Observations and Possible Function of the Striking Anterior Coloration Pattern of Galathea intermedia (Crustacea: Decapoda: Anomura)

Anomuran decapod crustaceans of the genus Galathea are widespread in rocky/gravelly subtidal areas around northwestern European coasts (Zariquiey Alvarez, 1968). Recent interest in the group has been stimulated by the growing commercial importance of squat lobsters as the target of an expanding European ¢shery. However, the biology and ecology of these animals remain poorly studied (De Grave & Turner, 1997). Of the six species of Galathea known to occur in coastal waters (to 200m) around the British Isles (Howson & Picton, 1997), ¢ve have been recorded in the Clyde Sea area (Allen, 1967). Our attention was drawn to Galathea intermedia Liljeborg, the smallest of these species (carapace length 58.5mm), during in situ observations between 6 and 15m depth on two maerl grounds in the Clyde Sea (Stravanan Bay 55845.33010N 0584.26010W and Creag Gobhainn 56800.60010N 05822.20000W) using SCUBA. Galathea intermedia were found in small groups, hiding in the interstices of maerl fragments or with up to three animals sharing the shelter of dead Dosinia exoleta (L.) shells. These bivalve shells were common on the sediment surface of the study sites having fallen prey to the star¢sh Marthasterias glacialis (L.) and Asterias rubens L. (J.M.H.-S., personal observation). A previously unrecorded feature of G. intermedia is its very conspicuous anterior coloration pattern consisting of six `neon' blue spots in the frontal region of the head (a triangular epistomial patch medially above a small labral spot, a pair of lateral spots beneath the eyes at the base of the antennae and a pair of spots on the geniculate carpopodite of the second maxilliped endopodite). Observations on the incidence of this coloration were made on 86 G. intermedia, with carapace lengths from 3.4 to 7.9mm, measured from the tip of the rostrum to the posterior middorsal margin. These animals were collected by removing maerl sediment and dead Dosinia shells from a 5m2 area at 10m depth in Stravanan Bay on 25 September 1997. The squat lobsters clung to the collected substratum and few escaped. Previous UK records of G. intermedia show that it can be very common on gravelly sediments in the shallow sublittoral, with few records beyond 50m depth (Marine Biological Association, 1957; Bruce et al., 1963; Crothers, 1966; Allen, 1967). The population density of G. intermedia recorded here on coralline gravel (at least 17 indm72) is within the range of 3.0^16.6 ind per 0.25m72 encountered by Samuelsen (1970) at a similar depth (7^9m) in Norway. He also found that this species was contiguously distributed with its abundance being positively correlated with the amount of coarse substratum. When placed in aquaria, the squat lobsters hid amongst the shell/maerl substrata. Animals of various sizes settled near to one another with equanimity and began a routine of feeding and self-grooming (recorded using a Panasonic F10 videocamera (¢lm rate 25 ¢elds s71) ¢tted with a 50mm Pentax macro-lens and connected to a Panasonic AG6200 videorecorder). The bright blue iridescent markings were strikingly apparent in en face view (Figure 1A). The largest and most conspicuous markings (visible in situ to the naked eye of divers) were on the carpopodite of the second maxilliped endopodite above due to limb £exure, but anatomically below a contrasting red/brown patch on the propodite (Figure 1A,B). These features are lost when the animals are ¢xed in alcohol or formalin, which probably explains why they have been overlooked in previous descriptions of the species (e.g. Liljeborg, 1851; Bouvier, 1940; Zariquiey Alvarez, 1968). The spacing and size of the markings increased with size of the animal but there were no obvious di¡erences in their pattern or intensity depending upon the sex, maturity or egg-bearing status of individuals as this con¢guration of markings was present on all collected specimens. A few individuals moulted in captivity and the markings were seen before and after ecdysis. Filmed observations in aquaria showed that G. intermedia used its chelae to tear food when presented with scraps of crab £esh but that scavenging was not its normal feeding mode, as shown with G. squamifera Leach, G. strigosa (L.) and G. dispersa Bate (Nicol, 1933). The ability of G. intermedia to detect carrion appeared to be limited, as scraps were ignored if lying beyond two body lengths from the animal. The animals fed primarily on detritus, sweeping ¢ne particles from the substratum using the setose third maxillipeds and passing material on to the second maxilliped and thence to the mandibles. The highly re£ective spots on the second maxilliped endopodites moved constantly as the animals fed or groomed the head region, this movement ceased when animals were disturbed with forceps and when conspeci¢cs met. Given the shallow-water and contagious distribution of G. intermedia, the highly re£ective blue spots may have a function in communication. Their pattern and colour (or contrast in colour) may provide a means of intraspeci¢c recognition