DISCUSSION

Panulirus regius is not considered as an alien species in the Mediterranean Sea (Holthuis, Reference Holthuis, Fischer, Schneider and Bauchot1987; Galil et al., Reference Galil, Froglia and Noël2002). The species is known from the Mediterranean Sea since the 19th Century. Risso (Reference Risso1816) was the first author to report it from Nice (France) as Palinurus fasciatus (nec Palinurus fasciatus Fabricius, 1798 = Panulirus polyphagus (Herbst, 1793)). Desmarest (Reference Desmarest1825) recognized Risso's misidentification and described it as a new species, named after Risso: Palinurus Rissonii. But later Risso regarded it as a simple variety of P. vulgaris (see Holthuis, Reference Holthuis1977, p. 487), followed in that by subsequent authors (Carus, Reference Carus1885).

De Brito Capello (Reference De Brito Capello1864) described a new species of spiny lobster from the Cabo Verde Islands, naming it Panulirus regius. Holthuis (Reference Holthuis1946) pointed out the identity of Panulirus regius de Brito Capello, 1864 with Palinurus Rissonii Desmarest, Reference Desmarest1825. Opinion 507 of the International Commission on Zoological Nomenclature in 1958, by suppressing the older name Palinurus Rissonii, made Panulirus regius the valid name for this species.

In the first years of the 20th Century French and Spanish fishermen started to exploit the large stock of P. regius present in the coastal waters of Western Sahara and Mauritania. Vessels, equipped with running water systems in special holds to store the catch, were able to carry thousands of living spiny lobsters back to their home ports (Gruvel, Reference Gruvel1911).

Darboux & Stephan (Reference Darboux and Stephan1907) reported a new capture (two specimens) of the royal spiny lobster in the Mediterranean Sea—Gulf of Marseille—in 1907. They also registered assertions of old local fishers to have occasionally caught that species in the past, but suggested the two specimens had been introduced by fishing vessels, based in Marseille, that operated off West Africa in previous years. This hypothesis of escape of specimens from holding tanks was supported by Bouvier (Reference Bouvier1917) and again suggested by Zariquiey Alvarez (Reference Zariquiey Alvarez1968) to explain the occasional capture of P. regius along the Spanish Mediterranean coast mentioned by Zariquiey Cenarro (Reference Zariquiey Cenarro1935).

The above hypothesis may hold true for the records of the first half of the 20th Century, when French and Iberian fishermen heavily exploited the spiny lobster stocks off West Africa. But the French fishery for the royal spiny lobster ended by the 1970s (Maigret, Reference Maigret1978; Pencalet-Kerivel, Reference Pencalet-Kerivel2008).

Nowadays only few Portuguese vessels are allowed to exploit the royal spiny lobster resource off Mauritania, within a fishery agreement between the European Union and the Islamic Republic of Mauritania (European Community, 2008). They land live spiny lobsters at their home ports (Luis & Calado, Reference Luis and Calado2009). Live spiny lobsters, from the West African artisanal fishery, are currently exported to European countries by airfreight. But, according to the Italian Coast Guard deputed to do fishery control in Italy, no import of live or fresh fish was registered in the airports of Tuscany in the past year (F. Paolillo, personal communication).

Moreover all the specimens herein reported are below the minimum landing size (21 cm from the tip of the rostrum to the end of the tail) fixed by the Protocol attached to the above mentioned EC Regulation and by Mauritanian national laws.

Therefore the hypothesis of the import of live animals and subsequent escape from a holding tank facility does not hold in the case of the present specimens, nor in the case of the specimens caught off Nice in the first years of the 19th Century, well before the start of the French fishery off West Africa, as already pointed out by Holthuis (Reference Holthuis, Fischer, Schneider and Bauchot1987).

Also an unintentional introduction of larvae with ballast waters, if it cannot be ruled out for the present records, does not apply to the material reported by Risso (Reference Risso1816) and Desmarest (Reference Desmarest1825), because at that time ships used only solid ballasts.

Lobsters of the genus Panulirus have a very long planktonic larval phase: over 300 days (Matsuda et al., Reference Matsuda, Takenouchi and Goldstein2006; Goldstein et al., Reference Goldstein, Matsuda, Takenouchi and Butler2008). Crosnier (Reference Crosnier1971) described 14 larval stages, inclusive of the ‘puerulus' stage, for P. regius. Palinurid larvae, phyllosoma, can be transported over long distances by oceanic currents (Rudorff et al., Reference Rudorff, Lorenzzetti, Gherardi and Lins-Oliveira2009). It is worth remembering that the western Atlantic Panulirus argus (Latreille, 1804) has been repeatedly collected in the Cabo Verde Archipelago (Freitas & Castro, Reference Freitas and Castro2006).

Therefore, even if the main current off Mauritania flows south-westward, we hypothesize that, under exceptional circumstances, coastal counter currents may drift larvae northward to the Strait of Gibraltar and these larvae may be trapped in the surface Atlantic water flowing into the Mediterranean Sea. The Atlantic water flows eastward in the Mediterranean Sea along the African continental slope, subject to intense mesoscale activity in the Algerian basin, then splits in several arms: one flows through the Strait of Sicily into the eastern Mediterranean whereas two others flow along the eastern Tyrrhenian coast and along the west coast of Corsica. These two arms enter the Ligurian Sea and continue to flow anti-clockwise (Millot & Taupier-Letage, Reference Millot, Taupier-Letage and Saliot2005; Cattaneo Vietti et al., Reference Cattaneo Vietti, Albertelli, Aliani, Bava, Bavestrello, Benedetti Cecchi, Bianchi, Bozzo, Capello, Castellano, Cerrano, Chiantore, Corradi, Cocito, Cutroneo, Diviacco, Fabiano, Faimali, Ferrari, Gasparini, Locritani, Mangialajo, Marin, Moreno, Morri, Orsi Relini, Pane, Paoli, Petrillo, Povero, Pronzato, Relini, Santangelo, Tucci, Tunesi, Vacchi, Vassallo, Vezzulli and Wurtz2010).

It is worthy of note that one of the Lagrangian drifters, equipped with ARGOS transmitter, launched in the Oran (Algeria) basin in October 1996, to investigate the flow of the Atlantic surface waters in the western Mediterranean Sea, was tracked by satellite until it reached the shores of Livorno in only 123 days (Font et al., Reference Font, Millot, Salas, Julià and Chic1998).

Thus a drift of larvae within the Atlantic water mass would explain the present captures of P. regius in the south-eastern part of the Ligurian Sea as well as the captures made in the north-western part (Nice) almost 200 years ago (Figure 3).

Fig. 3. Western Mediterranean: the main flow of surface Atlantic Water (AW) (adapted from Millot & Taupier Letage, 2005) and the records of Panulirus regius: •, present record, o, previous literature records.

The Mauritanian population of P. regius thrives in a coastal habitat strongly influenced by the upwelling of cold water masses, with annual temperatures ranging between 14° and 21°C (Maigret, Reference Maigret1978) comparable to those observed along the northern coast of Tuscany. Thus the species could get established in the western Mediterranean. Why it has not yet succeeded is open to further speculation.