title for limpet section of the Odyssey
  Habitats & ecology
  Topics on habitats & ecology of limpets include life in the intertidal zone, considered here, and COMPETITION, POPULATION & COMMUNITY DYNAMICS, HOMING & TERRITORIALITY, SHELL GROWTH (SHAPE) & COLOUR, and SEASONAL MOVEMENTS presented in other sections.
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  Life in the intertidal zone

Limpets, littorines, barnacles, and mussels are common representatives of mid- to high-intertidal regions on the west coast and, of these, limpets are often found as high or higher than any other invertebrates.  Physical factors affecting their survival include desiccation, insolation, and wave impact. 

Topics relating to "life in the intertidal zone" for limpets include zonation & critical tide factors considered here, and TEMPERATURE STRESS & HEAT-SHOCK PROTEINS, DESICCATION, and WAVE FORCES found in other sections.

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Zonation & critical tide factors

Research study 1

diagram of vertical distributions of four limpet species at Sunset Bay, Oregonschematic of a single 24-h tide cycleEarly ideas about factors that influence zonation of limpets and other intertidal organisms, include regulation at the lower limits of distribution by competition for space and food, and regulation at the upper limits by biological factors, including uppermost distribution of algal foods, and physical factors, most notably, desiccation.  The idea of desiccation being important in delimiting distributions of limpets comes from an observation in Sunset Bay, Oregon that upper limits of distribution of 4 limpet species, Acmaea mitra, Lottia digitalis, L. scutum, and L. pelta are in close agreement with two critical tide levels, namely, LHHW  (lowest higher high water) and LHLW (lowest higher low water; see diagram on Left). The topic of zonation1 of west-coast invertebrates is outside of the intended scope of the ODYSSEY, but perhaps a brief reference to critical tide levels is necessary before further discussing limpet distributions. 

The diagram above Right shows a typical day’s tidal pattern for the west coast, but with levels calculated for Sunset Bay, Oregon on a day in 1947. Note the semidiurnal pattern, with 2 high tides and 2 low tides, and mixed, that is, of unequal heights.  The critical levels for this 24-h tidal cycle are shown (HLW, HHW, LLW, and LHW).

schematic of one lunar day's tide cycle at Sunset Bay, OregonIf we now change this to a lunar day, which is approximately 28d in duration, we see the same pattern of tides and can identify 4 mean tidal levels as shown in the previous schematic - namely, MHLW (mean higher low water), MHHW, MLLW2, and MLHW and the component tides that make up these means (only MLLW is indicated in the schematic).  With a bit of noodling the terminology will become clear.  The MHHW level, for example, includes the highest tide level of the month, namely, HHHW3 (highest higher high water) and also all the other daily HHW tides of the month, down to the LHHW.  The HHHW level, or the highest spring tide of the month, is thought to be critical because above this level an organism will theoretically not be wetted at all during a given lunar day (excluding waves and spray). 

Now, how does this relate to the Sunset Bay limpet species? LHHW is one of the critical tide levels designated for the Sunset Bay limpets, as it represents the highest level at which a limpet will be wetted on every day of the month, and this appears to set the upper limit of distribution for L. scutum and L. pelta, and also partially for L. digitalis.  Another level, thought to be critical for A. mitra, is LHLW, the level at which a limpet will normally be emersed for at least a few minutes each lunar day; however, at summer solstice when high tides in Oregon are generally lower than at other times of the year, the highest tide of the day will fail to reach it.  During a summer spring tide this exposure could be lethal. 

Does the concept of critical tide levels really really help explain distributional limits of limpets and other intertidal organisms?  Yes, but only when considered in the fullest context of all other physical and biological factors that may be operating at a particular time in a given area.  On the specific rocks at Sunset Bay inhabited by the limpets during the specific period of study (not specified in the paper, but probably summer), the author records certain distributional patterns.  These distributions, however, are certain to be different in other nearby areas, ones with differing microhabitat conditions, including aspect to the sun, slope, food availability, and so on.  Such factors, in their myriad diversity, form the research basis for many hundreds of papers cited in the ODYSSEY. Shotwell 1950 Ecology 31: 647; charts of critical tide levels for Sunset Bay, Oregon adapted from Doty 1946 Ecology 27: 315.photograph of zonation on rocky shores around Bamfield, British Columbia

NOTE1  for a review of some of the older zonation references (up to 1975) see Carefoot 1977 Pacific Seashores Univ Wash Press, Seattle

NOTE2  MLLW (mean lower low water) represents Chart Datum or zero tide level on the U.S. system

NOTE3  this tide and LLLW are the spring tides of each month, when moon and sun are in alignment.  They are, respectively, the highest high tide of the month and the lowest low tide of the month.  In between are the monthly neap tides, when moon and sun are out of alignment, and these include the highest low tide of the month, HHLW, and the lowest high tide of the month, LLHW

Zonation on rocky shores in Barkely Sound, British Columbia

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