DISCUSSION

The sizes of the three adult specimens collected are on the same order of magnitude as those found by Ingle & Clark (Reference Ingle and Clark2006) in the southern coastal waters of the British Isles, which measured from 38.31 × 34.99 mm to 43.26 × 37.48 mm. They are close to the maximum carapace width measured in the southern Portugal populations (Flores & Paula, Reference Flores and Paula2002), which measured 43.6 mm for the biggest male and 38.8 mm for the biggest female. There is no doubt that the specimens collected at Blainville-sur-mer and Gonneville are adults: the presence of large barnacles on the dorsal surface of the carapace proves that these animals have not moulted in several months. As the female is non-ovigerous, it is not possible to state whether or not the species is able to reproduce in this northern area of its geographical distribution. For the central Portuguese coast, Flores & Paula (Reference Flores and Paula2002) suggest that the life span of the species may reach four years, probably more in males. So, the species has probably been present along the western coast of northern Cotentin for several years.

There is often disagreement about the new records of crabs along the seashore. For example, there is the case of the Pachygrapus marmoratus observed at Saint Vaast-la-Hougue on the northern Cotentin coast (Normandy). There are only two other published occurrences (Anthony, Reference Anthony1909; Fauvel, Reference Fauvel1929; Vincent & Le Granché, Reference Vincent and Le Granché1996, Reference Vincent and Le Granché2007) in this northern location, and Ingle & Clark (Reference Ingle and Clark2006) have suggested that both were probably introduced by human intervention. Poupin et al. (Reference Poupin, Davie and Cexus2005) in their revision of the genus Pachygrapsus evoke the possibility that the identification of Vincent & Le Granché may be an error, suggesting that this may have been a specimen of the non-native Hemigrapsus sanguineus. Later, Vincent & Le Granché (Reference Vincent and Le Granché2007) published a second note with a drawing of the crab, asserting that it is really a Pachygrapsus and not a Hemigrapsus and noting that the introduction of both Hemigrapsus species, H. sanguineus and H. takanoi (initially identified as H. penicillatus (de Haan, 1835) (Breton et al., Reference Breton, Faasse, Noël and Vincent2002)), is later than the observed occurrence of Pachygrapsus in March 1991 at Saint Vaast-la-Hougue.

Pachygrapsus may be able to establish temporary population during favourable warm conditions, as is suggested by d'Udekem d'Acoz (Reference Udekem d'Acoz1999) and discussed in Vincent & Le Granché (Reference Vincent and Le Granché1996).

Still, there is an unambiguous climatic change in the western part of the Channel with a clear increase of the sea surface temperature (see Genner et al., Reference Genner, Sims, Wearmouth, Southall, Southward, Henderson and Hawkins2004). The effects on faunal evolution of such increases in temperature and a long-term Russell cycle with its alternating warm and cold periods are well documented for the Plymouth area and other coastlines in Great Britain and Ireland (see Hiscock et al., Reference Hiscock, Southward, Tittley and Hawkins2004; Kendall et al., Reference Kendall, Burrows, Southward and Hawkins2004; Southward et al., Reference Southward, Langmead, Hardman-Mountford, Aiken, Boalch, Dando, Genner, Joint, Kendall, Halliday, Harris, Leaper, Mieszkowska, Pingree, Richardson, Sims, Smith, Walne and Hawkins2005). Sea surface temperatures in the western English Channel have risen 1°C since 1990, which is a larger increase than any other since temperature records began (Williams, Reference Williams2007). Warm water species are now able to extend their ranges into areas where the climate was too cold in the past; some species have extended their northern and eastern limits in the Channel more than 160 km (Williams, Reference Williams2007). Another point that must be underlined is that a lot of long-term observations are made only in locations near marine stations—for example, Plymouth, Roscoff, and Helgoland, to cite some of the more important stations along the English Channel and the North Sea. Consequently, long-term scientific observations are few and far between for large sections of the European littoral (Dauvin, Reference Dauvin2007).

Nevertheless, surveys of the areas surrounding the nuclear power plants on the French side of the English Channel, like the one at Flamanville (north-west coast of Cotentin), can provide useful information about the effects of climatic changes on benthic animals (Dreves, Reference Dreves2001). Hard bottoms are covered by several species of barnacles that have different temperature affinities. A boreo-arctic species Balanus balanoides (L., 1758) dominated a community containing the four barnacles, demonstrating a strong correlation between settlement patterns and sea temperatures with higher abundances after cold winters than warm winters (Dreves, Reference Dreves2001).

There is a crucial need for more observations. For this reason, I suggested in my 2007 editorial for Marine Pollution Bulletin that an opportunity exists today for researchers in the Humanities and the Social Sciences (e.g. sociology, history and ethnology) to conduct large surveys designed to gather information from the people living and working along littoral zones, including but not limited to fishermen. These people have many unrecorded memories that could help to improve our understanding of the primary changes in our littoral zones. Local studies by naturalist societies, often published in the grey literature, could also contribute to the body of observed changes in the seashore. In summary, a lot of climatic change affecting the composition of littoral and coastal marine fauna and flora has not been recorded by scientists and probably some interpretations of species records are erroneous. The fluctuation of the Pachygrapsus marmoratus north–south limit in the western part of the English Channel could be the result of climatic changes, with the northern locations being the result of a natural progression during the warm periods, as observed nowadays. Nevertheless, the presence of Pachygrapsus marmoratus remains rare in the Cotentin. During my prospection of non-indigenous crabs (i.e. both Hemigrapsus species, 15 sites visited around all the Cotentin) during the summer 2008 (July–August), P. marmoratus was collected only in two sites at Blainville-sur-mer (Blainville-sur-mer and Gonneville accesses to the sea).

The debate about the geographical distribution of indigenous species and the introduction of non-indigenous species along the north-eastern Atlantic coasts suggest that we need more observers—scientists and laymen, including professional and amateur fishermen and divers—to increase our knowledge of the actual distribution of species that are easily identifiable using digital photographs disseminated via websites. These records should, of course, be confirmed by specialists. For example, a surfcasting website related to the Opal coast (Dover Strait, eastern English Channel) notes the sighting of Pachygrapsus marmoratus on a dyke in Dunkirk harbour (inventory status—December 2007) (http://www.opalesurfcasting.net/). However, a very large non-indigenous population of brush-clawed shore crabs H. takanoi was recently confirmed in Dunkirk harbour (Dauvin et al., unpublished). The first specimen of this species was observed during a SCUBA diving operation in the autumn 2006 in the Dunkirk harbour basin (Y. Müller, personal communication). A large population of the Asian crab Hemigrapsus sanguineus is also present in this harbour (Dauvin et al., unpublished). I examined the photograph of the specimen sighted on the dyke and it appears that the species is in fact Hemigrapsus sanguineus, which confirms our own observations in 2008 on 16 sites all over the rocky shore of the Opal coast (Dauvin et al., unpublished), where no Pachygrapsus was found. Thus, the acknowledged northern limit appears to be the introductions reported by Ingle & Clark (Reference Ingle and Clark2006) on the western coast of the British Isles.