology
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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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