Marine Biology (2006) 148: 1357–1367 DOI 10.1007/s00227-005-0159-2
R ES E AR C H A RT I C L E
P. A. Todd Æ R. A. Briers Æ R. J. Ladle Æ F. Middleton
Phenotype-environment matching in the shore crab (Carcinus maenas)
Received: 21 January 2005 / Accepted: 6 October 2005 / Published online: 10 November 2005 Springer-Verlag 2005
Abstract The shore crab (Carcinus maenas) exhibits a range of carapace pattern polymorphisms, but little is known regarding their function or maintenance. If patterns represent some form of crypsis, then associations between carapace colouration and substrate are expected; to determine whether such relationships exist, frequency of crab morphs and quantity of substrate type were measured from fifteen 10·40 m2 quadrats at each of three sites along the southern shore of the Firth of Forth, Scotland. Five thousand one hundred and thirtyseven crabs and 3.6 km of line intercept transect data were collected during a 9-week period. Crab abundance, relative frequency of morphs and substrate type varied significantly among the three sites. Plain crabs were strongly associated with macro-algal substrates whereas patterned crabs were associated with mussel beds. This pronounced phenotype-environment matching, as well as various characteristics of the carapace patterns themselves, suggests that patterned crabs are cryptic on polychromatic backgrounds. The frequency of patterned crabs and the percentage of white pigment on the carapace both declined significantly with carapace width. The loss of pattern coincides with an ontogenetic shift in habitat use and we present evidence to suggest that individual crabs lose their pigment, rather than larger patterned crabs being preferentially removed from the Communicated by J.P. Thorpe, Port Erin P. A. Todd (&) Marine Biology Laboratory, Department of Biological Sciences, National University of Singapore, 14, Science Drive 4, Blk S1, #02-05, 117543 Singapore, Singapore E-mail:
[email protected] Tel.: +65-6874-1034 Fax: +65-6779-2486 R. A. Briers Æ F. Middleton School of Life Sciences, Napier University, 10 Colinton Rd, EH10 5DT Edinburgh, UK R. J. Ladle School of Geography and Environment, University of Oxford, Mansfield Rd, OX1 3TB Oxford, UK
population by predators. Throughout their life history, shore crabs encounter high variation in predation, food supply, and physical habitat; to survive they have evolved a strategy that includes elements of pattern polymorphism, crypsis, ontogenetic shifts, and plastic responses.
Introduction The shore crab (Carcinus maenas L.) is common in the intertidal throughout Europe and has been well studied due to the ease in which it can be found, identified, sexed, and measured. Nevertheless, one particular aspect of its life history, although often referred to, has not been examined in detail. The carapace of the mature shore crab tends to be a dull green/grey/brown with little alternative colouration other than some mottling. Many crabs, especially juveniles, however, display striking colours and patterns (Crothers 1966; Hogarth 1975, 1978; Bedini 2002). It has been suggested that this phenotypic variation is related to habitat and may confer some advantage against visual predators (Hogarth 1978; Bedini 2002). Polymorphism and crypsis has been reported for many marine invertebrates (see list in Palma and Steneck 2001), especially snails, bivalves, and isopods, but few studies have examined Brachyuran crabs (but see Palma et al. 2003). A polymorphic population of crabs can have an evolutionary advantage over a monomorphic one, with possible selection pressures including visual predation (Hogarth 1978; Palma and Steneck 2001), harmful ultraviolet radiation (UVR; Ekendahl 1998; Miner et al. 2000), thermal considerations (Etter 1988; Harris and Jones 1995), and sexual selection (Endler 1978). Alternatively, polymorphism can be a result of random evolutionary processes such as mutation, genetic drift, and founder effects (Goodhart 1987; Forsman and Appelqvist 1999; Johannesson and Ekendahl 2002). Polymorphism and crypsis are often linked because if selection is
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involved, and visual predation is the pressure, then correlations between morph and substrate are expected (Endler 1978, 1984, 1988; Ekendahl 1998) although not always found (Ekendahl and Johannesson 1997; Hull and Rollinson 2000). A variety of morphs makes it more difficult for a predator to create a search image (e.g. Pietrewicz and Kamil 1979). Variation in backgrounds can also interfere with the development of a search image (Kono et al. 1998) and there is evidence indicating that the more varied the background, the less accurate the camouflage has to be (Merilaita 2003). Most animals are expected to exhibit some level of habitat preference as survival is unlikely to be constant among all possible environments (Abramsky et al. 2002). Habitat choice can be driven by numerous factors, including the requirement to find shelter, food, and mates (Bostrom and Mattila 1999). Differences in habitat structural complexity have a significant effect on young crab survival (Moksnes et al. 1998) and juvenile crabs tend to be associated with the most heterogeneous substrate available, e.g. Cancer magister and broken shells (Eggleston and Armstrong 1995), Cancer borealis and cobbles (Richards 1992), Callinectes sapidus and seagrass (Hovel and Lipcius 2002). Predation rates in dense seagrass habitats are lower than in more exposed habitats for Callinectes sapidus (Hovel and Lipcius 2002; Orth and Montfrans 2002). Juvenile shore crabs have been shown to preferentially colonise Zostera noltii patches (Sprung 2001) or mussel clumps (Thiel and Dernedde 1994) as opposed to open areas such as tidal flats, suggesting they also gain protection from these habitats. Carcinus maenas megalopae pick complex substrates to settle in and habitat selection continues as the crab proceeds through its early instars to juvenile predatory crab (Crothers 1966; Hedvall et al. 1998). Although this behaviour demonstrates an active choice (Crowe and Underwood 1998), recruitment and/or mortality processes also play a part in the non-random distribution of juvenile crabs (Moksnes et al. 1998). Being highly mobile, Carcinus maenas can move among habitats throughout its life but there will be times when stochastic or predatory factors, for example, will force it onto suboptimal substrates. In such a situation a phenotypically plastic response, where an individual organism exhibits morphological, behavioural or physiological responses to differing environments (Bradshaw 1965; Schlichting 1986), may improve fitness. Plasticity can be expressed as a continuous range of phenotypes across environments or as a few discrete morphs. There are many examples where an organism assumes one of two or more phenotypes depending upon the environment, including: sex determination (Bull 1983), dimorphic barnacle shells (Lively 1986), seasonal polyphenisms in butterflies (Shapiro 1976), shift between the solitary phase to the gregarious phase in locusts (Uvarov 1966), social insects castes (Brian 1965; Nijhout 1999), and horn dimorphism in some beetles (Moczek 1998). Most shore crabs lose their patterns at the same
time that they move from polychromatic nursery grounds to more open areas (Hogarth 1975, 1978). As there is laboratory evidence indicating pigmentation changes in Carcinus maenas can be environmentally induced (Powell 1962; Hogarth 1983), it is possible this loss of pattern is a plastic response. Previous researchers have noted that juvenile shore crabs possess carapace patterns and colours (Hogarth 1975, 1978; Hayward and Ryland 1998; Bedini 2002), and others have shown that smaller shore crabs are associated with heterogeneous substrate patches (Thiel and Dernedde 1994; Sprung 2001). Hogarth (1978) identified a positive relationship between un-patterned shore crabs and seaweed cover. Here, we explore these relationships further by categorising carapace patterns into eight groups and substrate into eight types. Hogarth (1978) and Bedini (2002) also noted that carapace patterns disappear with increasing crab age/size. We use a large dataset to test three principal hypotheses: (1) that shore crab carapace morphs vary among sites, (2) that this variation is related to substrate, and (3) that carapace patterns are lost with age/size.
Materials and methods Study species The shore crab (sometimes referred to as the green crab) is extremely common in Northern Europe where, tolerating a large range of environments (Abello et al. 1997), it thrives throughout the littoral zone (Hayward and Ryland 1998). Carcinus maenas have the typical triphasic life cycle of many decapod crustaceans, that is: larvae to nursery grounds (via settling megalopae), nursery grounds to foraging home ranges, home ranges to spawning migrations (Crothers 1966, 1968; Pittman and McAlpine 2001). Adults are found in two principal colour morphs, green and red, which are thought to represent different life strategies (Reid et al. 1997; McGaw et al. 1992; McKnight et al. 2000); for example, green crabs are tolerant to a wide range of environmental conditions whereas red crabs are more commonly found in the lower intertidal. Males reach sexual maturity at a carapace width of approximately 25– 30 mm, females at 15–31 mm (Crothers 1966); large males fare better in reproduction (Reid et al. 1994). Shore crabs reach terminal anecdysis at 3 to 4 years old, after going through 18 moults (Crothers 1966). Study sites The three study sites are situated to the east of Edinburgh along the southern shore of the Firth of Forth, Scotland (Fig. 1). This large estuary flows into the North Sea with maximum tidal ranges during the sampling period in 2002 varying from approximately 4 m in August to 6 m in October. The sites were chosen based
1359 Fig. 1 The three study sites: Ferny Ness, Milsey Bay, and Long Craigs
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on their different substrate composition, and also because crabs recruited to them probably came from the same gene pool (Brian 2002; Roman and Palumbi 2004; Brian et al. 2005). The high degree of water movement among the three sites is reflected by the large tidal ranges (Greenwood and Hill 2003). All sites are under the protection of environmental legislation. Ferny Ness (5559.215¢N, 00254.091¢W) Ferny Ness is an exposed spit of low-lying rocky shore separating two large beaches: Seton Sands to the south and Gosford Sands to the north. The sample area is southwest facing and characterised by larges tracts of mussel beds and barnacle-encrusted rock. Rock pools and patches of seaweed are also common. Milsey Bay (5603.674¢N, 00243.016¢W) Milsey Bay is a north-facing bay overlooked by the town of North Berwick; most of the bay consists of sandstone protuberances interspersed by sandy beach. The sample area is at the westernmost point where there exists a long disused slipway, filled in with cobbles and boulders. Long Craigs (5600.317¢N, 00232.518¢W) Long Craigs is at the eastern end of Belhaven Bay, a large northeast facing stretch of sandy beach. The shore includes barnacle-covered rock, patches of mussel bed, loose rock slabs, and rock pools. The sample area faces northwest and is relatively low lying. Sampling strategy Due to their different life-history characteristics, the few red crabs (sensu Reid et al. 1997) encountered were excluded from the study. At each site a sampling area of 50·120 m2 perpendicular to shore was identified. At Milsey Bay and Long Craigs, this area corresponded approximately to high and low autumn spring tide
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marks but, at Ferny Ness, the top of the sampling area did not quite reach the high tide mark. The sampling area was positioned to encompass substrate components representative of the site in general and landmarks such as prominent boulders were integrated into this process so that the outlines could be reproduced on each visit. This large area was then subdivided into 15 smaller (10·40 m2) quadrats, arranged so that there were five adjacent replicate quadrats at each of three shore positions: high, middle, and low. Only one site was visited on any individual sampling day, but all three sites were visited within a 4-day period. The next period occurred 2 weeks later, and so on, until a total of five sessions were completed (from August 7 to October 10, 2002). Before sampling, quadrats were marked out with coloured nylon rope. One quadrat from each shore position was chosen randomly (using random number tables) before being thoroughly searched by one person for 45 min. Different individuals conducted the search (five in total) on different occasions, but all participants had similar levels of crab-collecting experience and P.A.T. was present at all times. During the search, rocks were turned over, seaweed disturbed, and crevices explored. Collected crabs were placed in sample pots containing fresh seawater before being transported back to the laboratory. Sampling using suction techniques (Palma and Steneck 2001; Palma et al. 2003) was not practicable, or desirable, at the scale of 100 s of m2. Removing cores and sieving the results (Moksnes 2002) from individual habitats was considered too destructive given the protected status of the sites. In the laboratory the maximum carapace width of all crabs was measured. They were also sexed (Shen 1935), digitally photographed, and categorised by their carapace pigmentation into eight different groups (Table 1, Fig. 2). The photographs were used to calculate the percent cover of pigment on 150 haphazardly selected carapaces. To limit the number of empty cells in the multivariate analyses, some pooling of intermediate forms was conducted arbitrarily via visual analysis by P.A.T. The most common intermediates can best be described as a mix of ‘spotted’ or ‘tan and grey’ and the
1360 Table 1 Pattern categories and their definitions Pattern category
Definition (description of carapace)
Plain Spotted Tan or grey 1-Spot 2-Spot 3-Spot Red and/or white Other marks
Dull green-grey-brown, i.e. typical adult colouration Small white spots not repeated among many crabs Complete cover of light to dark browns and greys One spot on or near the rostrum One spot at each of the hepatic regions One spot on the rostrum and at each of the hepatic regions Completely bright red or white, or a combination of red and white markings All other white markings
1-, 2-, and 3-spot categories. On all such occasions precedence was given to the 1-, 2-, and 3-spot designation. The various morphs that comprise ‘tan and grey’ and ‘red and white’ are explicit in the category definitions (Table 1). ‘Other’ includes all remaining, generally rare, morphs; these were often high-contrast with large areas of white pigment. Environmental measures To quantify the substrate composition in each quadrat, line intercept transect (LIT) methodology was applied (English et al. 1997), i.e. every substrate type Fig. 2 Examples of six morphs: a 1-spot, b 2-spot, c 3-spot, d spotted, e red and/or white, and f other marks
that was encountered under a calibrated tape was measured to the nearest 1 cm. To reduce the likelihood of clustered transects, each 10·40 m2 quadrat was first subdivided into four 10·10 m2 sections. Using random number tables a measuring tape was placed twice in each section; thus 8·10 m2 of LIT data were collected from every quadrat. As with the carapace categories, some pooling of groups was needed to reduce the number of empty cells in subsequent analyses. For example, initial groups such as ‘cobbles with barnacles’, and ‘rock, barnacles and limpets’, were subsumed into ‘rocks’, and ‘rock with barnacles’, respectively. Eight substrate categories were used in the final analyses.
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a 1136
= Ferny Ness = Milsey Bay = Long Craigs
858
Morph abundance
728 300 250 200 150 100 50 0
Morph frequency (pattern No/ plain No)
b 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0
Mean carapace breadth (mm)
c 22 20
Statistical analysis For all analyses, normality, homoscedascity, and homogeneity of variance were checked with diagnostic graphical displays and Kolmogorov–Smirnov, Levene’s, and Box’s M tests (Johnson and Field 1993; Grimm and Yarnold 1995). Assumptions were met after all crab data were log transformed and substrate data were both arcsine and log transformed. Size differences between sexes and patterned/unpatterned crabs were compared with T-tests on quadratlevel means. The relationships between the percentage of crabs with plain carapaces and carapace width, and the percent cover of white pigment and carapace width, were tested for with Spearman-rank correlations. Crab size among sites was tested with ANOVA and Chi-squared tests were used to examine sex bias, pattern frequencies among sites and ontogenetic shifts in pattern type. Variation in the number of crab morphs was examined using a three-factor multivariate analysis of covariance (MANCOVA) on categorised data (Table 1). As the presence/absence of carapace pattern is known to be related to crab size (Hogarth 1978) carapace width was included as a covariate. The three (fixed) factors were site, shore position, and sex. Random replicates were provided by the five quadrats at each shore position. A two-factor MANOVA was used to compare substrates among sites and shore positions (fixed factors), using the substrate categories as input variables. Redundancy analysis was used to examine the relationship between the frequency of different carapace morphs and the relative abundance of substrate types in the quadrats. Carapace width was entered into the analysis as a covariate to account for changing numbers of patterned crabs with size. Significance of the axes generated by the analysis was assessed using MonteCarlo simulation, as implemented in CANOCO version 4 (Ter Braak and Smilauer 1998).
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Results
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Crab morph Fig. 3 a The relative abundance among patterned morphs remains almost constant among sites. b The ratio of pattern:plain morph at each site. The order for every morph, in decreasing frequency, is Ferny Ness, Long Craigs, and Milsey Bay. c The crabs at Long Craigs are consistently larger than the crabs at the other two sites
In total 5,137 crabs were collected of which 2,932 (57.1%) were plain and 2,205 (42.9%) were patterned. Among sampling periods, the number of crabs collected ranged from 779 to 1,600 crabs. The mean male:female ratio was 1:1.17, but this is not unusually biased for Carcinus maenas populations (Hogarth 1983). T-test results (df=44, T= 5.60, P
