Its preferred salinity range of 27-41%o (Thomas .... g. 10. 11 12. Time (Houre). High acclimated (50ppt,). C) .° t 60. |. "E 4O m ao. J:l ..... Waterman, T. H., 1941.
J. Exp. Mar. Biol. Ecol., 155 (1992) 145-159 © 1992 Elsevier Science Publishers BV. All rights reserved. 0022-0981/92/$05.00
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Salinity preference of the shore crab Carcinus maenas in relation to coloration during intermoult and to prior acclimation l.J. M c G a w and E. Naylor School of Ocean Sciences, Marine Science Laboratories, Menai Bridge, Gwynedd, UK (Received 5 July 1991; revision received 23 September 1991; accepted 6 October 1991)
Abstract: lntermoult Carcinus maenas (Crustacea: Decapoda) exhibit a range of carapace colours from green through orange to red, reflecting increasing lengths of intermoult duration, with red crabs less tolerant than green crabs of low salinity. The salinity preference behaviour of red and green colour forms was investigated. Red crabs vacated low salinities earlier than green crabs and green crabs had a lower tolerance range. Acclimation to either low or high salinities affected red and green crabs in a similar manner; the lower the acclimation salinity the earlier the time of exit from the low salinity and vice versa; the higher the acclimation salinity, the longer the crabs remained in the low salinity. Carcinus of both colour forms were shown to be able to discriminate between salinities separated by a difference as little as 0.5%o. Key words: Acclimation; Carc#ms maenas; Color; Salinity
INTRODUCTION
The physiological responses of crustaceans and other organisms to changing salinities are well documented, whilst studies on behaviour in response to this environmental variable are rather more fragmentary (Davenport, 1985). However, whereas physiological changes associated with alterations of an animal's internal condition often take some time to complete (Lockwood, 1976), rapid behavioural osmotic responses have advantages which enable the animal to avoid or delay exposure to adverse conditions. Apart from preference for a medium within an organism's range of tolerance, behavioural selection of a medium may be influenced by the salinity to which the individual had previously been acclimated (Lockwood, 1976). The coconut crab Birgus latro was shown to control the osmotic pressure of its body fluids by behavioural selection of various seawater concentrations (Gross, 1955). Pachygrapsus crassipes chose salinities of < 34%0, after acclimation to 51%o, but salinity choice was unaltered by acclimation to 17%0 (Gross, 1957). The hermit crab Pagurus bernhardus exhibited isolation behaviour, by retreating into its shell, at a lower salinity after acclimation to dilute seawater (Davenport et al., 1980). Prior acclimation to either hypersaline or hyposaline media, Correspondence address: l.J. McGaw, School of Ocean Sciences, Marine Science Laboratories, Menai Bridge, Gwynedd LL59 5EY, UK.
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iaowever, had no effect on the salinity choice behaviour of Corophium volutator (McLusky, 1970)or Marinogammarus marinus (Bettison & Davenport, 1976). Carcinus maenas exhibits increased locomotor activity on exposure to hyposaline or hypersaline media, defined as halokinesis, by which it may avoid adverse salinity conditions (Taylor & Naylor, 1977; Thomas etal., 1981; Bolt & Naylor, 1985; Ameyaw-Akumfi & Naylor, 1987). Its preferred salinity range of 27-41%o (Thomas e~ ah, 1981), or 17-40%o (Ameyaw- Akumfi & Naylor, 1987), as determined by behavioural studies, is well within the limits of its physiological tolerance, as Carcinus is known to survive indefinitely in salinities as low as 5%0 (Broekhuysen, 1936). Subsequently, Reid et al. (1989) have reported that the osmoregulatory capacity and survival of Carcinus maenas in low salinities vary according to the colour of an individual. This can vary from pale green through orange to a deep red-brown colour, most apparent on the ventral side and limbs. The colour variation is associated with intermoult length with red crabs probably being in prolonged intermoult when compared with green crabs (Kaiser et al., 1990; McGaw et al., in press) and with the two colour forms having different ecologica! distributions (Crothers, 1968; Reid et al., 1989). The aim of the present study was to examine any differences occurring in salinity preference behaviour between the red and green colour forms of Carcinus, and to investigate the effects of salinity acclimation on such behaviour. MATERIALS AND METHODS
Crabs were collected intertidally and subtidally in the Menai Straits, North Wales (Grid Reference 722560, OS map 115), using baited pots. They were maintained in tiltered running seawater (34/00) ot at 12 + 2 °C on a normal light-dark regime for at least 1 wk prior to experimentation. All crabs used in experiments were intermoult males of 40-70 mm carapace width, and red and green crabs were tested separately. Salinities below full-strength seawater were prepared by diluting seawater with freshwater and checked using a Salinity Temperature Bridge Type M.0.5 salinometer. Salinities above ambient seawater were prepared by adding seasalts (Sigma Chemical) to seawater, and confirmed using a field salinometer refractometer. All experiments were carried out at temperatures of 12-14 °C. Two types of choice chamber were used in salinity choice experiments: (1) A multiple-choice chamber as used by Ameyaw-Akumfi & Naylor (1987) was constructed of waterproofed marine plywood 160 x 40 x 40 cm high. Four V-shaped chambers were constructed along its length using concrete. These were painted and sealed to prevent mixing between chambers. Each chamber sloped inwards to a maximum depth of 20 cm and was separately aerated. Salinities were found not to vary by > 0.5%0 during the experimental period. (2) A two-choice chamber was constructed within a fibreglass tank of dimensions 132 x 72 x 34 cm high. A divider was fibreglassed into the centre of the tank to prevent
SALINITY PREFERENCE OF CARCINUS
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mixing of water between the two sides. Two boards of painted marine plywood sloped up at ~ 30 ° to the centre of the tank and were held in place with concrete wedges. Perspex boards were placed over the tank to prevent crabs escaping. In the multi-choice tank, a set oflow salinities (5, 11, 17 and 22%0) and a set of higher salinities (22, 28, 34 and 40%0) were used in separate experiments. Batches of five crabs were introduced into each chamber. The number of crabs per chamber was recorded at 0.5-h intervals for the first 4 h, since crabs tend to make choices for a particular salinity in < 2 - 3 h (Thomas et al., 1981). Thereafter, the number of crabs was recorded at 1-h intervals for 6 h and subsequently at 2-h intervals up to a total time of 12 h. The crabs had been previously acclimated to salinities of less than (17%o), greater than (50%o) or equal to that of seawater (34%°) for 2-4 days before being introduced to the choice chambers. This acclimation time was chosen because it was found that Carcinus in the sample area is unlikely to encounter prolonged periods of exposure to lowered salinity for longer than a few hours even when resident in an estuary (McGaw & Naylor, in press). In addition, this selected acclimation period allows sufficient time for the haemolymph osmolality, heart-rate and cuticle permeability to reach new stable values (Theede, 1969; Spaargaren, 1975a,b, Hume & Berihid, 1976), and avoids a requirement to consider the effects of changes in enzyme levels such as ATPases which take 2-3 wk to equilibrate (Siebers et al., 1983; Winkler, 1986). The sums of four replicates, each with 20 crabs, for each colour and acclimation were used in the presentation of results and statistical analyses. Full-strength seawater (34%o) in each chamber was used as a control to ensure there was no bias for a certain chamber. Experiments were carried out in constant light provided by a fluorescent strip suspended 2 m above the tank. The two-choice chamber was used to investigate the effect of" acclimation on salinity preference. One chamber was filled with a salinity of 5%o and the other with full-strength seawater (34%0). Crabs were acclimated to low (17%o), normal (34%o)or high (50%o), salinities for 2-4 days prior to experimentation. Groups of 50 crabs (25 of each colour) were introduced into the lower salinity chamber and the time when the crabs vacated the low salinity was recorded. The experiment was run for 7 h and only crabs making a choice within this period were used in the analysis. The results represent pooled data of four repetitions at each acclimation salinity. Experiments were performed in constant light provided by a fluorescent strip suspended ~ 3 m above the tank. Activity detection switches as described in McGaw & Naylor (in press) were used to record the amount of locomotor activity over 48 h of eight arhythmic red and eight arhythmic green crabs in low (17%o), normal (34%0) and high (50%0) acclimation salinities after prior acclimation in each salinity for 2 days. To determine if Carcinus exhibits orientated behaviour in the form of a taxis along a salinity gradient, crabs were tested in the multiple-choice chamber tank. Each chamber was filled with a different salinity arranged in increasing strength and separated by a given increment. Five crabs were introduced into each chamber in each experiment, and approximately equal numbers of red and green crabs were used throughout. The
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numbers of crabs per chamber were recorded over 12 h, on the same time basis as described for the multiple-choice experiments. Three repetitions were performed at each set of salinities tested. Salinities separated by a difference of ~
