BIOTIC Species Information for Nucella lapillus
Click here to view the MarLIN Key Information Review for Nucella lapillus
Researched byLizzie Tyler Data supplied byUniversity of Sheffield
Refereed byThis information is not refereed.
Taxonomy
Scientific nameNucella lapillus Common nameDog whelk
MCS CodeW687 Recent SynonymsThais lapillus

PhylumMollusca Subphylum
Superclass ClassGastropoda
SubclassProsobranchia OrderNeogastropoda
Suborder FamilyMuricidae
GenusNucella Specieslapillus
Subspecies   

Additional InformationThe taxonomy of Nucella lapillus was reviewed by Crothers (1985) and Kool (1993)
Taxonomy References Howson & Picton, 1997, Hayward & Ryland, 1990, Hayward & Ryland, 1995b, Fish & Fish, 1996, Graham, 1988, Crothers, 1985, Kool, 1993, Fretter & Graham, 1985,
General Biology
Growth formTurbinate
Feeding methodPredator
Scavenger
Mobility/MovementCrawler
Environmental positionEpifaunal
Typical food typesBarnacles and mussels (see Crothers, 1985). HabitFree living
BioturbatorNot relevant FlexibilityNone (< 10 degrees)
FragilityRobust SizeSmall-medium(3-10cm)
HeightUp to ca 4 cm Growth RateHighly variable. See additional information
Adult dispersal potential1km-10km DependencyIndependent
SociabilityGregarious
Toxic/Poisonous?No
General Biology Additional InformationThe ecology, physiology and genetics of Nucella lapillus has been extensively studied. Therefore, the following review is based on more detailed reviews by Fretter & Graham (1994) and Crothers (1985), to which the user should refer for further detail. The original references are given where appropriate.

Growth rate
Growth rates vary depending on wave exposure, prey type and starvation. Sheltered shore populations grow faster than wave exposed shore populations, resulting in larger more elongate shells (Osborne, 1977; Crothers, 1985; Etter, 1989). Feare (1970b) reported that juveniles reached 10mm with a year, ca 15 mm at 2 years old and entered maturity at ca 20 mm at Robins Hood Bay in Yorkshire. Moore (1938a) reported that dog whelks reach 10-15 mm at I year, 21-26 at 2 years and 29.5 mm at maturity. Maturity was calculated to be reached at 2.5 years at which point shell growth stops (Fretter & Graham, 1994). However, Etter (1996) suggested that adults continued to grow but extremely slowly. Osborne (1977) noted that juveniles <12 mm grew at the same speed above which sheltered individuals grew faster than wave exposed individuals. Etter (1996) reported that juveniles grew 6 mm/150 days on wave exposed shores but 9 mm/150 days in sheltered conditions. Mussels supported the highest growth rates (Hughes & Drewett, 1985). Etter (1996) transplanted juveniles between shores of different wave exposure, and concluded that growth was determined by environmental factors and depressed by wave exposure since it reducing foraging or feeding time.
Crothers (1985) suggested that, although crabs select the largest first year class dog whelks, rapid growth may allow dog whelks to grow beyond the predators preferred size range and decrease their susceptibility to predation.

Feeding
Nucella lapillus is an important intertidal predator and preys mainly on barnacles and mussels but may also prey on cockles, other bivalves and gastropods.

  • As in many neogastropods, the mouth and radula are born on an extensible proboscis, which in Nucella lapillus is approximately the same length as the shell in each individual (Barnes, 1980; Crothers, 1985).
  • Nucella lapillus feeds by either
    1. pressing the proboscis between the valves of bivalves or plates of barnacles and removing flesh by the rasping radula, or
    2. by boring a hole in the shell of its prey and inserting the proboscis through the hole.
  • The victims shell is bored by a combination of mechanical rasping by the radula in the proboscis and chemical attack by secretions of the accessory boring organ (ABO) situated in the sole of the foot.
  • Once penetrated, the prey is narcotized by secretions of the accessory salivary glands, which also secrete a cement like substance that may help keep the proboscis attached to the prey (Andrews, 1991; Fretter & Graham, 1994). The hypobranchial gland also secretes a pharmacologically active choline ester that may be involved in narcotization (Carriker, 1981; Crothers, 1985), however its function is disputed by Fretter & Graham (1994).
  • The dog whelk secretes digestive enzymes into the body of the prey and then ingests the resultant tissue 'soup' (Crothers, 1985).
  • The 'gape' attack method is energetically more effective (Fretter & Graham, 1994) and is more likely to be used by dog whelks with experience of handling the prey than inexperienced dog whelks and results in a lower prey handling time than boring (Rovero et al., 1999). Large dog whelks can also force the proboscis between the opercular plates of barnacles to apply the narcotic (Carriker, 1981; Fretter & Graham, 1994). The 'gape' attack is presumed to rely on successful application of the narcotic (Rovero et al., 1999).
  • Rovero et al. (1999) reported that the 'gape' attack method resulted in prey handling time (including inspection, narcotization and ingestion) of 49-51 hrs depending on experience, compared with a handling time of ca 100 hrs by boring. Morgan (1972) reported that boring could take 3 days to complete. Therefore, handling time can span several tidal cycles during which the dog whelk is vulnerable to desiccation, wave exposure and predation (Hughes & Drewett, 1985; Rovero et al., 1999).
  • It has been shown that experience of a particular food type reduces handling time (Dunkin & Hughes, 1984; Hughes & Dunkin, 1984), which may partly explain dog whelks preference for a particular type of prey even in the presence of others.
  • Once fed the dog whelk rests in a crevice or other shelter, up to 2-4 tides after feeding on barnacles and 7-9 tides after mussels (Hughes & Drewett, 1985). However, larger dog whelks have higher energy demands and rest for shorter periods (Bayne & Scullard, 1978).
Factors affecting feeding
  • Crothers, (1985; Table 1) lists 24 potential recorded food species. Nucella lapillus usually favours Semibalanus balanoides > Balanus spp. > Mytilus edulis > Eliminius modestus > (Crothers, 1985).
  • Larger dog whelks tend to handle larger prey than small dog whelks. Hughes & Burrows (1993 ) found that dog whelks avoided mussels <5 mm and preferred 10-20 mm mussels (Hunt & Scheibling, 1998). Hunt & Scheibling (1998) noted that although juveniles (<3 mm) rarely fed on mussels <2 mm and post recruits (<5 mm) only rarely consumed mussels <5 mm both were capable of feeding on the full size range of mussels presented. Mussels > 40 mm seem to be safe from dog whelk attack, and Crothers (1985) suggested that 20 mm long mussels were optimum for a 30 mm long dog whelk (Bayne & Scullard, 1978; Crothers, 1985).
  • Feeding rates vary, depending on size (hence shell thickness) of prey, temperature and season.
  • Crothers (1985) suggested a mean annual consumption of 15-40 mussels per dog whelk (Largen, 1967a; Bayne & Scullard, 1978) and reported rates of 0.5 or 0.59 mussels/day or 1.1 Semibalanus balanoides /day in summer (Connell, 1961; Fretter & Graham, 1962; Anala, 1974).
  • Largen (1967b) noted that feeding rates decreased with temperature, from an average of 16 barnacles or 0.7 mussels per week at 20 °C to 10.2 barnacles and 0.4 mussels per week at 15 °C.
  • Connell (1961) noted that the time spent feeding on open rock surfaces decreased from about 60% in July to September to only 13% in January to March.
  • Although foraging patterns on a given shore are similar, they vary between locations depending on the type of shore, its wave exposure, and local weather. Dog whelks from sheltered shores forage less in sunny, warm weather, whereas animals from wave exposed shores favoured calm periods even when sunny (Burrows & Hughes, 1989; Fretter & Graham, 1994).
  • Stickle et al. (1985) demonstrated that starvation could overcome the dog whelks tendency to avoid stressful conditions e.g. low salinity. Feeding rates were reduced at low salinity and temperature, e.g. only 25% of dog whelks examined fed at 25 psu and 5°C or 15 psu and 8.5°C. When exposed to air (emersion) ingestion rates were affected by the air and water temperatures, the difference between these temperatures, salinity, humidity, weather conditions and appetite (Stickle et al., 1985) (see sensitivity).
  • Dog whelks avoid dense mussel beds, preferring the diffuse margins between the mussel bed and the surrounding barnacle dominated substratum, or solitary mussels (Petraitis, 1987; Fretter & Graham, 1994; Davenport et al., 1996). This was partly because mussels can immobilise gastropods (Nucella lapillus and Littorina littorea) crossing the mussel bed with their byssus threads. Davenport et al., (1996) found that although Littorina littorea broke free of at least 14 byssus threads within 45 mins, Nucella lapillus attached by 1-18 byssus threads took 4-12 hrs to escape. Some specimens, however, were found immobilised by at least 30 byssus threads. Dog whelks take a long time to feed, hence, increasing the chance of them being immobilised. Petraitis (1987) suggested that mussels co-operated to flip over predatory dog whelks. However, Davenport et al., (1996) found that byssus attachments occurred to areas of the shell closest to the substratum, their was no evidence of selective attachment to flip the shell over. Petraitis (1987) calculated that nearly 30% of dog whelks in a mussel bed perished due to being immobilised.

Shell shape, colour and sculpture variation
Nucella lapillus is highly variable in the appearance of its shell, depending on wave exposure and location (see Crothers, 1983; 1985 for review).
  • Shell colour may be white, brown shading to black, mauve grading to pink, yellow shading to orange and rarely true orange, pink or black (Moore, 1936; Berry & Crothers, 1974; and Crothers, 1985, Plate 2). The white form predominates in the UK, but coloured shells predominate in the southern limits of its range (Portugal and Northern Spain) and in northern populations in Iceland (Crothers, 1985).
  • Nucella lapillus also exhibits a variety of banded forms, with a mixture of un-banded, thin or thick banded (see Crothers, 1985 for discussion).
  • Palmer (1984) demonstrated the inheritance of colour, banding and spiral shell sculpture using breeding experiments in Nucella emarginata. Similar breeding experiments have not been carried out in Nucella lapillus (Crothers, 1985), however it seems likely that colour and banding are under genetic control.
  • In some populations, mainly sublittoral or from the intertidal in North Kent, the growth lines extend outwards to form flounces or ruffles, and this variety of dog whelk is called Nucella lapillus var. imbricata. The imbrication is genetically determined but may appear less marked due to abrasion (Largen, 1971; Crothers, 1985).
  • The shell may also bear white dentiform tubercles on the inside edge of the shell lip, which develop once the shell has stopped growing (ca 2 years ). However, interruption of growth earlier in life, possibly due to starvation or parasitism may result in additional rows of teeth (Crothers, 1985).
Shell shape variation
Variation in shell shape and length has been extensively studied (see Crothers, 1985 for a review). Key points follow.
  • Sheltered shore animals grow faster than wave exposed individuals (Osborne, 1977) and sheltered shore populations have longer shells than wave exposed populations. Populations of exceptional length (up to 60 mm) occur subtidally or at extreme low water at Porlock Weir in the Severn Estuary, between Swanage and Kimmeridge, Dorset and at some sites in western Scotland (Crothers, 1985).
  • Nucella lapillus from wave exposed shores tend to have shorter, squatter shells than those from sheltered shores, which are more elongate. A progression from squat to elongate form is seen with decreasing wave exposure (Cooke, 1895; Kitching & Ebling, 1967; Crothers, 1985). The shape of the shell can be expressed in terms of the shell length relative to the aperture length (see Crothers, 1985 for review). The possible reasons for the relationship between wave exposure and shell shape are noted below.
    • Short squat shells offer less resistance to water flow and wave action and exhibit a larger aperture and larger foot and hence increased pedal surface area, which increases their adhesion the substratum (Cooke, 1895; Kitching & Ebling, 1967; Crothers, 1985; Etter, 1988).
    • Etter (1988) noted that the foot grows faster in wave exposed conditions rather than in sheltered conditions and that foot size increased in dog whelks transplanted to more wave exposed shores but change little in the reciprocal transplant (Etter, 1988; Fretter & Graham, 1994).
    • Longer, elongated shells have a relatively smaller foot but can hold a significantly greater volume of water within its mantle cavity when emmersed and are more tolerant of desiccation (see sensitivity) (Osborne, 1977, Kirby, 1994a).
  • Short squat shells are more prone to predation, their rounded shape makes them easier to swallow for birds such as gulls and eider duck. In addition, the animal is not able to withdraw completely into the shell making them susceptible to crabs and oystercatchers whereas the elongate shell form can withdrawn completely and the narrow aperture does not allow crabs to gain adequate purchase on the shell. (Osborne, 1977) (see sensitivity to wave exposure).
  • Crothers (1985) reported that the squat shell shape was absent from wave exposed sites in south-east England, the north coast of Wales, the Solway Firth and the Severn Estuary. Crothers (1985, Figure 33) suggested that the UK population of Nucella lapillus was divided into two groups, a south western group bearing the genes for the squat shell shape and another north-eastern from, which lacks the genes for the squat form and can only develop as the elongate form.
Genetic variation
In addition to the colour variation mentioned above, Nucella lapillus has been shown to demonstrate clines in allozyme (Day & Bayne, 1988; Day, 1990; Kirby, 1994a, b) and mitochondrial DNA polymorphisms (Kirby et al.,1997), Robertsonian translocation (Staiger, 1957; Bantock & Cockayne, 1975; Page 1988; Pascoe & Dixon, 1994; Pascoe et al., 1996), peri-and para-centric chromosomal inversions (Page 1988; Pascoe & Dixon, 1994; Pascoe et al., 1996; Pascoe, 2002). Variation in chromosome number was found to vary greatly between different populations, within some populations and even within some individuals (Pascoe, 2002).
Biology References Fish & Fish, 1996, Graham, 1988, Morgan, 1972, Hughes & Drewett, 1985, Berry & Crothers, 1968, Coombs, 1973, Moore, 1938a, Fretter & Graham, 1994, Crothers, 1985, Barnes, 1980, Anala, 1974, Bayne & Scullard, 1978, Fretter & Graham, 1962, Connell, 1961, Feare, 1970b, Hughes & Burrows, 1993, Davenport et al., 1996, Petraitis, 1987, Stickle et al., 1985, Moore, 1936, Moore, 1938b, Etter, 1989, Largen, 1971, Cooke, 1895, Kitching & Ebling, 1967, Osborne, 1977, Etter, 1988, Crothers, 1983, Hunt & Scheibling, 1998, Etter, 1996, Kinne, 1980, Etter, 1988b, Berry, 1983, Baker, 1974, Bantock & Cockayne, 1975, Staiger, 1957, Page, 1988, Day & Bayne, 1988, Day, 1990, Kirby et al., 1997, Pascoe et al., 1996, Pascoe & Dixon, 1994, Pascoe, 2002, Fretter & Graham, 1985, Hayward & Ryland, 1990,
Distribution and Habitat
Distribution in Britain & IrelandCommon on all rocky coasts of Britain and Ireland.
Global distributionFound throughout the littoral zone of the North Atlantic from the Arctic to the Algarve in the east, Iceland and the Faroes, and from Long Island north to south west Greenland in the west.
Biogeographic rangeNot researched Depth range0 - 40 m
MigratoryNon-migratory / Resident   
Distribution Additional Information
  • Nucella lapillus is widely distributed approximately between the 19 °C summer isotherm in the south and the -1 °C winter isotherm in the north (Moore, 1936), except in areas of reduced salinity such as the Baltic Sea (Crothers, 1985).
  • Dog whelks occur below of mid tidal level, approximating to 10-75% emersion (Fretter & Graham, 1994).
  • Nucella lapillus may form aggregations on the shore. In summer (May - October) aggregations of 20-500 individuals of mixed ages may form on the open rock surface of extensive shores (e.g. at Robins Hoods Bay) giving the appearance of a hunting pack (Feare, 1971; Lewis, 1964; Crothers, 1985). In winter individuals aggregate in crevices and pools, presumably to avoid dislodgement, since they have difficulty re-attaching in cold weather. Winter aggregations may form into pre-breeding and breeding aggregations in which the juveniles leave to feed but the adults remain (Feare, 1971; Crothers, 1985).

Substratum preferencesBedrock
Large to very large boulders
Small boulders
Artificial (e.g. metal/wood/concrete)
Rockpools
Under boulders
Caves
Crevices / fissures
Overhangs
Physiographic preferencesOpen coast
Strait / sound
Sealoch
Ria / Voe
Estuary
Enclosed coast / Embayment
Biological zoneMid Eulittoral
Lower Eulittoral
Sublittoral Fringe
Wave exposureExtremely Exposed
Very Exposed
Exposed
Moderately Exposed
Sheltered
Very Sheltered
Extremely Sheltered
Tidal stream strength/Water flowVery Strong (>6 kn)
Strong (3-6 kn)
Moderately Strong (1-3 kn)
Weak (<1 kn)
SalinityFull (30-40 psu)
Variable (18-40 psu)
Habitat Preferences Additional Information
Distribution References Hayward & Ryland, 1990, Hayward & Ryland, 1995b, Fish & Fish, 1996, Graham, 1988, Morgan, 1972, Berry & Crothers, 1968, Fretter & Graham, 1994, Crothers, 1985, Feare, 1970b, Moore, 1936, Moore, 1938b, Lewis, 1964, Kirby et al., 1994a, Kirby et al., 1994b, Feare, 1971, Hunt & Scheibling, 1998, Wilson et al., 1983, Hayward & Ryland, 1990,
Reproduction/Life History
Reproductive typeGonochoristic
Developmental mechanismOviparous
Reproductive SeasonAll year but max spirng and autumn Reproductive LocationAs adult
Reproductive frequencyAnnual protracted Regeneration potential No
Life span6-10 years Age at reproductive maturity1-2 years
Generation time3-5 years Fecundity
Egg/propagule size10 mm diameter Fertilization typeInternal
Larvae/Juveniles
Larval/Juvenile dispersal potential<10m Larval settlement periodNot relevant
Duration of larval stage   
Reproduction Preferences Additional InformationBreeding occurs throughout the year but is maximal in spring and autumn.
Spawning
Adult Nucella lapillus may be seen spawning or copulating in spawning aggregations. Pre-spawning and spawning aggregations develop in early spring, sometimes summer, and may comprise 30 or (many) more individuals, dominated by adults. Pre-spawning aggregations may be difficult to distinguish from winter aggregations, except that the winter aggregations consist of all age classes. Winter and spawning aggregations occur in sheltered areas of the shore (e.g. crevices or under hangs and leeward faces), which are also perfect sites for spawning. Adults do not feed during mating and spawning, and may remain in their winter aggregation sites for 4-5 months without feeding or moving significantly (Crothers, 1985).

Nucella lapillus lays its eggs in protective egg capsules on hard substrata in damp crevices and under stones. Copulation is repeated at intervals, between which a few egg capsules are laid, one at a time (Fretter & Graham, 1994). Larger females lay larger capsules, however most capsules are vase shaped, 9 -10 mm high, 3 -4 mm across and yellow to brown in colour. Capsules are cemented to the substratum by the ventral pedal gland and foot and is sealed with a 'plug' at the opposite end. (Crothers, 1985; Graham, 1988; Fretter & Graham, 1994). The gametogenesis, ovoposition and structure of egg capsules is discussed in detail by Ankel (1937), Fretter (1941), Feare (1970a), and Fretter & Graham, (1985, 1994).
Fecundity
The number of capsules laid depends on the female's food reserves, age and temperature, e.g. populations in the White Sea lay ca 20-30 capsules per season, while temperate Atlantic populations may lay 5 times this number. Although each capsule may contain ca 600 eggs, 94% of the eggs are unfertilized and function as 'nurse eggs' and are fed upon by the developing embryos (Fretter & Graham, 1994; Crothers, 1985). Capsules have been reported to release 12 -15 'crawl-away' hatchlings per capsule (Crothers, 1977), 13-36 hatchlings per capsule (Feare, 1970b) or 25-30 hatchlings per capsule (Graham, 1988). Fretter & Graham (1994) estimated that each female could produce 1030 hatchling per year. Etter (1989) noted that, in Massachusetts, adults from wave exposed shore laid about twice as many egg capsules and released about twice as many hatchlings per capsule (albeit ca 20% smaller) as adults from sheltered shores. The number and size of offspring produced was dependant on wave exposure, and formed a cline across the wave exposure gradient (Etter, 1989).

Impact of TBT on reproduction
The effects of tributyl tin (TBT), used in anti-fouling paints, on Nucella lapillus have been extensively documented and represent one of the best known examples of the effects of chemical pollution (see sensitivity). The following is based upon reviews by Hawkins et al. (1994) and Bryan & Gibbs (1991) to which the reader should refer for further detail.
  • TBT is thought to increase the levels of testosterone in the female causing the development of male sexual characteristics, termed 'imposex' (Smith, 1980).
  • With increasing TBT concentration a penis and vas deferens develop in the female, until the vas deferens occludes the genital papillae of the female, preventing release of egg capsules and effectively rendering the female sterile. The aborted capsules eventually build up until they rupture the capsule gland of the female, and kill the individual. The different stages of development are described by the vas deferens sequence (VDS) (Gibbs & Bryan, 1983). The degree of imposex may also be measured by the relative size of the female and male penises and termed the relative penis size (RPS).
  • Both RPS and VDS have been used to estimate the degree of TBT contamination to which a population has been exposed and environmental monitoring of TBT (Bryan & Gibbs, 1991; Evans et al., 1991; Moore et al., 2000)

Larval development
The equivalent of the veliger stage occurs within the capsule. Nucella lapillus larvae feed on the nurse cells in the late veliger stage, during which development is halted for about 1 week. Development is slow and temperature dependant, taking ca 4 months in temperate areas but up to seven months in the White Sea, where the eggs over-winter (Fretter & Graham, 1994). Once larvae have become miniature adults they leave the capsule via the terminal plug, although if this exit is blocked by other hatchlings they may bore through the capsule wall. Hatchlings may be termed crawl-aways (Crothers, 1985; Fretter & Graham, 1994).

Longevity and mortality
Feare (1967) suggested that a large proportion of the 69% mortality of Nucella lapillus observed on the Yorkshire coast in the winter of 1965 -66 was due to predation by oystercatchers (Haematopus ostralegus). Adults are also preyed on by gulls and eiders, which swallow the dog whelk whole. Adults dog whelks of 40mm or more long are probably safe from birds (Crothers, 1985). Juveniles are eaten by rock pipits, turnstones, and purple sand-pipers. Feare (1970b) estimated juvenile mortality to be 90% within the first year, ca. 50% in the second year and 27% in the third. Feare (1967; 1970b) reported that the purple sand-piper favoured 2-5 mm long dog whelks (occasionally 8 mm) and accounted for most of the 90% mortality in juvenile dog whelks in the winter of 1965-66 in Robin Hood's Bay. Juveniles are also susceptible to crab predation. Feare (1967) reported that most of the juvenile mortality between summer and autumn 1966-67 (Robin Hood's Bay) was due to crabs. Carcinus maenas can handle dog whelks up to 15 mm in length whereas Necora puber can handle up to 25 mm (Crothers, 1985). Crothers (1985) suggested that lobsters, which can crush any size adult, may be a significant predator below low water. Feare (1970b) estimated a life expectancy of at least 6 years, although Crothers (1985) suggested that this may be an under-estimate.

Dispersal
Nucella lapillus lacks a dispersive pelagic larval phase. They are relatively inactive as adults, moving mostly at night (males more than females) but rarely far. Several movement estimates have been reported, for example, an average of 100 mm /tidal cycle (Connell, 1961), or 123 mm/day over barnacles and 329 mm/day over a cockle bed (Morgan, 1972; Fretter & Graham, 1994). Crothers (1985) reported that marked specimens were recovered within 30 cm of their release site after one year, and suggested that with an abundant food supply there was little stimulus to move far from their site of birth. Castel & Emery (1981) reported that adults do not move more than 30 m in their life-time. Nucella lapillus recolonizing Watermouth Cove in north Devon, following the effects of TBT pollution (Crothers, 1998), have advanced at least 30 m in a minimum of 13 years (Crothers, in prep). Palmer (1984) also noted that few Nucella emarginata moved more than 10 m in a year in the USA. Similarly, Gosselin & Fu-Shiang Chia (1995) reported that dispersal was limited to a few meters from the egg capsules in Nucella emarginata (in the USA). Poor dispersal as adult and hatchling results in low rates of recruitment from or migration between adjacent populations, and may lead to relatively high levels of genetic isolation and variation within the population (population sub-division).

However, Martel & Fu-Shiang Chia (1991) collected two hatchling Nucella emarginata drifting in the intertidal, suggesting that dispersal by passive transport by currents can occur occasionally. Gosselin & Fu-Shiang Chia (1995) point out that occasional drifting by small numbers of hatchling, while rare, may still result in significant gene flow, and that since dislodgement increases with wave exposure, more gene flow (hence less population subdivision) may occur in wave exposed rather than sheltered shores.
Reproduction References Graham, 1988, Berry & Crothers, 1968, Feare, 1970a, Coombs, 1973, Moore, 1938a, Fretter & Graham, 1994, Crothers, 1985, Connell, 1961, Moore, 1938b, Etter, 1989, Palmer, 1984, Ankel, 1937, Fretter, 1941, Gosselin & Fu-Shiang Chia, 1995, Martel & Chia, 1991b, Castle & Emery, 1981, Etter, 1996, Hawkins et al., 1994, Smith, 1980, Moore et al., 2000, Feare, 1967, Bryan & Gibbs, 1991, Gibbs & Bryan, 1987, Evans et al., 1991, Evans et al., 1996b, Fretter & Graham, 1985, Crothers, 1998,
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