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

photograph of a breeding aggregation of whelks Nucella lamellosaWhelks and other neogastropods have internal fertilisation and encapsulate their eggs. Drawing of late-stage veliger larva from LeBoeuf 1971 Veliger 14: 205.drawing of veliger shell of whelk Nucella emarginata

 

 

Early veliger shell of whelk
Nucella emarginata

 

 

Breeding aggregation of
whelks Nucella lamellosa 0.3X

  black dot
  Egg production & encapsulated development
  A free-living larval stage is absent. Topics in this section include egg production & encapsulated development, considered here, and NURSE EGGS, HATCHLING ECOLOGY, DISPERSAL GENETIC HETEROZYGOSITY & GLACIAL REFUGIA, and IMPOSEX, considered in other sections.
  black dot
 

A whelk’s egg capsule provides not only the proper chemical environment for its embryos to develop, but also protection from predators, storms, disease, UV light, and other harmful environmental conditions. 

NOTE the capsule referred to here is a small wheat grain-shaped container, sometimes also referred to as a case, within which the female deposits  a few dozen eggs.  The eggs develop in this protective capsule or case, then hatch into crawling embryos that complete their development within the capsule.  Later, when the juveniles have completed their development, they hatch from the capsule via an opening at the top and crawl out onto the sea bottom as, what else?…hatchlings.  This currently used terminology is obviouslly confusing.  Better that the embryos hatch from their egg capsules into the fluid within the case, then later emerge from the case onto the sea bottom as juveniles.  Nonetheless, the terminology used in the studies below and elsewhere for other whelks is that used by the authors in their research papers

  black dot
 

Nucella spp.

  The topic of egg production & encapsulated development is divided into a section on Nucella spp. considered here, and a section on OTHER GENERA, presented elsewhere.
  black dot
Research study 1
 

drawings of egg capsules of whelks Nucella lamellosa and N. canaliculataIn Bodega Bay and Tomales Bay, California Nucella spp. have different breeding patterns. Females of N. ostrina are ripe and deposit egg capsules throughout the year, but mostly in Nov-Feb.  In comparison, 80% of female N. canaliculata are ripe in Jan-Feb, and eggs are encapsulated from the end of March to mid-May.  In both species males are ripe thoughout the year.  Sex ratios in a collection of 252 N. ostrina are 1.3 female: 1 male, with 3 hermaphroditic individuals, while in 252 N. canaliculata they are 1 female: 1.2 male, with no hermaphroditic individuals.  Egg capsules of the 2 species are slightly different.  In both species, only 3-5% of eggs develop into adults, while the rest are nurse eggs.  Houston 1971 Veliger 13: 348.

 
Research study 2
 

graph showing capsule-deposition time in the life cycle of whelks Nucella lamellosaWhelks Nucella lamellosa aggregate for several weeks during copulation and egg-laying.  During this time, decrease in size of the visceral mass provides a reliable indication of time of spawning.  A 19-mo study at Friday Harbor Laboratories, Washington shows that snails aggregate during November and deposit capsules during Jan-Mar, with build-up of testes in males and capsule-albumin gland in females occurring during the following summer (see accompanying graph). Stickle 1973 Biol Bull 144: 511.

NOTE in females about 80% of the visceral mass is represented by reproductive products

 
Research study 3
 

photograph of whelks Nucella lamellosa depositing egg capsulesNucella lamellosa has a distinct reproductive period occurring in late autumn.  In San Juan Islands, Washington breeding groups of several hundred individuals are formed and these are maintained over a period of a few weeks.  Over a several week period, a female will deposit 4-5 capsules per day, up to a 100 or more in total, each capsule yielding about 20 juveniles.  Spight & Emlen 1976 Ecology 57: 1162; Spight 1974 Ecology 55: 712; Spight 1975 Oikos 26: 9.

 

Whelks Nucella lamellosa depositing egg capsules
in Stanley Park, Vancouver near the end of November

 
Research study 4
 

A whelk’s egg capsule provides not only the proper chemical environment for its embryos to develop, but also protection from predators, storms, disease, and other environmental effects.  For example, egg capsules of the whelk Nucella spp. are often found in intertidal areas directly exposed to sunlight.  Investigations at the Bamfield Marine Sciences Centre, British Columbia into the possible deleterious effects of UV components in sunlight on graph showing survival of embryos of whelks Nucella ostrina when exposed to UV lightdeveloping embryos in Nucella spp. show that >90% of damaging UV-B wavelengths are screened out by the capsule wall. photograph of egg capsules of whelk Nucella ostrina courtesy Tim Rawlings

Exposure of N. ostrina capsules to natural sunlight over a 23-d period results in 50% mortality of the contained embryos.  Removal of the outer 2 layers of the capsule wall under the same conditions results in 78% mortality.  Control treatments in the dark or with UV wavelengths filtered out lead to little mortality.  Rawlings 1996 Mar Ecol Progr Ser 136: 81. Photograph courtesy Tim Rawlings, Cape Breton University, Nova Scotia.

Several egg capsules of the whelk Nucella
ostrina
deposited in or near a mussel shell 2X

 
Research study 5
 

drawings of hatchling whelks Nucella lamellosa emerging from an egg capsuleLarge size clearly is advantageous for survival in Nucella spp.  Observations in San Juan Islands, Washington show that a newly hatched N. lamellosa have a 1-2% chance to reach 3mo of age.  An individual reaching 3mo has a 35% chance to reach 1yr of age.  Older, larger, individuals have a 40-60% chance to survive through subsequent years.  Spight 1975 Oikos 26: 9.

 
Research study 6
 

table showing sizes of some west-coast whelks Nucella spp. and Ceratostoma foliatum at hatching relative to how many nurse eggs each consumes during developmentIn a review of hatching sizes of marine gastropods, including several west-coast species, the author concludes that rather than being selected for as a response to predation, hatching size in whelks Nucella spp. and Ceratostoma foliatum in San Juan Islands, Washington reflects a general trend for upper-shore species to hatch at a larger size than lower-shore ones.  Note that the highest west-coast intertidal species, Nucella ostrina, is equally large at hatching as the other Nucella species listed, yet hatch from the smallest-diameter eggs.  Nucella ostrina is one of the species listed that is known to consume nurse eggs within the capsule.  Spight 1976 Oecologia 24: 283.

NOTE  in a later publication the author provides additional review of life-history patterns in Nucella spp., specifically, N. lamellosa and N. ostrina: Spight 1979 p. 135 In, Reproductive ecology of marine invertebrates (Stancyk, ed.) U South Carolina Press, Columbia, SC.

   
 
Research study 7
 

photograph of whelk Nucella ostrinaIn west-coast populations of Nucella ostrina, females deposit capsules throughout the year.  A female may produce a hundred or more capsules per year, each yielding about 15-20 juveniles.  The embryos feed on nurse cells provided by the parent and the number consumed largely determines the size of the hatchling.  An egg of about 0.2mm original diameter can produce a hatchling 10 times larger (0.9-2.3mm shell length).  Spight & Emlen 1976 Ecology 57: 1162; Moran & Emlet 2001 Ecology 82: 1597.

NOTE  capsules at deposition in a population of N. ostrina in Dillon Beach, California each contain about 16 embryos and 450 nurse eggs.  At the end of development all nurse eggs have been consumed. LeBoeuf 1971 Veliger 14: 205.

 

 

Whelk Nucella ostrina resting on a
bed of sea lettuce Ulva sp.1.2X

 
Research study 8
 

With developmental times of 3-5mo, survival of whelk embryos in their capsules relies heavily upon the quality of the site the parents choose for their deposition.  But do the parents choose the best sites?  A study of survival of embryos of Nucella lamellosa at several sites in San Juan Island, photograph of whelks Nucella lamellosa with their egg capsulesWashington suggest that they do not, at least not based strictly on embryo survival.  On average, only 57% of embryos survive to hatching, and it is not unusual for none to survive. Adults return to the same or nearby sites in subsequent years, and a female may return to a site that is relatively unfavourable.   Interestingly, sites that lead to good survival of embryos are often less suitable for survival of the spawning adults and newly hatched snails.  No category of site identified by the author appears to satisfy all 3 important requirements: minimum exposure to physical stress, minimal exposure to predators, and ready access to food.  Sites used most commonly usually satisfy only 2 of these requirements.  Sites used uncommonly are suitable in only one respect or not at all.  The author concludes that site-usage patterns represent a compromise between the conflicting needs of the 3 life history stages and, while sites used may ultimately be the right places for the snails, they may not be for their embryos.  Spight 1977 Evolution 31: 682.

 

Whelks Nucella lamellosa with egg capsules. Sites
where whelks aggregate to copulate are usually the
same as where they deposit their egg capsules

 
Research study 9
 

graph showing effect of low salinity seawater on osmotic concentration of egg-capsule fluids in whelks Nucella lamellosaResearch at Friday Harbor Laboratories, Washington on Nucella lamellosa and N. lima show that the capsules provide some degree of  protection to the embryos against low-salinity stress.  Although the capsule walls are ultimately permeable to ions, their passage is slowed somewhat, and this may provide short-term protection as, for example, during periods of rainfall (see graph).  Note in the graph that the osmotic concentrations of the intracapsular fluids decline to near ambient levels after transfer of the egg capsules to dilute medium (5‰).  Pechenik 1982 J Exp Mar Biol Ecol 63: 195.

NOTE  the specimens of Nucella lima are collected in Alaska

 
Research study 10
 

photo/drawing composite of whelk Nucella lamellosa with egg capsulesThrough a long and productive research career the Danish marine biologist Gunnar Thorsen assembled a large collection of invertebrate specimens at the Zoological Museum of the University of Copenhagen.  Among the collection are samples of egg capsules of several species of west-coast whelks.  Presented here are drawings of egg capsules for 3 species of Nucella assembled by a researcher from the University of West Florida using original specimens and drawings from the Zoological Museum.  D’Asaro 1991 Ophelia 35: 1. Photograph of N. canaliculata courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  drawings by the same author of egg cases of other west-coast whelks can be found in the section OTHER GENERA. Note that the photographs are not in scale with the drawings of the egg capsules

 

photo/drawing composite of whelk Nucella ostrina with egg capsulesphoto/drawing composite of whelk Nucella canaliculata with egg capsules

 
Research study 11
 

photograph of an egg capsule of whelk Nucella ostrina with capsule walls stripped off to view interior, courtesy Tim Rawlings, Cape Breton University, Nova ScotiaFemale whelks Nucella spp. enclose their fertilised eggs in protective capsules which they glue to the substratum.  A single female will deposit many capsules, each containing several hundred eggs, during breeding period.  Most of the photograph of an egg capsule of whelk Nucella ostrina with capsule walls stripped off to view interior, courtesy Tim Rawlings, Cape Breton University, Nova Scotiaeggs contained within a capsule are infertile nurse eggs, and these are later eaten by the developing veligers for nutrients and energy.  There is no free-living veliger-larva stage and metamorphosis to the juvenile stage takes place within the capsule. The capsule walls consist of layers of protein/carbohydrate material that form a tough, 2-layered, leathery sac.  A hole in the top of the capsule is blocked at first with a hard, clear, gelatinous plug, but the plug degrades over the approximately 80-d developmental period.  At the end of development the shelled juveniles break through the remains of the plug and crawl out of the hole onto the substratum where they take up the adult way of life.  Rawlings 1995 The Veliger 38: 54. Photos of capsule of N. ostrina courtesy Tim Rawlings, Cape Breton University, Nova Scotia.

 

Egg capsule of whelk Nucella ostrina with
walls partially stripped away to see inside.
The 3rd-stage veligers are about 36d of age and
are consuming the many smaller nurse eggs 10X


Egg capsule of whelk Nucella ostrina containing
4th-stage veligers (about 60d of age). Note that all
the nurse eggs have been consumed and the veligers
are larger and close-packed within the chamber 10X

 
Research study 12
 

graph showing growth of hatchling whelks Nucella ostrina in autumn at Coos Bay, OregonAdvantage of large size in hatchling whelks is investigated with Nucella ostrina at the Oregon Instutute of Marine Biology, Coos Bay, Oregon.  Here, large and small hatchlings1 are "outplanted"2 in intertidal locations to examine the effect of size generally on growth and survival, and specifically on tolerance to different temperature regimes.  The results show that larger hatchling size at the start graph showing effect of hatchling size on their survival in whelks Nucella ostrina at Coos Bay, Oregonresults in: 1) faster growth3 (see graph upper Left), 2) quicker maturation, and 3) increased survival (see graph on Right).  Note that it takes a small hatchling almost a month to reach the starting size of a large hatchling.  This is a large and perhaps significant fraction of Nucella’s 2-year life span.  Overall, then, and as one might expect, increased maternal investment leading to larger hatchling size generally improves hatchling performance. 

Under certain environmental conditions, however, such as heat stress/desiccation, being large confers no advantage, and mortality is largely random with respect to size.  Thus, large hatchlings survive better than small ones in outplantings on the cooler4 west-facing side of a 2-m wide surge channel, but size confers no advantage on the hotter east-facing side of the same surge channel (data not shown here). Moran & Emlet 2001 Ecology 82: 1597.

NOTE1  “large” hatchlings have a mean shell length of 1.6mm, while “small” hatchlings have a length of 1.1mm

NOTE2 the authors present data for 3 "outplantings" in spring, summer, and autumn, but only the autumn set is shown here

NOTE3  the hatchlings are marked with a dye (that also enables growth rates to be measured) and are outplanted on plastic “astroturf” that has been seasoned in the field for a minimum of 5mo.  The turf contains abundant barnacles, mussels, and algae for food and cover, and bears a sticky boundary that deters escape of hatchlings.  The astroturf panels are retrieved at 9-d intervals for monitoring of the hatchlings

NOTE4 the authors do not measure heat stress/desiccation directly but, rather, infer these from their temperature-logger data which show morning temperatures to be 5-10oC higher on the east-side of the surge channel than on the west side

 
Research study 13
 

histogram of predator effects on hatching in whelks Nucella lamellosaEgg cases (capsules) of whelks Nucella are generally diffusible, allowing free exchange of gases and release of waste products of metabolism.  Water-borne substances can also diffuse inwards, and this raises the question as to whether metabolic-waste substances of predators would enter the capsules, be perceived by the embryos and juveniles developing within, and perhaps delay their development and/or time of emergence of the young snails.  This is investigated by researchers at Western Washington University by positioning capsules of N. lamellosa downstream of caged predators, crabs Hemigrapsus oregonensis  and isopods Idotea sp.  Exposure times are 36-65d.  Results show that exposure to crabs significantly delays hatching by 3.4d, while exposure to isopods delays hatching by 3.4d (non-significant; see histogram).  Cumulative effects of both yield a significant 6.1d delay.  What about exposure to conspecific adult snails?  These results indicate that hatching is accelerated by 6.3d, a finding that, according to the authors, is unexpected based on the results of other studies on crustaceans and amphibians.  The authors remark that theirs is the first study to document predator-induced hatching plasticity in a marine mollusc.  Miner et al. 2010 Oecologia 163: 69.

NOTE  these would include soluble components of feces, digestive secretions, kidney excretions, and so on

 
  RETURN TO TOP