The ochre star Pisaster ochraceus is commonly found higher in the intertidal zone than other species and therefore is subject to more extremes of weather conditions than other species.  Studies at Friday Harbor Laboratories, Washington reveal a seasonality of feeding in ochre stars, with 60-80% of the population feeding during July-August, and only 5% feeding during January-February.  Barnacles, limpets, and mussels predominate in the summer diet, and chitons in the winter diet.  The author suggests that, because the gonads grow during winter with spawning in the late spring, summer is a time of stockpiling of nutrients and energy, with conversion into gametes during winter.  During the period of late autumn/winter to spring, the sizes of pyloric caeca and gonads are inversely related.  It is not known whether colder weather in winter is a factor involved in the feeding cycle.  Mauzey 1966 Biol Bull 131: 127.

Summer-time ochre star Pisaster ochraceus with a
mostly consumed mussel Mytilus californianus 1.5X

The intertidal sea stars Pisaster ochraceus and Leptasterias hexactis exhibit reduced or no feeding in winter. Although it is not known why Pisaster does not feed in winter, the behaviour in Leptasterias may owe to several factors: 1) females are breeding at this time, so about half the population is incapable of feeding, 2) food is less available in winter, 3) winter storms disrupt feeding, and 4) low temperatures depress metabolism. Menge 1972 Ecol Monogr 42: 25.

A summer-feeding ochre star Pisaster ochraceus
is involved with several mussels, including one largeish
Mytilus californianus whose shell valves are being
pulled opened by the oral tube feet 0.6X

The sea star Leptasterias hexactis and the pulmonate Onchidella (Onchidoris) borealis occupy similar shore regions, but are rarely found together in the same microhabitats.  Scientists studying interactions of the two species in San Juan Islands, Washington and Barkley Sound, British Columbia suggest that repugnatorial secretions from Onchidella may repel the sea stars.  The secretions are released from about 20 small papillae and associated repugnatorial glands located peripherally on the upper body surface (see photo on Right). Physical contact seems to be necessary for the secretion to be expelled, and each gland can operate independently and discharge repetitively.  The secretion is clearly visible as a milky, translucent, viscous substance extending several millimeters from the animal.  The secretions have the strongest and most consistent effect on intertidal sea stars, including Pisaster ochraceus, L. hexactis, and H. leviuscula.  On contact with the secretion Leptasterias curls its arms upwards while Onchidoris makes its escape (see photo series below).  Other potential predators such as whelks, polyclad flatworms, nemerteans, and certain fishes are not deterred by the secretions.  Crabs such as Hemigrapsus nudus will consume dead Onchidella but appear to be deterred by the secretions of living individuals.  Young et al. 1986 Biol Bull 171: 391. Photograph of living Onchidella borealis courtesy Dave Cowles, Walla Walla University, Washington rosario.wallawalla.edu.

NOTE  this species is mainly a suspension-feeder and it not known to be a predator of snails

Most vertebrate predators such as birds, fishes, and mammals respond quickly to localised recruitment of their prey.  But what about sea stars Pisaster ochraceus?  Do these slow-moving invertebrate predators possess the behavioural responses that would enable them to exploit episodes of massive prey recruitment?  A 7-yr study at Bamfield, British Columbia tells us that the answer is yes, that sea stars aggregate relatively quickly in spots with large recruitment of mussels, and disperse as local abundances of their prey decline (see graph on Right).  Note that when recruitment of mussels Mytilus californianus is low, Pisaster density is low, and when recruitment is high, Pisaster density is high.

Additionally, in areas of the shore where numbers of juvenile mussels are experimentally augmented, sea stars aggregate in significantly higher numbers than they do just above, or just below, the area by a factor of about 10. At the same time, densities of Pisaster in nearby control areas do not change significantly.  The authors note that the sea stars are surprisingly motile when the tide is high and that aggregations form in a matter of days in response to addition of juvenile mussels to a test site. Robles et al. 1995 Ecology 76: 565.

NOTE recruitment index in the graph is based on a percentage of quadrats set out in mussel beds (1.8-2.4m tide level) containing modal values of mussel sizes <4cm in length.  Mussels of this size are in their first year of life.  Similar significant results are shown for Mytilus trossulus, as well, but are not presented here

Sea stars Pisaster ochraceus along the coasts of Oregon and California are seasonally exposed to temperature drops of 3-5^oC during periodic upwelling events lasting several days to a few weeks.  Little is known of the effects of these cold-water events on feeding, growth, and energetics of sea stars and other invertebrates.  An experiment at the Hatfield Marine Science Centre, Oregon in which P. ochraceus are maintained for 18wk¹ under temperature treatments of constant 9^oC, constant 12^oC, and alternating every 2wk from 12 to 9^oC (i.e., to mimic upwelling effects) produces some interesting and somewhat unexpected results.  During the first three 3 intervals, consumption of mussels Mytilus trossulus is higher in the warm-water treatments but, later, during the last 2 biweekly intervals, total mussel consumption is not significantly different in the 3 treatments (see graph). Interestingly, however, overall growth, conversion efficiency, and energy stored in the pyloric ceca² are significantly higher in the variable temperature (=upwelling) treatment than in the 12^oC treatment, and are not significantly different from the 9^oC treatment. Thus, there appears to be an energetic advantage to living in a temperature regime that is characteristic of intermittent upwelling.  The author suggests that sea stars and other benthic consumers³ will feed more intensely during warm-water periods, yet possibly benefit from reduced metabolic costs during cold-water periods.  The implication from this study is that P. ochraceus in upwelling conditions will have greater reproductive outputs than ones living in constant warm conditions. Sanford 2002 J Exp Mar Biol Ecol 273: 199; for review see Sanford 2002 Integr Comp Biol 42: 881.

NOTE¹ the study commences in early June, following spawning, and runs for the period of most active feeding and energy storage.  The 18-wk period also corresponds with the major period of upwelling on the Oregon coast

NOTE²  the ceca store nutrient reserves in the form of lipids, proteins, and glycogen that are used in growth and reproduction.  Generally, there is an inverse relationship between size of the pyloric ceca and size of gonads in sea stars. 

NOTE³  similar data are obtained for the whelk Nucella canaliculata in the same study, but not shown here

Although exposure to high or low temperatures during low-tide periods is thought to be a major factor in limiting the vertical distribution of ochre stars Pisaster ochraceus, experimental evidence for the mechanism by which this may work is lacking.  What effects, for example, do high body temperatures during air-exposure of this keystone predator have on feeding rates on mussels Mytilus californianus during subsequent high-tide periods. And what effects might high body temperatures have on later growth?  These questions are investigated in a combined field and laboratory study at Bodega Marine Laboratory, California where substratum temperatures of sea stars at various intertidal heights are monitored over the course of several days in Jun-Jul, and body temperatures of laboratory animals are measured over simulated tidal cycles over periods of 8d exposed to heat lamps. 

Results show that field animals in summer may typically experience relatively high body temperatures on successive days (reaching about 27^oC), but that before the situation becomes critical they move to lower intertidal positions or into crevices.  Laboratory trials indicate that a single exposure to environmentally realistic aerial body temperatures of about 23^oC significantly increases feeding, but has no effect on growth, while chronic exposure over 8 successive days, in the example shown here, to the same temperature significantly reduces both feeding (see graph on Left) and growth (see graph on Right). Thus, although temperature is thought to be important in limiting upper limits of distribution of many intertidal organisms, the results of this study suggest, at least for P. ochraceus in Bodega Bay, that intolerable temperature conditions may not set upper limits to foraging, since field body temperatures never approach the species’ lethal threshold.  Pincebourde et al. 2008 Limnol Oceanogr 53: 1562.

NOTE  the researchers use a “biomimetic” data-logger, consisting of a disc of plastic foam containing a thermistor, for estimating body temperatures of the sea stars in the field.  These mimics provide estimates with an average absolute error of about 1^oC from true body temperatures (checked by direct measurements in selected field specimens located near to the mimics)

NOTE  determined by the authors in laboratory tests to be about 35^oC

In a later study at the Bodega Marine Laboratory, California, aimed specifically at identifying physiological effects of climate-induced temperature stress, the same research group investigates effects of feeding in ochre stars Pisaster ochraceus on sea mussels Mytilus californianus under conditions of varying levels of difference between emersed and immersed temperatures, both constant and fluctuating.  During low tide, an organism’s body temperature can fluctuate widely over relatively short time-intervals (minutes), but during high tide and owing to the high heat conductivity of seawater, body temperature equilibrates quickly and fluctuations are dampened essentially to zero.  Although one intuitively thinks that the stresses in each medium would have similar physiological effects, this may not be the case.  Also, and this is what the researchers test here, what happens when the 2 conditions change from constant to fluctuating, or when one condition fluctuates and the other is constant, or when the 2 conditions fluctuate – both in phase and out of phase?  The researchers first measure body temperatures of sea stars in the field to set realistic limits on non-lethal levels to be used in the laboratory experiments.  The overall laboratory results even under constant conditions are complex, as shown in the accompanying figure.  Higher rates of feeding are shown in purple and lower rates in red.  Lowest rates overall are associated with higher emersion temperatures, but ameliorated to some degree by cold immersion temperatures (10-12^oC).  In contrast, and as expected, highest feeding rates are associated with lower emersion temperatures alternating with intermediate immersion temperatures (see purple in graph).  Other results are more complex, and the reader will need to study them directly.  The approach is innovative and novel, and the researchers should be complimented on tackling a project so ambitious, but so potentially valuable, as this one.  Pincebourde et al. 2012 Ecology Letters 15: 680.

NOTE  the “sea stars” are actually solid foam data-loggers known from previous work (see Research study X above)  to mimic accurately the thermal properties of P. ochraceus

NOTE  results are averaged for treatments of 6h emersion and 18h 50min immersion cycling over a period of 20d.  Heat lamps are used to regulate air temperatre during emersion.  The sea stars mainly feed when immersed, so the main influence is likely to be the emersion temperature immediately preceding each immersion treatment