title for whelk section of A SNAIL'S ODYSSEY
   
title for whelk section of A SNAIL'S ODYSSEY
  Habitat & community ecology
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  Shore-level size gradients
  Topics in this diverse section include shore-level size gradients, considered here, and SHELL MORPHOLOGY & HABITAT, AGENTS OF COMMUNITY STRUCTURE, and DEMOGRAPHY & HABITAT COMPLEXITY considered in other sections.
 
Research study 1
 

photograph of whelks Nucella lamellosa from Puget Sound, Washington courtesy Linda Schroeder, Pacific Northwest Shell Club, SeattleIn one of the earliest studies on population biology of whelks in Puget Sound, Washington the author speculates on factors limiting the extent of upward and downward movements of Nucella lamellosa and N. ostrina, and on relationships of shore-level position to sizes.  In the case of Nucella ostrina, the author speculates that it is probably length of emersion by the tides, with associated drying, UV irradiance, and so on, that sets upward limits in the intertidal zone.  In the case of N. lamellosa, it may be competition for food or, more likely, abundance of predatory sea stars and rock crabs, that sets the lowermost limits of distribution.  Continuous immersion seems not to be a problem for this species, as subtidal populations are common, in which individuals can reach extra-large size.  Kincaid 1957 Local Races and Clines in the Marine Gastropod Thais lamellosa Gmelin  A population study  The Calliostoma Company, Seattle 75 pp. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle Washington PNWSC.

 

 

A variety of whelks Nucella lamellosa collected
intertidally from Puget Sound region. Note
their generally small size. Subtidal N.
lamellosa
can reach 11cm shell length

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Research study 2
 

Studies at several sites in Puget Sound, Washington, including San Juan Island and Shannon Point, disclose that whelks Nucella spp. and Lirabuccinum dirum exhibit shore-level size-gradients, with size of shell increasing in snails further down the shore.  This is shown graphically for the 2 whelk species, N. ostrina and N. lamellosa, that exhibit the strongest trends, including their prey barnacle species Balanus glandula and Semibalanus cariosus (see graph on Left). Maintenance of the gradient appears to be by a combination of orthokinetic responses to light combined with geotaxes that differs between species and between sizes of individual snails.   Thus, small N. ostrina tend to crawl upwards in both light and dark, while large N. ostrina are neutral to gravity in the light, but crawl upwards in the dark.  In comparison, small N. lamellosa crawl downwards in the light and upwards in the dark, while large N. lamellosa crawl downwards in the light and are neutral to gravity in the dark. This is a bit complex to try to visualise, but the author sums it up nicely by saying that the behaviours lead to vertical separation graph showing shore-level size gradients in whelks Nucella spp. and their prey barnacles at Shannon Point, Washingtonof the 2 whelk species and establishes shore-level size-gradients as shown in the graph.

graph showing shore-level size gradients in whelks Nucella spp. and their prey barnacles at Shannon Point, WashingtonAdditional influence in the size gradients of the whelks has been the evolution of similar size-gradients in their major food sources, Balanus glandula and Semibalanus cariosus. The strategy saves energy in terms of predator-prey size selection, as well as facilitates resource partitioning both within and between the species.  Bertness 1977 Ecology 58: 86.

NOTE  although the regressions in the author's graph are correctly drawn, with the dependent variable (shell size) on the Y axis, and independent variable (tidal height) on the X axis, an easier way to visualise the relationships might be to reverse the axes. In this way tidal height becomes the Y axis, as shown in the redrawn graph on the Right

NOTE  behaviour in which the rate of movement of an animal changes in direct proportion to the strength of a stimulus, in this case, light, Thus, the behaviour is a photo-orthokinesis. Geotaxes are movements in response to gravity.  A negative geotaxis results in the snails crawling upwards

 
Research study 3
 

graph showing shore-level size gradients for whelks Nucella lamellosa, N. ostrina, and N. canaliculata in San Juan Island, WashingtonA somewhat different account of shore-level size gradients of Nucella spp. in San Juan Island, Washington is given in a later study.  The author measures sizes of snails on various shorelines and does experiments using cages attached at specific tidal levels on the dock at the Friday Harbor Laboratories, and also at various tidal heights on several evenly sloping shorelines. The results of field surveys leads the author to conclude that the relationship between size distribution and shore-level height is highly variable in the 3 species N. lamellosa, N. ostrina, and N. canaliculata, and that snails of different sizes do not exhibit behaviours correlative with an intertidal gradient as described in Research Study 2 above (see graph upper Left). Note that of the 3 species depicted in the graph, only N. lamellosa exhibits increasing shell length with increasing depth (the outlier point represents just a single individual), while the data for the other whelk species suggest that sizes actually decrease with increasing depth.

Data from another collection of N. lamellosa, moreover, from dock pilings at Friday Harbor Laboratories, actually indicate decreasing shell length with depth, although even the author agrees that the high variability in the data does not inspire confidence.  Because of the number of  experiments and observations presented by the author, including some data sets from observations on Thais orbita in South Australia, the results tend to be hard to follow.  However, the author concludes that where a size gradient exists, it is the product not of size-dependent behaviours to light and gravity as described earlier, but of direct responses to factors such as shelter, temperature, desiccation, and food.  The author uses a “conceptual model” as a kind of summary statement, the entries proceeding in the order: 1) a Nucella when graph showing size of whelks Nucella lamellosa on dock pilings in San Juan Island, Washingtonplaced underwater will move upwards unless it encounters food,  2) heat stress causes it to move downwards unless the stress is mild and food is available, in which case it photograph of 3 species of whelks Nucella lamellosa, N. ostrina, and N. canaliculatatends to stay put,  and 3) ridges and crevices are selected as resting spots especially if conditions are wavey.  These factors are thought to impose a size selection on Nucella that would account for the size distributions observed. On this basis, the author concludes that it is unnecessary to invoke specific responses to light and gravity as proposed in the earler study.  Butler 1979 J Exp Mar Biol Ecol 41: 163.

NOTE  this is only one of several data sets obtained from sites in the San Juan archipelago and 2 on the Olympic Peninsula, Washington.  The Eagle-Cove site featured here is near to the one used in Research Study 2 above


The 3 whelk species arrayed in vertical pairs. Left: Nucella
lamellosa;
Middle: N. canaliculata; Right: N. ostrina 1.4X

 
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