title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Physiology & physiological ecology
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  Physiological challenges to life of an acorn barnacle intensify in direct relation to height in the intertidal zone.

Life in the high intertidal region presents numerous physiological challenges to an acorn barnacle.  Chief among these are temperature, desiccation, insolation, salinity, and requirements for feeding, reproduction, and gas exchange.

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Research study 1

The high intertidal position of barnacles means that they may be subjected to extremes of salinity conditions, on the low end from rainfall and, on the high end, from evaporation of mantle-cavity fluid.  Studies on other non-west-coast species show that in salinities less than 25‰ the opercular plates are shut tight and, in times of high evaporative loss, that an individual may enter a state of “salt sleep”.  Here, the opercular plates are closed, cirral photograph of barnacles Balanus glandula at the extreme top of the shoreactivity ceases, and respiration is maintained at a low level.  Studies on salinity tolerance of Balanus glandula at the Hopkins Marine Station, Pacific Grove, California reveal that adults are able to survive immersion in test salinities of up to 300% seawater for periods of up to 72h.  Embryos, in contrast, are killed in salinities of less than 25% and and in salinities greater than 250%.  Immersion of adults in a range of test salinities from 10-200% shows that individuals remain closed in salinities of 50% and less, in which case the salinity of their mantle-cavity water remains high.  However, in test salinities of 125-200%, where the barnacles actively feed, the mantle-cavity salinity is the same as that of the test salinity.  The author notes that the opercular-closing behaviour of adults in low salinities would likely protect the embryos, but this would not occur in high salinities.  Bergen 1968 Crustaceana 15: 229.

NOTE  it is not clear from the author’s results how old the embryos are, other than “early stage”, nor how long they survived in the test salinities, perhaps as little as 1h. 100% seawater is not defined by the author, but is probably about 32‰

NOTE  assessed by drilling a small holes in the shell plates of several individuals, removing some of the fluid with a small syringe, pooling the samples, and measuring salinity with a refractometer

Barnacles Balanus glandula (with some Chthamalus sp.
intermixed) at the extreme vertical limits of distribution 0.5X

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Research study 2

photographs of tergal plates from barnacles Amphibalanus amphitrite grown in different locations, including the Salton SeaAn interesting paper to consider in this section on physiological ecology is one dealing with the introduction of the low-intertidal/subtidal-inhabiting barnacle1 Amphibalanus (Balanus) amphitrite from southern California into the landlocked Salton Sea in the early 1940s, and its subsequent change in adult and larval morphology.  Laboratory and field studies at the Marine Science Institute, UC Santa Barbara involve larval rearing and some translocation2 experiments. The major morphological difference in the adults of the 2 populations is in tergal morphology (see drawing on Left). While the plates from adults in the 2 locations are significantly different, when grown under similar conditions (both in a semi-isolated lagoon and in the laboratory) they do not differ significantly (see photographs on Right). This suggests that the morphological divergence in adult form owes to environmentally induced phenotypic plasticity and not genetic divergence.  Development3 of A. amphitrite in the Salton Sea seems to be typical of marine barnacle species, with basically 6 feeding naupliar stages and drawing of tergal plate of barnacle Amphibalanus amphitrite for study of salinity effects on growth in the Salton Sea, Californiaone non-feeding cyprid stage.  Major differences in larvae include; 1) coloration of cyprids (unpigmented and white in Salton Sea vs. greenish-brown in Mission Bay), 2) cyprid size (larger in Salton Sea = 475µm, Mission Bay = 435µm), and 3) naupliar duration (longer time to cyprid stage in Salton Sea).  The author hypothesises that the first difference, loss of pigmentation, may owe to typically turbid water conditions in the Salton Sea resulting in less potential damage by UV radiation and consequent selection for lack of pigmentation.  The second difference, cyprid size, likely relates to the longer time spent as nauplii by the Salton Sea population, but the reason for this is unclear.  On the basis of these results, the author concludes that the differences in larval morphology are best explained by evolutionary selection.  Raimondi 1992 Biol Bull 182: 210.

NOTE1  this barnacle species apparently originated in the southern hemisphere and is common on California shores from San Francisco Bay south.  It is thought to have been introduced into the Salton Sea from the San Diego area as adults on mooring buoys or ropes, or as larvae in bilge water of naval flying boats conducting naval exercises.  The Salton Sea has a comparatively recent history, being formed in the early 1900s by disruption of  an irrigation channel, and maintained since by irrigation water.  Its salinity is presently about 43‰, or about 8‰ above that of oceanic water

NOTE2  interestingly, the most obvious experiment, namely, reciprocal translocations of Mission-Bay and Salton-Sea individuals is not done in the study, for legal and ethical reasons.  Instead, the author rears newly settled spat of both populations in a semi-isolated lagoon near the marine laboratory of UC Santa Barbara

NOTE3  the species is apparently world-wide, including the Atlantic coast of North America.  As part of the study the author includes a population of A. amphitrite from Beaufort, North Carolina, mainly to be assured that the Mission Bay larvae are typical of the species. They are

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Research study 3

map showing study sites in Oregon for barnacle Balanus glandula researchgraph comparing survival of barnacles Balanus glandula at 3 sites in the  South Slough EstuaryThe intertidal barnacle Balanus glandula is common in open coast areas of the west coast, but also frequents estuarine conditions.  The question arises as to what effect lower salinity may have on their fitness, measured as growth and survival.  This is assessed by researchers at the Oregon Institute of Marine Biology, Charleston by settling spat onto slate panels, then “outplanting” these panels at 3 locations in South Slough Estuary, an offshoot from graph comparing survival of barnacles Balanus glandula at 3 sites in the South Slough EstuaryCoos Bay characterised by a marked salinity gradient from riverine to ocean conditions (see map). Growth, measured as increase in basal-plate area, and survival are monitored at regular intervals over a 20d period.  Results show, somewhat surprisingly, that juveniles perform better at the mid-estuarine location than at the mouth of the estuary in oceanic conditions (see graph and histogram on Right).

Performance is least good at the riverine end of the estuary, manifested in low growth rate and high mortality.  Both measures of fitness decrease with increasing intertidal height at all 3 sites (see histogram lower Right).  The authors note that their study is the first to examine life-history traits of a single species across an estuarine gradient.  Berger et al. 2006 J Exp Mar Biol Ecol 336: 74.

NOTE  6 replicate panels (6 x 6cm), each bearing newly settled barnacles, are set out at each of the 3 sites at 3 different tidal heights (1.3, 1.4, and 1.5m above MLLW).  The plates have 96 shallow pits to encourage settlement and, at the time of deployment of the plates, the spat are thinned so that each plate has 32 juveniles.  Four such outplantings of panels are done during spring/summer, but only results from the one done in May 2003 are presented here as representative of the data

NOTE  only growth, however, shows a significant effect of intertidal height in the particular set of data presented here, and then only at the oceanic site

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Research study 4

histogram comparing levels of heat-shock proteins in barnacles Balanus glandula after exposure to temperatures of 14 and 34 degrees CelsiusBarnacles Balanus glandula should be an ideal candidate species for studies on effects of thermal stress on heat-shock proteins.  It is not unusual on a hot summer’s day for tissue temperatures in barnacles of several west-coast species to reach 30-34oC.  Researchers at the Oregon Institute of Marine Biology, Charleston test the effects of thermal stresses on B. glandula by exposing them to temperatures ranging from 11-34oC for periods of up to 8.5h.  Some acclimation experiments are also done, as well as some observations in the field, and some measurements on moulting effects (data from these experiments show mostly non-significant effects).  Results from the main experiment show non-significant changes in Hsp70 levels in individuals exposed to 20 and 28oC.  Only at 34oC are levels significantly higher than in control animals at 14oC (see histogram). Although anticipated by the researchers, accompanying levels of conjugated ubiquitin fail to disclose irreversible protein damage at temperatures up to 34oC.  Interestingly, acclimation to temperatures ranging from 10-22oC fails to shift the induction temperature for heat-shock proteins upwards, as would be expected.  The authors conclude that B. glandula appears to be well adapted to thermal stresses of high intertidal life, but that an ability to modify tolerance levels through acclimation is lacking.  Berger & Emlet 2007 Biol Bull 212: 232.

NOTE  heat-shock proteins are a type of molecular chaperone that bind to and help repair damaged proteins.  Their action reduces the need for new protein synthesis and thus reduces the metabolic cost of thermal and other stresses.  The designate “70” refers to the molecular mass, here including proteins in the 70-80kDa range.   Other studies on heat-shock proteins can be found at LEARN ABOUT MUSSELS: LIFE IN THE INTERTIDAL ZONE: HEAT-SHOCK PROTEINS, and in several other locations in the ODYSSEY

NOTE  ubiquitin is another protein that identifies and binds to irreversibly damaged proteins.  The amount present in conjugated form, that is, bound to other proteins, signifies the level of metabolic stress being borne by an individual 

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Research study 5

In a companion paper to the one reviewed in Research Study 3 above, information is provided on reproduction in Balanus glandula along an estuarine gradient in South Slough Estuary, Oregon.  Results show that reproductive output decreases by 4-fold along 6km of the estuary from fully marine to brackish (7400 and 1600 embryos per year, respectively). Interestingly, salinity has no significant effect on percentage of population brooding, possibly because the high-salinity period overlaps to some extent with seasonally low temperatures.   Berger 2009 Mar Ecol 30: 346.

NOTE  defined as number of offspring produced per individual over a 1yr period (# of embryos per brood x number of broods)

NOTE  3 sites sampled monthly from ocean to mid-estuary to riverine end of the estuary over a 15mo period.  Salinity varies from highs of 28-32ppt during late summer/autumn (low value at the riverine end) to 8-28ppt during late winter/spring

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Research study 6

graphs showing cirral-beating behaviour under different conditions of water flow and temperatureAn investigation1 into the effects of water-flow velocity and temperature on oxygen uptake in barnacles Balanus glandula  at Fiday Harbor Laboratories, Washington focuses on physiological limitations at extreme levels of each parameter.  Results show that gas-exchange is limited at 2 extremes of water flow and temperature.  One of these occurs at high-velocity flow and low temperature when the metabolic capacity of barnacles to take up and process oxygen is slower than its physical delivery (kinetic limitation2), and the other at low-velocity flow and high temperature when metabolic demand out-strips its delivery by the current (mass-tranfer limitation).  In the former conditions the barnacles engage less in continuous beating and more in abbreviated beating consisting of quiescent gaping or periodic testing beats (see Right-hand side of upper graph3).  In the latter conditions the barnacles fully extend their cirri and may engage in pumping behaviours in an apparent attempt to increase ventilation. (see Left-hand side of bottom graph).  Generally, effects of elevated temperatures appear to be ameliorated in conditions of faster flow.  None of these observations in itself is particularly surprising, but in the identification and quantification of mass-transfer and kinetic limitations in barnacles, the authors make an important contribution.  Nishizaki & Carrington 2014 J Exp Biol 217: 2101.

NOTE1  experiments are done in a recirculating temperature-controlled flow chamber where oxygen concentrations are measured along with behavioral changes in cirral-beating activity

NOTE2  metabolic functions such as gas exchange depend upon the uptake of dissolved oxygen from the seawater medium.  Rate of uptake in a barnacle is potentially limited by transport rate of dissolved oxygen to the epithelial surface (mass-transfer limitation) and by kinetic processes that control the actual uptake and transport of oxygen across the epithelial surface (kinetic limitation).  In barnacles, kinetic limitation is largely related to slow diffusion across the cuticle, and to reliance of cirral and body movements for hemolymph circulation and an absence of oxygen-transporting respiratory pigments

NOTE3 these graphs represent only the 2 extreme temperatures (other temperatures not shown are 10, 15, and 20oC). The points represent barnacles that are beating their cirri continuously (black dots) and barnacles that are beating in a discontinuous or abbreviated way. The sum of these for a given flow velocity represent all barnacles in the test population that are not closed up. Under these extreme conditions only a small proportion of barnacles are active; at intermediate temperature and flow conditions up to 50% may be active

  Research study 7

Ecophysiologists believe that poor growth and survival of barnacles at high intertidal levels is a product of temperature and desiccation stresses combined with limited food availability during low-tide periods. This last has been investigated under controlled laboratory conditions but not in the field. Scientists from McKenna, Scripps, and Pitzer Colleges in Claremont, California attempt an ambitious field experiment at an unused pier in Los Angeles Harbor involving 8 racks of adult barnacles Balanus glandula attached to PVC plates at 4 shore heights that span the species’ upper vertical range (see drawing). Each rack consists of plates arranged in 3 vertical stacks of 4 supported on a framework of PVC piping. The total height of 45cm spans the species’ natural upper vertical range. Each stack can be removed by pulling it up and out of the framework. For each of 19d of the 39d study period the “experimental” stack is removed and immersed for an average of 2.5h in food-containing seawater, while the “manipulated control” stack is immersed for the same time in clean filtered seawater. The"control" stack remains diagram of rack used to support barnacle-bearing plates in study of intertidal-height effects on growth of barnacles Balanus glandulain situ over the entire study period. The authors’ expectation of enhanced growth on the experimental plates is met, with an average 24% greater growth over all tidal heights, but there is no significant interaction between intertidal height and feeding treatment (the authors expected higher shore individuals to benefit more from the food supplement than lower shore ones), suggesting that most or all upper-level barnacles are food-deprived. There is no significant effect of food supplementation on survival. As expected, barnacles in control plots exhibit slower growth rates and higher mortality rates at higher intertidal levels than than at lower ones. The study is the first to document that upper limits of distribution of barnacles is governed not only by levels of tolerance to physical stressful conditions, such as temperature and desiccation, but also to energy/nutrition stresses caused by food-deprivation. Gilman & Rognstadt 2018 J Exp Mar Biol Ecol 498: 32.

NOTE food consists of 2-3d-old brine-shrimp Artemia spp. nauplii, known to promote good growth and reproduction in B. glandula. Supplementary feeding increased available food by up to one-third over the food present naturally. As shell growth is negligible over the 39d study period, growth is estimated from change in tissue mass