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  Feeding, growth, & regeneration
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Environmental effects on feeding

  Environmental effects on feeding are considered here, while LARVAL FEEDING, ADULT FEEDING, PREY RESOURCES, INGESTIVE CONDITIONING & OTHER FOOD-BASED LEARNING, and GROWTH & REGENERATION are considered in other sections.
Research study 1

graph showing seasonal variation in feeding of ochre stars Pisaster ochraceusThe 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 photograph of an ochre star Pisaster ochraceus with recently consumed musseldiet, 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

Research study 2

photograph of ochre star Pisaster ochraceus eating musselsThe 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

Research study 3

photograph of a pulmonate Onchidella borealis crawling on sea lettuce Ulvaphotograph of pulmonate Onchidella borealis showing locations of repugnatorial glands along the body margin courtesy Young et al. 1986 Biol Bull 171: 391The 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

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

photograph of predatory sea star responding to repugnatorial secretions from an intended prey the pulmonate Onchidella borealis (first of a series of 4)
Leptasterias touches Onchidella with its tube feet
photograph of predatory sea star responding to repugnatorial secretions from an intended prey the pulmonate Onchidella borealis (second of a series of 4)
Repugnatorial glands discharge and the sea star responds by curling its arm away
photograph of predatory sea star responding to repugnatorial secretions from an intended prey the pulmonate Onchidella borealis (third of a series of 4)
Onchidella reverses its direction of crawling while Leptasterias remains still
photograph of predatory sea star responding to repugnatorial secretions from an intended prey the pulmonate Onchidella borealis (fourth of a series of 4)
Onchidella crawls away while Leptasterias remains stationary
Research study 4

graph showing relationship between mussel recruitment and density of ochre stars Pisaster ochraceusphotograph of ochre stars gathered in a mixed assemblage of mussels & barnaclesMost 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

Research study 5

graph showing effect of temperature on various aspects of feeding, growth, and energetics in ochre stars Pisaster ochraceusSea stars Pisaster ochraceus along the coasts of Oregon and California are seasonally exposed to temperature drops of 3-5oC 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 9oC, constant 12oC, and alternating every 2wk from 12 to 9oC (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 12oC treatment, and are not significantly different from the 9oC 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

Research study 6

graph showing effect of body temperatures on feeding in ochre stars Pisaster ochraceusgraph showing effect of body temperatures on growth in ochre stars Pisaster ochraceusAlthough 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 27oC), 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 23oC 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 1oC 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 35oC

Research study 7

illustration showing alternating temperature effects during immersion/emersion on feeding in ochre stars Pisaster ochraceusphotograph of ochre star Pisaster ochraceus feeding on a mussel during low tideIn 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 (10-12oC).  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 as ambitious, and 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 19h immersion cycling over a period of 20d.  Heat lamps are used to regulate air temperature 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

Research study 8

graph showing model predictions for growth of ochre stars Pisaster ochraceus compared with actual experimental dataHow handy would it be to have a mathematical model of an organism so well constructed that it replicated every aspect of energy intake and allocations so perfectly as to make the living entity redundant? Well, this may come some day, but for now we have a well-conceived and executed mathematical construct formulated by researchers at the Universities of South Carolina and Northeastern University for ochre stars Pisaster ochraceus that sets a high benchmark for future modelling efforts for west-coast invertebrates. It is based on a Dynamic Energy Budget Model framework that employs all available data for physiological processes relating to feeding, growth, reproduction, and development, and it applicable to any taxon. In the case of Pisaster, where published information is lacking for essential entries the authors conduct experiments to fill in the blanks. Although the Dynamic Model employs the same energy-allocation parameters of ingestion, absorption, assimilation, respiration, growth, reproduction, and so on, as other traditional models, it differs in being time-based, rather than static. The model’s basic premise that assimilated energy is first stored as reserves, in the case of Pisaster in the pyloric caeca, then allocated to physiological processes. It can also treat the life cycle of the modeled organism as a whole, without the partitioning through various stages of ontogeny as seen in traditional models. In the dynamic model, larval, juvenile, and adult energetics are may be considered separately or as a continuum. Checks on predictive accuracy of a Dynamic Model are done routinely to identify areas needful of further work, as well as new research avenues to explore. Examples of the model’s predictive accuracy provided by the authors include growth of different life stages, where larval growth differs from real data by only 12%, and early post-metamorphic growth by only 9% (see graph for the latter). However, predictions for adult growth are much less precise (25% error), owing mainly to variability in spawning times and durations, and to variable levels of catabolism of body tissues during times of food shortage. The authors maintain that dynamic models of this sort allow increasingly reliable predictions of physiological responses to changing environmental conditions to be made, enabling better understanding of a species population dynamics. This may be critical for ochre stars and other asteroids facing future climate change combined with wasting disease that has so decimated west-coast asteroid genera. Monaco et al. 2014 PLoS ONE 9 (8): e104658. doi:10.1371.

NOTE this modeling format has been around for several decades, but this appears to be a first application for a west-coast invertebrate species