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  Life in the intertidal zone

photograph showing several ochre stars Pisaster ochraceus clustering together at low tideOnly a few west-coast sea-star species live intertidally. These include Pisaster ochraceus and Leptasterias hexactis in the mid-intertidal zone, and Evasterias troschelii in the mid-low intertidal zone. Some other species, such as Dermasterias imbricata, Pisaster giganteus, and Pycnopodia helianthoides prefer to live subtidally, but may find themselves occasionally emersed in the lower part of the intertidal zone. Conditions in the intertidal region relate not only to factors associated with periodic absence of seawater, including drying, UV irradiation, and rainfall, but also to factors associated with waves, including dislodgement, impact from floating objects, and sediment abrasion.

Ochre stars Pisaster ochraceus cluster
together at low tide possibly to conserve
moisture and to protect from avian predators

  The topic of life in the intertidal zone includes a section on temperature & desiccation considered below, and sections on WAVES & CURRENTS, SALINITY & OSMOTIC REGULATION, SEASTAR WASTING DISEASE, OCEAN ACIDIFICATION, OTHER PHYSIOLOGICAL STRESSES, COLOUR MORPHS OF PISASTER, and SYMBIONTS presented elsewhere.
photograph of a few ochre stars Pisaster ochraceus being washed by the waves

CLICK HERE to see a video of ochre stars Pisaster ochraceus being washed by the waves.

NOTE  the video replays automatically

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Temperature & desiccation

Research study 1
photograph of an ochre star Pisaster ochraceus
Pisaster ochraceus 0.4X
photograph of sea star Pisaster giganteus courtesy NOAA and Channel Islands Marine Sactuary
Pisaster giganteus 0.5X
photograph of a sea star Pisaster brevispinus eating a clam
Pisaster brevispinus eating a clam 0.3X

Three Pisaster species inhabit west-coast shores with the ochre star P. ochraceus living highest in the intertidal zone on rocky shores.  Pisaster giganteus occurs lower in the intertidal zone on rocky shores, but is more common subtidally, while P. brevispinus is strictly subtidal on sand/mud flats.  A comparison of water loss during air exposure in the 3 species in Santa Barbara, California shows, as expected, that the higher-dwelling P. ochraceus is graph showing comparative water loss after 1h in the sun for three species of sea stars Pisasterslightly more resistant to water loss than P. giganteus or P. brevispinus (12%, 15%, and 15% losses in body mass after 1h exposure to direct sun, respectively).  Although the data seem highly variable and overlapping, the author reports the means to be significantly different, but only for P. ochraceus.  The author notes that relative water loss in the 3 species is not related to body size and suggests that differences in structure of the body wall may account for the different abilities to withstand air exposure.  Landenberger 1969 Physiol Zool 42: 220.

NOTE  air temperature 22oC, 53-61% RH.  Also included in the paper is a graph showing comparable losses in the shade but, as these shade data show a similar pattern, they are not included here

NOTE  the losses are based on live mass, so it is puzzling why the data are converted into dry-mass units (how does one visualise the equivalent live size of a 100g dry-mass individual?).  However, other data provided by the author show that water contents are similar in the 3 species, at 75-78% so, to answer the question, simply multiply dry masses by a factor of 1.3 to get live mass in g.  Nonetheless,
the author agrees with other researchers that live mass gives a reliable measure of size

Research study 2

schematic showing heat budget of an ochre star Pisaster ochraceusIn anticipation of impending climate change and its potential effect on intertidal organisms, there has been increasing attention on development of heat-budget models for various intertidal organisms, and several of these studies have included “thermal mimics” of organisms. The budgets use several parameters to predict, in this case, the body temperature of sea stars Pisaster ochraceus at various states of tidal exposure, air and body temperature, solar radiation, wind velocity, humidity, and so on, to determine heat flux (see accompanying model). The predictive accuracy of the model is tested by comparing model-generated data against actual measured body temperatures of air-exposed Pisaster at several sites in the field during 26d in summer.  Environmental data such as wind speed, solar radiation, relative humidity are collected from a small weather station set up each day adjacent to the test sea stars in the intertidal zone, while water temperature, cloud cover, and other climate data are assembled from local weather stations and weather-satellites.  Ochre stars with implanted thermocouples are used for comparison with model predictions. 

graph comparing body temperatures of ochre stars Pisaster ochraceus measured in the field with those predicted from a heat-budget modelThe graph on the lower Left shows good correlation (to within about 1oC) between model-output data and actual body temperatures.  Note that the flat part of the curve in the graph represents the high-tide water temperature.  At the 120-min mark the thermocouples are emersed by the receding tide, and actual body-temperature data commence at the 282-min mark.  The variability between daily maxima model-output temperatures and measured Pisaster temperatures is provided by the authors, but not shown here. 

Interestingly, and representative of the kinds of useful data that can be generated by such graph showing the relationship between body temperatures of sea stars Pisaster ochraceus on a certain day and the numbers of sea stars that are present in the same area on the following daya model, the experimenters find a negative correlation between the maximum body temperatures on Day n and the number of sea stars present in the same part of the intertidal region on Day n + 1.  What this means is that the previous day’s low-tide body temperatures may be a good predictor of where in the intertidal zone the sea stars will be found on subsequent days.  Szathmary et al. 2009 Mar Ecol Progr Ser 374: 43.

NOTE  these take the form of actual models, such as limpets or, in the case of the present study on sea stars, of “biomimetic” temperature loggers constructed of solid foam discs that thermally mimic a live Pisaster.  The model used here employs some 30 parameters

NOTE  rocky shore areas near the Bamfield Marine Sciences Centre, Bamfield, British Columbia

Research study 3

It is generally believed that echinoderms, along with most other marine invertebrates, excrete ammonia as their primary by-product of protein catabolism and, hence, require immersion in seawater to rid the body of this toxic substance.  However, a survey of sea stars at Friday Harbor Laboratories, Washington shows that 90% excrete significant quantities of urea (>10% of total nitrogenous products excreted) and, in some species, for example, Orthasterias koehleri (66% urea), Pisaster ochraceus (54%), Pteraster tesselatus (73%), and Luidia foliata (50%), urea respresents the major form of N excretion.  The author notes that while the subject needs to be further investigated, simple “eye-balling” of the data reveals no obvious correlation with extent of intertidal exposureStickle 1988 Comp Biochem Physiol 91A: 317.

NOTE  several other urea-excreting (ureotelic) echinoderms are identified, but not sea urchins, which are primarily ammoniotelic

NOTE  ureotely is considered to be a water-saving strategy in terrestrial and semi-terrestrial animals. In general, ammonia is most soluble and most toxic, and requires the most water for its elimination. Uric acid, in comparison, is least soluble and least toxic, and requires no water for its elimination. it is excreted as harmless crystals in those animals that use it: for example, birds, lizards, insects, and some snails

Research study 4

Biological effects of changing seawater temperatures through El Niño and climate change are becoming  increasingly evident on a global scale.  But what about small-scale temperature effects, such as on the intensity of a species’ interaction with its prey, competitors, predators, and so on?   As evidenced graph showing effects of upwelling on population metabolism of sea stars Pisaster ochraceusby the example of ochre stars, a few key interactions may contribute disproportionately to maintaining the composition and functioning of a community. If these interactions are especially sensitive to temperature, then even small climatic change could effect broad-scale changes in community dynamics.  As an example, field studies in Oregon show that even a slight drop of 3-5°C in water temperature through upwelling greatly modifies the impact of a keystone predator Pisaster ochraceus on its principal prey, the mussels Mytilus californianus and M. trossulus.  The effects of the upwelling are 2-fold: first, a 68% reduction in individual feeding rates of the sea stars and, second, a reduction in the number of sea stars actually present on the shore (they remain inactive in channels or in shallow subtidal waters).  Combined, the temperature drop through upwelling reduces predation on the mussels by about 75%.  Sanford 1999 Science 283: 2095; see also Sanford 2002 J Exp Mar Biol Ecol 273: 199. 

NOTE  this is where deeper water that is usually colder, often more saline, and mostly richer in nutrients moves to the surface through displacement of the surface waters, usually by winds.  Upwelling is a seasonal event in many regions of the Pacific west coast and is associated with such things as colder inshore waters, localised fogs, and seasonal phytoplankton blooms

Research study 5

But isn’t temperature one of the principal effects on activity in living organisms? If seawater temperature decreases during upwelling wouldn’t it be expected that feeding activity of the sea stars would decrease commensurately?  Yes, but temperature affects an organism generally by doubling or halving its metabolic rate for every increase or decrease, respectively, of 10°C.  The Pisaster in Oregon respond to a drop of only 3-5°C with a 75% decrease in feeding activity, much more than would be predicted on the basis of metabolic response to temperature. The study demonstrates that relatively small temperature shifts and also perhaps their seasonal timing can markedly influence the local strengths of a keystone interaction. Sanford 1999 Science 283: 2095.

NOTE  this is known as temperature coefficient or Q10, where a value of 2 represents a doubling of metabolic rate resulting from a 10°C rise in temperature. While a handy “rule of thumb”, Q10 should be interpreted cautiously, as values measured may be specific to an organism, or to the particular chemical reactions being assessed, and may be applicable only to a particular temperature range.  The magnitude of Q10 also varies with the thermal history of an organism, with its activity level, feeding history, body size, reproductive condition, and so on

Research study 6

map showing Santa Cruz Island study siteFor reasons, perhaps convenience or expediency, researchers interested in modeling different climate-change scenarios often assume that habitat temperature is equivalent to an organism’s body temperature.  At least, that is the contention of a group of southern-California researchers who set out to test whether it is true for sea stars Pisaster ochraceus and their major prey mussels Mytilus californianus at 4 sites around Santa Cruz Island, California (see map).  The sites are selected for their extreme differences in temperatures, the western-most sites experiencing cooler ocean temperatures than the southern-most sites, some 15-20km distant.  The researchers do not implant temperature sensors directly into study organisms; rather, they use “sea star” and “mussel” biomimetic loggers, set out in pairs in air-exposed mid-intertidal locations at the 4 sites.  Temperature data are recorded every 30min or so over a 40dd period in summer.  Results show that “body” temperatures at the 2 western-most sites follow markedly different trajectories, even though the 2 “animals” are occupying identical “micro”habitats (the data loggers are separated in some cases by as much as 20cm). At the 2 southern-most sites the data from the 2 biomimetics are more closely coupled.  As well as providing actual temperature data recorded by each type of logger, the authors show informative graphical representations of Kendall-correlation coefficients that show the extent to which the temperature profiles of the  2 “species” are coupled over time lags of up to 4h.  The authors conclude that “the well understood predator-prey interaction between Pisaster and Mytilus cannot be predicted based on habitat-level information alone, as is now
commonly attempted with landscape-level models”.  Broitman et al. 2009 Oikos 118: 219.photograph of sea stars Pisaster ochraceus eating sea mussels Mytilus californianaus

NOTE  the authors cite several references to support their contention, but several recent studies including Research Study 2 above have employed more precise temperature-recording methodology, including implanted thermocouples and biomimetic temperature loggers apparently similar or even identical to the type used in the present study

NOTE  no details are provided as to the physical nature of the instruments, nor their precise position relative to one another in the intertidal zone. For example, Pisaster often humps up over its prey, which presumably insulates the latter from temperature extremes, and the predator itself may shift position in hot weather from more exposed to more sheltered crevice positions.  For convenience, the researchers refer to “body” temperatures of “sea stars” and “mussels” throughout their paper. However, without some indication that the “biomimetics” are actually doing their job in simulating the thermal characteristics of the real thing, a certain scepticism may be justifiable.  In this regard, would it not have been possible to implant temperature loggers directly into the organisms themselves, even if only over a single tidal cycle? 


Sea stars Pisaster ochraceus clustered with prey mussels
Mytilus californianus
and barnacles Balanus glandula 0.4X

Research study 7

photograph of a juvenile sea star Pisaster ochraceus air-exposedWhen the tide recedes, intertidal sea stars can seek shelter from sunlight and warm air temperatures by moving down the shore and into protective crevices.  Another novel way for them to thermoregulate in hot weather, as shown in a study on ochre stars Pisaster ochraceus at Bodega Marine Laboratory, California, is for an individual to draw cold water into its coelomic cavity prior to the next low tide, that is, prior to its next exposure to higher temperature conditions.  The buffering capacity of the behaviour is most effective when the seawater is cold during the previous high tide.  To test this the researchers maintain 3 groups of sea stars in seawater baths of 10, 13, and 16°C over a 6-d period, exposing them for 6h out of each day to air temperature of 27°C.  Results over 5d of treatment show that exposure to cooler water during “high tides” leads to lower body temperature during subsequent low tides.  As coelomic fluid may constitute up to 30% of the live body mass of P. ochraceus, the buffering potential of its exchange may be considerable.  Increase in live body mass measured at the end of each high tide is significantly greater in individuals maintained in the warmer “high-tide” treatments.  In one experiment, these “warm-water” individuals gain about 15g of coelomic fluid, representing about 8% of their initial starting live masses.  The authors note that the physiological mechanism for body-fluid regulation in sea stars is not known, although the madreporite has been implicated in other studies.  Pincebourde et al. 2009 Amer Nat 174: 890.

NOTE  for information on the function of the madreporite in sea stars, see LEARN ABOUT TUBEFEET & LOCOMOTION: FUNCTION OF THE MADREPORITE


Juvenile Pisaster ochraceus beginning to dry during tidal
exposure. The smooth texture of the skin around the central
disc owes to the presence of partially inflated dermal
not yet retracted into the coelomic cavity 1X

  Research study 7.1

graph comparing aerial and aquatic oxygen consumption in ochre sea stars Pisaster ochraceusphotograph of ochre stars Pisaster ochraceus exposed at low tideWhat are the physiological costs of stresses associated with intertidal life in a sea star?  This is assessed by a group of researchers at  Bodega Marine Laboratory, California and Friday Harbor Laboratories, Washington for different degrees of air-exposure and temperatures in ochre stars Pisaster ochraceus.  The researchers use oxygen consumption (VO2) as a measure of metabolic cost, recording rates in the laboratory over a range of ecologically relevant temperatures in both water and air.  These data are then integrated with field temperatures of individuals to obtain estimates of metabolic rates over 10d periods at different vertical locations (low, mid, and high intertidal).  Results indicate that different durations of emersion at different vertical heights on the shore have a much smaller metabolic cost than year-to-year temperature effects.  Intertidal-height temperature effects are made almost negligible by the fact that VO2 in air is so much lower than that in water (see graph).  In comparison, inter-year temperature differences have a much greater relative effect, halving or doubling metabolic costs.  Fly et al. 2012 J Exp Mar Biol Ecol 422/423: 20.

NOTE  temperatures used are ones normally encountered by sea stars during summer in the Bodega Bay area, over 3 different years, namely 10, 15, 20, and 25°C

NOTE  temperatures are not recorded directly from individual sea stars; rather, from biomimetic models constructed to mimic thermal properties of the real thing

Research study 8

graph showing temperature discontinuities in a sea star Pisaster ochraceus under experimental conditions and their effects on arm lossgraph showing water loss in relation to temperature and its effect on arm lossIn follow-up research at Bodega Marine Laboratory the same group of researchers as noted in Research Study 7 above investigate thermoregulation and temperature tolerances in ochre stars Pisaster ochraceus.  By use of heat lamps in the laboratory the researchers are able to mimic sunny days in the field, and find that most sea stars die when their core temperatures exceed 35°C (see graph on Left). In these experiments the sea stars that survive are found to have arms that are hotter than their cores, apparently because they move cooler coelomic fluid into their central areas to cool down.  The authors speculate that in creating heat sinks in the arms, thermoregulation is aided by the greater relative surface area of the arms conducting heat away more effectively.  However, when the cores reach sublethal temperatures of 31-35°C, the arms are at 33-39°C, and the sea stars begin to shed them, consistently losing the hottest ones first.  Although lost arms can be readily replaced, the process is nonetheless energetically costly and may incur other risk/cost tradeoffs.  Water loss from tissues is also a factor, with losses of up to 15-25% being recorded in individuals with core temperatures of 31-35°C (see graph above Right). Note that at temperatures above this range, greater water loss may be a critical factor leading to arm loss and death. The study is of value in demonstrating that parts of the body of a sun-exposed invertebrate, especially those peripheral to the core, may be subject to thermal stresses quite different from other parts, with different physiological responses (see illustration bottom Right).  Such matters will be important in future considerations of effects of climate change.  The study is the first to show a direct relationship in an echinoderm of temperature of a body region and arm loss Pincebourde et al. 2013 J Exp Biol 216: 2183.  Infrared photograph courtesy Allison Matzelle & Mackenzie Zippay.

photo composite showing body-temerature discontinuity of a sea star Pisaster ochraceus in the fieldNOTE  known as heterothermy

NOTE  the authors comment that while autotomy of arms is usually instantaneous, arm loss as a result of temperature stress usually occurs
2 or more days later

Infrared photograph shows thermal heterogeneity
in Pisaster ochraceus in the field during low tide


Research study 9

graph showing effect of air temperature on body temperatures of ochre stars Pisaster ochraceus graph showing effect of air temperature on oxygen consumption of ochre stars Pisaster ochraceusGiven that ochre stars Pisaster ochraceus live intertidally where air emersion is a daily event, we might expect a high level of physiological compensation for temperature extremes, but to what extent is this true? This is investigated by researchers at the Bamfield Marine Sciences Centre, British Columbia who monitor effects of 6h air-emersion on oxygen uptake1, CO2 production, and potential acidosis resulting from elevated CO2 levels. The experiment graph showing effect of air temperature on carbon-dioxide production of ochre stars Pisaster ochraceusinvolves maintaining sea stars in late-summer temperature water (13°C), then subjecting them to 6h2 air-exposure at 3 temperatures (5, 15, and 25°C, at 58%R.H.) before returning them to their original holding conditions. Measurements of body temperatures before, during, and after the experimental treatment show equilibration with air temperature at 5°C, but several degrees less in the 15°C and 25°C treatments, the last two possible results of evaporative cooling (see graph on Left). Oxygen consumption increases somewhat during the 25°C air-emersion and is depressed in the 5°C treatment (see graph upper Right), but this pattern is not matched for CO2 production, which generally increases when in air in all treatments (see lower graph on Right). Oxygen uptake rates continue to be higher than expected following air exposure. That this is simply repayment of an oxygen debt is discounted by the researchers on the basis of absence of evidence of anaerobic metabolism occurring durine the air-exposure period and, instead, provide possible alternate explanations. The authors conclude from their overall results that Pisaster ochraceus is “ideally suited3 for life in the intertidal zone”. McGaw et al. 2015 Exp Mar Biol Ecol 468: 83.

NOTE1 also examined are effects on ammonia production, calcium-ion metabolism, pH, and other metabolic indicators, but not considered here

NOTE2 6h represents average air-emersion time for ochre stars during average summer low-tide conditions in Barkley Sound, B.C.

NOTE3 temperatures used in the experiment are within the extremes that occur naturally in the Barkley Sound area, so the physiological processes reported on would fall within the species’ tolerance levels and, well, they live there…so why wouldn’t we expect them to be “ideally suited”?
This is akin to comments sometimes seen in the literature that the intertidal zone is a harsh, demanding
area in which to live. No it isn’t, not for the plants and animals that live there…they love it!

Research study 10

Shore-level size gradients of distribution are not uncommonly found for intertidal species, sometimes, as with limpets, with larger individuals living higher up the shore; other times, as with ochre stars Pisaster ochraceus, with smaller individuals living higher. Why1 this should be with Pisaster is investigated at sites in Strawberry Hill, Oregon and Bodega Marine Reserve, California. The researchers first test the effect of size on thermal and desiccation tolerances2 in the laboratory, then relate these data to habitat temperatures using biomimetic sensors3 set out in a variety of protected and exposed shore habitats. Results show that the proportion of individuals inhabiting shaded microhabitats, as would be photograph of ochre stars Pisaster ochraceus clustered in gulley at low tideexpected, increases with degree of sun exposure and with air temperature (seawater temperature, wind speed, and wave height have nonsignificant effects). Moreover, the researchers find that only in sheltered microhabitats does a size gradient exist, with smaller individuals living higher on the shore, a pattern not present in sun-exposed microhabitats. The explanation ultimately posited by the authors is that small-sized individuals can occupy small crevice refuges high on the shore, but big ones cannot. This behaviour not only potentially provides insulation from drying and thermal stresses, but also possibly from predation by sea gulls. Monaco et al. 2015 Mar Ecol Prog Ser 539: 191; for more on effects of environmental stresses on behaviour of ochre stars and how these influence their interactions with prey sea mussels Mytilus californianus see Monaco et al. 2016 Ecol Monogr 86 (4): 429.

NOTE1 the authors initially purport to test an hypothesis that smaller individuals can withstand higher temperatures than larger ones owing to their relatively larger respiratory surface-area-to-body-volume ratios, but they ultimately never measure or calculate these values. In any case, ochre stars mostly withdraw their respiratory dermal papulae and tube feet when conditions get hot and/or dry. The authors also consider the more usual hypothesis that small intertidal individuals would not be found higher than large because of their relatively greater surface-area-to-body-volume ratios leading to greater desiccation but, again, no data are presented to test this. Later, perhaps to cover even more possibilities, the authors hypothesise that greater energy expenditure in coping with thermal stress will favour a behavioral strategy of seeking out protective microhabitats (again, requisite data not presented). Ultimately, they drop all these possibilities and go for the idea that little sea stars can fit into higher-level crevices that are too small to accommodate big sea stars

NOTE2 lethal-temperatures (LT50) recorded in these lab experiments are, unexpectedly as noted by the authors, lower for small individuals (about 32°C, 25-75g live mass) than for large individuals (33°C, 250-400g)

NOTE3 these are temperature-sensors embedded in hydrophilic polyurethane-ester foam (Aquazone sponge) or “robo sea stars” whose thermal properties mimic closely those of live ochre stars

As noted by the authors, ochre stars Pisaster ochraceus
are mostly found clustered in crevices, in tidepools, under
seaweeds, or other similar habitats during low-tide periods