title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
  Reproduction & development
 

photograph of egg capsules of whelks Nucella lamellosa and Ceratostoma foliatumWhelks and other neogastropods have internal fertilisation and eggs that are laid within cases and glued to the substratum.  A free-living larval stage is absent.  The juvenile hatch from their egg capsules, crawl within their cases for a short time, then emerge from the cases as walk-away juveniles (known as hatchlings) to take up the adult mode of life.

NOTE most authors that work on neogastropod reproduction err by terming the larger containers egg "capsules" instead of "cases". Of course, the eggs themselves are contained in their own egg capsules from which they hatch before they emerge from the egg cases. However, recognising that not ALL of these authors are likely to be crazy, their original terminology will be retained in the ODYSSEY

 

Egg cases of whelks Nucella lamellosa (Left)
and Ceratostoma foliatum (Right) 1X. Neither
species encloses nurse eggs within their egg cases

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  Nurse eggs
  Topics in this section on reproduction & development include nurse eggs, considered here, and EGG PRODUCTION & ENCAPSULATED DEVELOPMENT, HATCHLING ECOLOGY, DISPERSAL GENETIC HETEROZYGOSITY & GLACIAL REFUGIA, and IMPOSEX, considered in other sections.
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Research study 1
 

histogram showing effect of crowding on size in embryonic whelks Nucella ostrinaphotograph of whelk Nucella ostrina courtesy Tim Rawlings, Cape Breton University, Nova Scotiatable showing hatchling sizes of several species of west-coast whelks along with data on numbers of nurse eggs available for the embryosHatchling size in whelks Nucella ostrina is largely dependent upon the number of embryos within the capsule and, thus, on the availability of nurse eggs for them to eat during development. The histograms show that embryos in capsules containing 1-4 eggs hatch at about twice the size of embryos in capsules containing 17-23 eggs.  Young embryos are able to engulf a nurse egg whole, but this ability diminishes in older embryos, which cannot ingest such large particles. The author notes that nurse eggs are distributed “regularly” amongst the capsules in N. ostrina and does not think that competition for nurse eggs, even in the most crowded capsules, contributes much to differences in hatching size.  However, without knowing whether nurse eggs are actually in surplus in the crowded capsules this conclusion is difficult to accept. Spight 1976 Biol Bull 150: 491; see also Lyons & Spight 1973 Veliger 16: 189. Photograph courtesy Tim Rawlings, Cape Breton University, Nova Scotia.

NOTE  other accounts suggest that up to 97% of the eggs may be nurse eggs, although there appears to be much regional variation. Note in the tabulated data (lower Left) that N. lamellosa and C. foliatum do not appear to provide nurse eggs to their embryos. The author provides comparable data for other whelk species, not presented here

 
Research study 2
 

photograph of whelk Nucella canaliculata courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington A related study by the above author at Friday Harbor Laboratories, Washington shows that the proportion of infertile eggs in Nucella canaliculata is highly variable, ranging from essentially zero in some areas to 16% in others.  In these latter populations the infertile eggs are used as nurse eggs.  The author proposes that N. canaliculata may be “abandoning one mode of reproduction in favor of another”; in other words, is in the process of evolving nurse eggs.  The author discusses several evolutionary ‘scenarios’ and the idea probably justifies further research attention.  Spight 1977 The Nautilus 91: 74. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

 
Research study 3
 

map showing sites on the west coast where female whelks Lirabuccinum dirum are collected, including ratios of nurse eggs: embryos in their egg capsulesgraph showing the ratio of nurse eggs to embryos in whelks Lirabuccinum dirum in relation to how many embryos there are in an egg capsuleFemale dire whelks Lirabuccinum dirum (= Searlesia dira) provide nurse eggs within a single capsule in average proportion of 18-21 nurse eggs to 1 embryo.  At laying the nurse eggs are indistinguishable from normal eggs. The nurse eggs are swallowed whole by embryos and, at first swallowing, the embryo is only slightly larger than its food item (each being about 240µm in diameter).  There is considerable variation geographically in the ratio of nurse eggs to embryos, but without any clear pattern (see map on Left). 

There is also considerable intracapsule variability in number of embryos, and in the proportion of nurse eggs to embryos.  At San Juan Island, Washington, for example, one collection of 54 capsules varied from 150 nurse eggs in capsules with one embryo, to about 12 nurse eggs in capsules with 11-12 embryos (see graph upper Right).  This represents a huge difference in potential food available for each embryo.

graph showing relationship between hatchling size in whelks Lirabuccinum dirum and number of hatchlings per capsuleBecause of food competition within capsules, then, hatchling size is strongly dependent on number of hatchlings per capsule (see graph lower Right). This represents an almost 2-fold difference in size between solitary hatchlings and hatchlings with 9 or more hatchling mates.  Even within a single clutch, then, hatchling size varies widely.  The author suggests that this may be adaptive for a species that lays its eggs months ahead of when juveniles will emerge into an unpredictable environment. in other words, is a bet-hedging strategy. The author adds that, alternatively, it could simply be a consequence of an imperfect mechanism for "increasing" hatchling size.  Rivest 1983 J Exp Mar Biol Ecol 69: 217.

NOTE  the mechanism preventing development in nurse eggs is not known

 
Research study 4
 

graph showing variable hatchling sizes in different populations of whelk Nucella ostrina at Cape Arago, OregonVariation in extent of nurse-egg feeding in whelks Nucella ostrina at Cape Arago, Oregon is thought to be a major contributor to variation in hatchling size between clutches graph showing difference in growth rates between large and small hatchlinigs of whelks Nucella ostrinaproduced by different females (see histograms at Left). So, as a whelk, is it better to be a large hatchling or a small one?  Like runts in any litter, small hatchling whelks start with a strike against them, and they may never catch up with their larger clutch-mates.  Note in the graph on the Right that small hatchlings remain about 2wk behind the large hatchlings with respect to growth. Moran 1999 Am Zool 39: 304.

NOTE  hatchlings from 6 clutches are fed ad libitum on small barnacles of an unidentified species for 65d.  Mean starting size of large hatchlings is 1.5mm shell length and, small hatchlings, 1.1mm

 
Research study 5
 

map showing study sites for investigation of nurse-egg provision in whelks Nucella ostrinaThe relationship between hatchling size and maternal provisioning in the form of nurse eggs in whelks is further investigated in whelks Nucella ostrina at the Bamfield Marine Sciences Centre, British Columbia (see map on Left). In 9 populations sampled at different sites with varying degrees of wave exposure, variation in hatchling size owes almost entirely to the ratio of nurse eggs to embryos.  Egg size itself is not a reliable predictor of hatchling size. 

histogram showing nurse-egg allocation to embryos  in whelks Nucella ostrina in relation to wave exposureThe authors remark on a tendency for females in wave-protected areas to allocate more nurse eggs per embryo than females in more wave-exposed sites but, as shown by the data on the Right, there is really too much overlap in the data to justify such a conclusion.

The authors conclude that within-population variation in hatchling size is controlled mainly by differences in allocation of embryos per capsule, while among-population variation is controlled mainly by differences in allocation of nurse eggs per capsule (i.e., volume of yolk per embryo).  Lloyd & Gosselin 2007 Biol Bull 213: 316.

NOTE  egg capsules in wave-protected areas are larger than in wave-exposed areas, possibly an adaptation to reduce surface area-to-volume ratios leading to better protection of the embryos from desiccation and/or osmotic shock, and this may be another factor to consider. see Rawlings 1995 Veliger 38: 54

NOTE it is also difficult to obtain reliable data on wave exposure on rocky shores

 
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