title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
  Predators & defenses
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photograph of whelks Nucella lamellosa courtesy Dan Yoshimoto, Vancouver Island, British ColumbiaPredators of whelks include crabs, isopods, and other snails during embryonic development, and crabs, sea stars, birds, and fishes during adulthood.  Defenses include shell strength and sculpturing, and burying. Photograph courtesy Dan Yoshimoto, Vancouver Island, British Columbia www.manandmollusc.



Collection of whelks Nucella lamellosa made
at Port Hardy, British Columbia 0.5X

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Shell colours & camouflage


This section on predators & defenses is divided into topics of shell colours & camouflage, considered here, and sections on
NOXIOUS SECRETIONS, considered in other sections.

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Research study 1

drawing of whelk Nucella lamellosa showing banding pattern of coloursphotograph of a whelk Nucella lamellosa feeding on barnacles Balanus glandulaTwo species of west-coast whelks, Nucella lamellosa and N. ostrina are notable for their shell-banding patterns and colours, and the question arises as to whether the colours and/or patterns perform a function as, for example, protective camouflaging.  In collections of N. lamellosa around Edmunds, Washington and on offshore island reefs, shells with up to 20 bands can be found, with predominant colours being orange, chestnut brown, purple-gray, and white.  In the 20-band specimen featured here, bands 11-20 over the apertural lip are in the process of being deposited by the underlying mantle tissue; hence, should be a repeat of bands 1-10, which swing around the main body whorl in a continuous spiral. The blue demarcations are an attempt to show the 2 sets of bands.  The colours occur mostly in juveniles, while adults tend to be dull-gray in colour without bandings.  Interestingly, while shell coloration in many different kinds of snails and limpets is thought to be dependent upon diet, it seems not to be in N. lamellosa.  Of 90 surviving juveniles of an initial batch of 763 hatchlings reared on the whelk’s favourite field diets of barnacles and mussels for 7mo, 63 or 70% become white in colour, while 22 or 24% stay the same colour as at the start.  Only 5 of 90 or 6% change colour.  The author concludes that there is no evidence that transfer of pigments from food accounts for any colour in N. lamellosa

As to the function of colours and banding patterns, it seems unlikely that they camouflage against colour-visioned predators such as birds, fish, and crabs, at least based on what the human eye sees.  Also, owing to different background patterns of different environments, a given colour morph will have a different visibility at each site.  Frequency of occurrence of colour morphs at bird anvils is no help because, while Nucella spp. are commonly represented at such anvils (for example, 79% of 91 shells collected by the author at several anvils on Ripple Island, Washington are Nucella spp.), the shell fragments are mostly drably-coloured adults. Juveniles, if eaten by gulls, are apparently swallowed whole.   Also, as noted by the author, the shells may be picked up elsewhere and brought to the killing ground. Chemical camouflage associated with different “whole-shell” colours is a possibility, but no research has been done.  Another factor relating to visual predation is that snails tend to remain hidden under rocks at low tide, and even at high tide if feeding on barnacles beneath or between adjoining rocks.  Clearly, more research needs to be done on the functional significance of colour morphs in N. lamellosa.  Spight 1976 Res Popul Ecol 17: 176.

NOTE instead of clarifying the banding pattern, the author's interpretation has confused it. For example, Bands 11-20 are NOT a repeat pattern of Bands 1-10. Even if Band 1 is actually a merged pair of dark bands, it does not correspond with Band 11, which is a single white band. Also, although it is not clear in the author's drawing, Bands 3-4 appear to be 2 white bands merged into a single one, but there is no counterpart in Bands 13-14. There are also an unexpected Band 0 and an unlabeled Band somewhere between 11 and 14, indicated by the blue question marks on drawing. These comments may seem overly "pickey", but they underscore the need for care when dealing with such a tricky subject

NOTE but is this true?  The data in the experiment described show that the shells of most juveniles are becoming white, which is clearly a change in colour, although not involving carotenoid or other plant-bearing pigments.  Also, is it possible that a laboratory diet may lack the micro-diversity of potential pigment-bearing food items that would be available in the field? More research is needed on this topic

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Research study 2

photographs of parents and progeny of whelks Nucella ostrina: cross between a white male and a black-banded femaleNucella ostrina inhabits rocky shores with fairly high wave exposure and moderate currents.  Embryos develop in capsules and hatching is to crawl-away juveniles.  Adults move only small distances during their lifetimes.  For these reasons, little gene flow is expected between populations.  Crosses of individuals from widely distant populations produce viable offspring, and this allows tests to be made of the heritability of colour and colour patterns from these populations. 

photographs of parents and progeny of whelks Nucella ostrina: cross between an orange-banded male and a black-unbanded femalePredominant field colours of N. ostrina are black, white, and orange.  To test for heritability of colours and banding patterns, immature individuals are collected, sexed, and held at the Bamfield Marine Sciences Centre, British Columbia until they mature.  Numerous crosses, some between individuals from Alaska and British Columbia, yield Mendelian ratios that suggest 3 discrete shell-colour alleles.  These include a cross of an orange male (Torch Bay, Alaska) and a black female (Bamfield, British Columbia) yielding half black and half orange-brown progeny.  A cross of a white male (Bamfield) and black-banded female (Torch Bay) yields approximately half white and half black offspring (see photos of this cross on Left). A cross of an orange-banded male with a black-unbanded female (both from Barkley Sound, British Columbia) yields approximately one-quarter orange unbanded, one-quarter black unbanded, one-quarter orange banded, and one-quarter black banded, suggesting that banding is controlled by two alleles at a single locus (see photos on Right).  Results of these and other crosses initially suggest that black coloration is dominant to orange but, in a note added to the paper in proof stage, the author suggests that in some situations the reverse may be true. This will require clarification.

Feeding experiments at Friday Harbor Laboratories , Washington involving diet switching from barnacles to mussels indicate that diet has little effect on shell colour in N. ostrina and N. canaliculata, in support of what has been described for N. lamellosa in Research Study 1 above.  Palmer 1984 Malacologia 25: 477.

NOTE  marked individuals in one field experiment moved less than 5m in a 12mo period

NOTE  other colours such as orange/grey, brown, grey/brown, and grey also occur, but are more variable and harder to score

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Research study 3

graph showing incidences of occurrence of banding in whelks Nucella ostrina in relation to wave exposureInterestingly, a study on the distribution of banded individuals of Nucella ostrina at the Bamfield Marine Sciences Centre, British Columbia shows that the proportion of banding increases with increasing exposure to waves (see graph).  The author remarks that the adaptive value of this polymorphism is not known at present, but also adds that the study is a preliminary one. The banding appears to result from regularly spaced zones of suppressed pigmentation in the outer shell.  Genetic crosses show that inheritance of shell banding is controlled by a single autosomal locus (OB) with two alleles (OBB = banded, and OBU = unbanded), with banding being dominant.  Banding is also assorted independently of shell colour. If the banding polymorphism in Nucella ostrina is selected for by visual predators such as surfperches and birds, the species responsible remain unknown.  Palmer 1985 Biol Bull 169: 638.

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Research study 4

With respect to colour banding, what is the relationship between shell ridging and colour banding? It often seems that banding is associated with ridging. If this is true, then it may be something to consider when theorising about the function of colour banding. For example, a ridged shell is expected to be stronger than an unridged one (has this been tested?) so, if ridging is associated with colour banding, then another factor is added to the equation. These photos show an assortment of Nucella ostrina collected from photographs of whelks Nucella ostrina courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattleareas of Puget Sound, Gulf of Georgia, Barkley Sound, and other photographs of whelks Nucella ostrina courtesy Dave Cowles and Nathaniel Charbonneau, Walla Walla University, Washingtonlocations in northern Washington and southern British Columbia, so you can see for yourself whether the idea is worth pursuing.

Photographs on the Left courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC; photographs on bottom row Right: Dave Cowles and Nathaniel Charbonneau, Walla Walla University, Washington www.wallawalla.

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Research study 5

histogram comparing brightness changes in hatchling whelks Nucella ostrina being fed diets of mussels and barnacles, and being kept in sunlight or shadeSome interesting findings relating to shell brightness/colour in whelks Nucella ostrina are presented in a study conducted at the Bamfield Marine Sciences Centre, British Columbia. In the main experiment, snails are reared from 3d-old hatchlings on diets of mussels Mytilus trossulus or barnacles Balanus glandula/Chthamalus dalli while being exposed to either sunlight or shade. After 21d there is no significant effect of diet on shell brightness, but snails maintained in sunlight are significantly (28%) darker than ones kept in the shade (see histogram). In the field hatchling snails initially prefer crevice-inhabiting mussels over all other prey species, and only begin eating open-rock-inhabiting barnacles at about 2-4mo post-hatching. The authors remark that similar ontogenetic (during the lifetime) shifts in brightness also occur in 3 other motile species included in the study (Nucella lamellosa, Pagurus granosimanus, and Mytilus trossulus; 14-29% decreases in brightness from smallest- to largest-sized individuals), ones also noted to undergo ontogenetic microhabitat changes. Such phenotypic plasticity in pigmentation/brightness may have selective advantage when young individuals move out of structurally complex microhabitats onto open-rock habitats where darker pigmentation may help to camouflage them from visual predators such as fish or birds and/or protect them from UV radiation. De Bruyn & Gosselin 2014 Mar Ecol Progr Ser 498: 147.

NOTE a measure of brightness (lightness or luminosity) rather than hue provides a more standardised method for comparing species and additionally relates to all visual predators whether they see in colour or not

NOTE the authors examine ontogenetic colour changes in size-ranges of a total of 15 species of field-collected intertidal marine invertebrates

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