Journal of Experimental Marine Biology and Ecology 255 (2000) 215±227 www.elsevier.nl/locate/jembe Upper thermal tolerances of the beach¯ea Orchestia gammarellus (Pallas) (Crustacea: Amphipoda: Talitridae) associated with hot springs in Iceland David Morritta,* , Agnar Ingolfsson b aSchool of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK bInstitute of Biology, University of Iceland, Grensavegur 12, IS-108 Reykjavik, Iceland Received 5 June 2000; received in revised form 12 August 2000; accepted 18 September 2000 Abstract The upper thermal tolerance (CTmax ) of beach¯eas Orchestia gammarellus (Pallas) collected from a number of different locations in Iceland was determined. Differences were recorded between ®eld populations associated with thermal springs and those from non-thermal sites. A number of reciprocal acclimation experiments (where animals from thermal and non-thermal sites were acclimated to the measured ambient temperatures of thermal (17 and 228C) and non-thermal (118C) sites) were performed. Differences between at least one thermal population and a non-thermal population were maintained following this reciprocal acclimation, supporting the hypothesis that population differences were due to non-reversible genetic differences and not local acclimatisation. Animals from one thermal site (Reykjanes) had a mean CTmax 5 37.160.58C when acclimated at 118C and 38.660.38C when acclimated at 228C, whereas animals from a non-thermal site (Hvassahraun) had CTmax values of 35.960.5 and 37.960.38C, respectively. In other cases, differences are best explained by local acclimatisation. Results are discussed in relation to ambient local conditions and the degree of isolation of the different populations. 2000 Elsevier Science B.V. All rights reserved. Keywords: Amphipoda; Crustacea; Iceland; Talitridae; Thermal springs; Upper thermal tolerance 1. Introduction The common beach¯ea Orchestia gammarellus (Crustacea: Amphipoda: Talitridae) (Pallas) is the dominant detritivore in strandline algae on a wide range of rocky, boulder and shingle shores and saltmarshes in NW Europe. Talitrid amphipods play an important *Corresponding author. Tel.: 144-1784-443971; fax: 144-1784-470756. E-mail address: [email protected] (D. Morritt). 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00299-9 216 D. Morritt, A. Ingolfsson / J. Exp. Mar. Biol. Ecol. 255 (2000) 215 ±227 role in breakdown of organic debris and subsequent recycling of nutrients in these intertidal habitats, as well as being important food items for a range of invertebrate and vertebrate predators (Wildish, 1988, for review). The ecophysiology of talitrid am- phipods, especially O. gammarellus, has been extensively studied (Spicer et al., 1987; Morritt and Spicer, 1998, for reviews), especially aspects of respiratory physiology, osmo- and ionic regulation and developmental ecophysiology. However, we know relatively little about the thermal tolerances of talitrid amphipods, including O. gammarellus, with the exception of a recent study by Gaston and Spicer (1998). The presence in Iceland of populations of O. gammarellus associated with both thermal springs and non-spring sites (Ingolfsson, 1996) offers a unique opportunity to test ideas on fundamental thermal tolerances of populations experiencing different thermal regimes. Numerous studies have addressed the upper thermal tolerances of closely related groups of species and shown that species from warmer habitats have higher upper thermal tolerance (CTmax ) than those from colder habitats. This may be related to latitude (e.g., Moulton et al., 1993, in North American caddis¯ies), altitude (e.g., Gaston and Chown, 1999, for African dung beetles), microhabitats within a narrow geographical range (e.g.,Van der Merwe et al., 1997, for sub-Antarctic weevils), zonation on the shore (e.g., Davenport and MacAlister, 1996, for South Georgian intertidal invertebrates) or even depth distribution in the deep ocean (e.g., Young et al., 1998, for bathyal echinoid larvae). Perhaps more interesting is evidence that different populations within a particular species may have different thermal tolerances depending on their geographic origin, e.g. introduced North American and European populations of the zebra mussel, Dreissena polymorpha (McMahon, 1996). Indeed, Gaston and Spicer (1998) have recently demonstrated differences in the upper thermal tolerances of two latitudinally separated populations of the beach¯ea Orchestia gammarellus within the UK. Further- more, these differences were maintained following acclimation to a range of en- vironmentally realistic temperatures. It is well known that the CTmax of many arthropod species can be increased by experimental acclimation to elevated temperatures (e.g., Edney, 1964, for isopod species), although this is not always the case (e.g., Quinn et al., 1994). Furthermore, this experimental acclimation can occur relatively quickly, some- times within 24 h (e.g., Colhoun, 1960). A similar process can also occur in nature and here the process is correctly referred to acclimatisation to the prevailing environmental conditions. The main aim of the present paper was to determine the upper thermal tolerances (CTmax ) for a number of populations of O. gammarellus from Iceland and, by acclimation at different temperatures, attempt to reverse any differences in tolerance in order to determine whether differences are due to irreversible genetic adaptation or reversible, phenotypic acclimatisation. Experiments were designed with close reference to a similar study (Gaston and Spicer, 1998) on two latitudinally separated UK populations of O. gammarellus in order to allow comparison with those data. 2. Materials and methods 2.1. Upper thermal tolerance in Icelandic populations of Orchestia gammarellus Animals were collected by hand from a number of locations around Iceland from D. Morritt, A. Ingolfsson / J. Exp. Mar. Biol. Ecol. 255 (2000) 215 ±227 217 where Orchestia gammarellus had previously been recorded by Ingolfsson (1996). At each site, animals were collected with substratum from the natural habitat along with decaying macroalgal debris to provide food. Animals were transported to the Sandgerdi Marine Centre in a sealed plastic container on their natural substratum and maintained in a cooled laboratory (158C) until required. Animals were fed on fucoid algae and chopped carrot ad libitum. During collection of animals, temperature was recorded to the nearest 0.18C with a digital temperature probe (Ama-Digit Precision, Amarell Electronic, Kreuzurstheim, Germany) in the microhabitats from where animals were collected as well as recording sea temperature and thermal spring temperature (where appropriate). Two sites were sampled on the Reykjanes peninsula in southwestern Iceland: a non-thermal site on Puccinellia maritima saltmarsh at Hvassahraun (648019110N, 228099190W) and a saltmarsh site with thermal in¯uence at ReykjanestjornÈ (638479520N, 228439120W). In retrospect it may have been inaccurate to describe the ReykjanestjornÈ population as being exposed to thermal in¯uence: this is discussed later in more detail. Both these sites in southwestern Iceland are situated on Holocene basic and intermediate lava bedrock between 1100 and 11,000 years old. A further three thermal spring sites were sampled in northwestern Iceland at Hveravik, SteingrimsfjordurÈ (658419470N, 218339550W), Bjarnarstadir, IsafjordurÈ (658499160N, 228299190W) and Reykjanes, ReykjafjordurÈ (658559250N, 228259530W): no non-thermal sites were sampled in this area as Orchestia gammarellus are only found at sites associated with thermal springs in this part of Iceland. The northwestern sites are situated on upper and middle Miocene bedrock (10±15 million years old). Other characteristics of collecting sites and collection dates are given in Table 1. Within 24±48 h of collection the wet mass of 150 randomly selected animals was measured 60.1 mg using a microbalance (Scaltec SBC22, Heiligenstadt, Germany) for each population: ovigerous females were debrooded prior to weighing. Debrooding can be performed routinely without damage to the female; indeed, broods can be returned to the marsupium without damage to either female or brood (see Morritt and Spicer, 1996, for details). Within 48 h of collection the upper thermal tolerance (CTmax ) in air of 30 individuals Table 1 Site characteristics for populations of Orchestia gammarellus collected in Iceland. Note: the peculiar nature of the ReykjanestjornÈ site which is adjacent to a sea water inlet warmed by geothermal energy but the temperature of the microhabitat at which animals were collected was not appreciably warmed Site Collection Substratum type/ Microhabitat Thermal spring Sea water date microhabitat temp. (8C) temp. (8C) temp. (8C) Hvassahraun 6 July 1999 Saltmarsh/under lava stones 11.5±12.5 N/A 11.2 ReykjanestjornÈ 6 July 1999 Saltmarsh/under lava stones 11.3±12.9 Spring warms 17.3 sea water inlet from beneath Hveravik 10 July 1999 Bedrock-gravel/under stones 13.3±15.0 73.4 6.3 Bjarnarstadir 10 July 1999 Bedrock-gravel/under stones 15.0±25.0 41.8 6.0 Reykjanes 10 July 1999 Bedrock-gravel/under stones 21.962.4 78.0 6.5 (n 5 13) 218 D. Morritt, A. Ingolfsson / J. Exp. Mar. Biol. Ecol. 255 (2000) 215 ±227 from each population, representing as wide a size range as possible, was determined, based on the technique described by Gaston and Spicer (1998). Each individual was weighed and then transferred