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  Life in the intertidal zone
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  Other physiological stresses

The topic of life in the intertidal zone includes a section on other physiological stresses considered here, and sections on TEMPERATURE & DESICCATION, SALINITY, COLOUR MORPHS OF PISASTER, and SYMBIONTS presented elsewhere.

Research study 1

It is generally believed that echinoderms, along with most other marine invertebrates, excrete ammonia as their primary by-product of protein catabolism.  However, a survey of sea stars in San Juan Island, Washington shows that 90% excrete significant quantities of urea (>10%) and, in some species, e.g., 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 the subject needs to be further investigated, but remarks that simple “eyeballing” of the data reveals no obvious correlation with extent of intertidal exposure.  Stickle 1988 Comp Biochem Physiol 91A: 317.

NOTE  several other urea-excreting, or 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

Research study 2

histogram comparing preferences of sea stars Pisaster ochraceus for various light conditionsDoes exposure to UV radiation play a role in selection by sea stars Pisaster ochraceus of “safer” microhabitats as the tide is dropping?  This is tested at Friday Harbor Laboratories by containing sea stars in shallow seawater in 5 plastic aquarium tanks outdoors and exposing them to ambient summer sunlight (see diagram).  Half of each of 4 tanks is covered with a light-filtering polyester or plastic material to create 4 choice treatments: no UVR, no UVB, shade, and all components of light (UVA, UVB, and PAR).  In each case half of the tank is left open to ambient sunlight.  A fifth tank is left uncovered as a no-filter control treatment.  Tests are run over 4h from 1000h, and by far the majority (93%) of individuals have made their choice of resting spot within 20min.  Results show that while P. ochraceus does not avoid UVB or UVR, it does avoid PAR.  This is shown by a significant preference for shade, but no preference for any of the other treatments (see histogram).  Field surveys show that of 283 individuals emersed during low tide, 85% are in complete shade, either cast by rocks, or crevices, or from canopies of sea-cabbage kelp Hedophyllum sessile.  The authors conclude that is exposure to PAR that leads Pisaster to seek out shaded microhabitats at low tide.  They further suggest that P. ochraceus may be unable to detect UVR wavelengths.  Burnaford & Vasquez 2008 Mar Ecol Progr Ser 368: 177.

diagram of aquariums used in UV radiation study on sea starsNOTE  this is Photosynthetically Active Radiation (400-700nm).  UVR or UltraViolet Radiation includes UVB radiation from 290-315nm and UVA radiation from 315-400nm.  Temperatures are lower under the filters, but only by <1oC

NOTE  the researchers do 2 major experiments, differing only in past feeding history of the test sea stars.  The results are basically identical and only one data set is included here

Test aquariums are 60cm in length. Sea stars to be tested are placed on the upper
platform. This is to reduce the effect of light attenuation through the walls
of the aquarium were the sea stars to be placed at the bottom of the tank

Research study 3

graph showing growth of juvenile sea stars Pisaster ochraceus at different temperatures and carbon-dioxide concentrationsgraph showing feeding rates of juvenile sea stars Pisaster ochraceus at different temperatures and carbon-dioxide concentrationsEcological effects of climate change are increasingly becoming evident, but what about physiological effects related to increasing ocean temperature and acidification?  This is investigated by researchers at the University of British Columbia, Vancouver using growth in sea stars Pisaster ochraceus feeding on mussels as an indicator of physiological response to different combinations of  temperature and carbon-dioxide concentration.  Interestingly, and contrary to results from similar types of studies on other marine invertebrates, not only does feeding and growth increase with increasing temperature, as expected, but both parameters respond positively to higher carbon-dioxide concentration (see graphs).  Content of calcified ossicles actually decreases over the course of the study, but still is accompanied by soft-tissue growth.  The authors think the difference in response to carbon-dioxide concentration between their study and other similar studies on echinoderms and molluscs may relate to a lesser relative dependence upon a calcified endoskeleton in P. ochraceus as compared with, say, in heavily calcified ophiuroids and echinoids.  The study represents a valuable addition to the growing scientific literature on effects of climate change in the ocean environment.  Faster future growth of this keystone predator could have significant consequences on the dynamics of intertidal-mussel communities. Gooding et al. 2009 Proc Nat Acad Sci 106: 9316.

NOTE  a value of 380ppm is used to represent current carbon-dioxide levels in the ocean, and this is approximately doubled to 780ppm in the growth experiments to represent a climatically realistic value based on predictions for the year 2100. The study suffers from a fault common in other such investigations on predicted future climatic events; namely, the tests are done at acute levels, whereas Pisaster in the future would have many generations of past experience with gradually increasing carbon dioxide levels, and consequently there may be an acclimatisation factor not being accounted for 

NOTE  the authors cite a value of 11% for mass of calcareous ossicles relative to total live mass in P. ochraceus, which corresponds to a dry-mass value of about 60-65%.  On the basis of this, one might argue that the species is not “lightly” calcified in comparison with other invertebrates.  The authors’ contention, however, is that unlike in sea urchins, mollusks, and brittle stars that have containment-type skeletons, the comparatively loosely arranged system of endoskeletal ossicles in Pisaster and other asteroids possibly would not be so physically restrictive on soft-tissue growth

Research study 4

histogram showing survival of brachiolaria larvae of ochre stars Pisaster ochraceus to seawaters of differeing oxygen contentsphotograph of a brachiolaria larvaLater research on ochre stars Pisaster ochraceus by a group of Oregon university and fisheries scientists at the Hatfield Marine Science Center, Newport also addresses an issue related to climate change, in this case tolerance of coastal invertebrates to hypoxia (low levels of dissoloved oxygen). Incidences of hypoxia are increasing in coastal areas worldwide, notably in areas of strong and persistent upwelling, and predictions for climate-change effects suggest that the frequency of these incidences will increase.  Hypoxic conditions can appear as “pockets” that may be quite transient, as large deep-water “dead zones” that may be quite stable, or as a result of upwelling where deeper hypoxic waters are transported to the surface.  Causes are often unknown, but in coastal areas may be associated with agricultural and industrial input of nutrients that cause local increases in oxygen demand.  Hypoxic events can cause large-scale death of fishes and invertebrates. Of special interest to the present researchers is whether there is differential tolerance of west-coast intertidal invertebrate species to hypoxic conditions.  Only larvae are used in the study, as this life stage is considered to be the most sensitive to environmental stresses.  Larvae are reared from eggs and, at various stages of development  (brachiolaria in the case of these sea stars), are exposed to seawater treatments with different oxygen contents, including control, upwelling, hypoxic, and anoxic for periods of 12h-6d.  Surprisingly, results for all species tested are variable and unpredictable.  As an example, note that 100% of brachiolaria larvae of P. ochraceus survive for up to 6d in all 4 treatments (see histogram). Other results range from low (zoeae of crabs) to high tolerance (sea star, sea urchin, acorn barnacle), but with most larvae being in the high-tolerance category.  The authors conclude that, contrary to expectation, open-coast invertebrate larvae are highly tolerant of hypoxic and near-anoxic conditions over periods of up to 6d exposure.  Eerkes-Medrano et al. 2013 Mar Ecol Prog Ser 478: 139.

NOTE  the researchers test larvae of 10 common species included in which are sea anemones, barnacles, crabs, mussels, nudibranchs, sea stars, and sea urchins.  Only tests on brachiolaria of P. ochraceus are presented here, just as an example

NOTE  normal seawater is bubbled with nitrogen gas to produce desired levels of oxygen content.  Average oxygen contents of each of the 4 treatments are control (6.0ml . l-1), upwelled (2.7ml . l-1), hypoxic (1ml . l-1), and anoxic (<0.5ml . l-1).  The “upwelled” value represents the dissolved-oxygen level recorded in waters upwelled into the inner Oregon-coast shelf during summer.  Nitrogen-bubbling has an added effect of increasing pH (from a control level of 8.1 to 8.3 in the “anoxia” treatment); however, an extra set of experiments to test this indicates no significant effect on larval survivorship