Dog Whelk
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| Dog Whelk Conservation status: Lower risk | ||||||||||||||||||||||||
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| Nucella lapillus Linnaeus, 1758 |
The Dog Whelk or Dogwhelk, Nucella lapillus, is a carnivorous rocky shore mollusc found around the coasts of Europe, between the 0°C and 20°C isotherms. It feeds on a variety of animals, including barnacles, limpets, mussels, tubeworms, and other dog whelks.
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Feeding
Its adaptations include a modified radula (a toothed chitinous structure) to bore holes in the shells of prey, complemented by an organ on the foot which secretes a shell-softening chemical. When a hole has been formed paralysing chemicals and digestive enzymes are secreted inside the shell to break the soft body down into a ‘soup’ which can be sucked out with the proboscis. The plates of barnacles can be pushed apart with the proboscis, and the entire individual is eaten in about a day, although larger animals such as mussels may take up to a week to digest. Feeding only occurs when conditions are conducive to such an activity, and during these times the dog whelk consumes large quantities of food so that the gut is always kept as full as possible. This allows shelter until more food is required, when foraging resumes. If waves are large or there is an excessive risk of water loss the dog whelk will remain inactive in sheltered locations for long periods.
Predators
Predators of the dog whelk include various species of crabs and birds. Protection against predation from crabs which attempt to pull the soft body out through the shell aperture can be afforded by growing teeth around the edge of the aperture. Many predators cannot smash the strong shell of an adult dog whelk, but juveniles are vulnerable to attack from many predatory species. Eider ducks and various other birds simply swallow the entire body with its shell, while oystercatchers and various crustaceans are often capable of crushing or breaking the shells. In the winter they endure more predation from purple sandpipers and similar wading birds, but in the summer crabs represent a greater threat. In general, the dog whelk can be thought of as being vulnerable to birds when emersed, and to crabs when immersed.
Reproduction
Dog whelks reproduce by aggregating for the mating season in the spring. Eggs are laid in yellow capsules in suitable crevices on the shore in April and May, and the young feed on unfertilised eggs before emerging as almost fully-formed but very small replicas of adults. Juveniles are susceptible to drying out when emersed, and a large proportion are killed in this way. Full maturity is attained after three years.
Habitat
The dog whelk can be identified by a pale grey colouration and a groove on the underside of the shell. Shell shape varies widely according to the exposure to wave action of the shore on which a particular population lives. Wave action tends to confine the dog whelk to more sheltered shores, but can be counteracted, both by adaptations to tolerate it such as the shell and muscular foot, and by the avoidance of direct exposure to wave action afforded by making use of sheltered microhabitats in rocky crevices. The preferred substrate material of the dog whelk is solid rock and not sand, which adds to its problems at lower levels on the shore where weathering is likely to have reduced the stability of the seabed. Water loss by evaporation has to be tolerated (by means of the operculum which holds water in and prevents its escape as vapour), or avoided (by moving into water or a shaded area).
The peak in dog whelk population density is approximately coincidental with the mid tide level. In general it can be said that at high vertical heights on the shoreline the dog whelk is most threatened by biotic factors such as predation from birds and interspecific competition for food, but abiotic factors are the primary concern, creating a harsh environment in which it is difficult to survive. At low vertical heights it is biotic factors, such as predation from crabs and intraspecific competition, which cause problems. The upper limit of the range in which the dog whelk is generally found is approximately coincidental with the mean high water neaps line, and the lower limit of the range is approximately coincidental with the mean low water neaps line, so that the vast majority of dog whelks are found on the middle shore.
Tidal pools and comparable microhabitats extend the vertical range of organisms such as the dog whelk as they provide a more constant environment, but they are prone to increased salinity because evaporation concentrates dissolved substances. This can create toxic conditions for many species.
The dog whelk can only survive out of water for a limited period, as it will gradually become desiccated and die if emersed for too long. Metabolic processes within cells take place in solution, and a decrease in water content makes it impossible for the organism to function properly. In experiments it has been shown that 50 per cent of dog whelks die at 40.0ºC, and it can be assumed that at temperatures lower than this a smaller proportion will be killed off. Furthermore, the dog whelk has to excrete ammonia directly into water, as it does not have the adaptation possessed by many Upper Shore species which would allow it to produce uric acid for excretion without loss of water. When kept emersed for seven days at a temperature of 18ºC, 100% of dog whelks die, in contrast to many periwinkle species which can lose even more water than the dog whelk (i.e. more than 37 per cent of their total body mass) but survive as a result of their ability to excrete toxic waste products more efficiently.nl:Purperslak