Only 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
for protection from avian predators

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 slightly 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 22^oC, 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

In 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. 

The graph on the lower Left shows good correlation (to within about 1^oC) 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 a 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.  This means that the previous day’s low-tide body temperatures may be a good predictor of where in the intertidal zone the sea stars may 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

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 Labortories, 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 exposure.  Stickle 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

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 by 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

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

When 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 branchiae
not yet retracted into the coelomic cavity 1X

For 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 40-d 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.

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