Introduction
Until now, 23 sub-Antarctic insect species have been described from the Iles Kerguelen (Schermann-Legionnet et al. Reference Schermann-Legionnet, Hennion, Vernon and Atlan2007), and 46 species from Iles Crozet (Ile de la Possession) (Vernon et al. Reference Vernon, Vannier and Tréhen1998). Despite the high level of adaptation to the specific environmental conditions evolved by sub-Antarctic insect species (Vernon et al. Reference Vernon, Vannier and Tréhen1998, Schermann-Legionnet et al. Reference Schermann-Legionnet, Hennion, Vernon and Atlan2007), their thermal biology has been little studied. At iles Crozet and Kerguelen, organisms must complete their entire life cycle at mean monthly temperatures ranging from 1.3°C (SD 0.7°C) in July (mean minimum temperature reaches -5.0 ± 1.6°C) to 9.0°C (SD: 0.9°C) in January (Lebouvier et al. Reference Lebouvier, Laparie, Hullé, Marais, Cozic, Lalouette, Vernon, Candresse, Frenot and Renault2011).
Chill coma onset, the temperature at which locomotor activities are interrupted (Hazell & Bale Reference Hazell and Bale2011, Macmillan & Sinclair Reference Macmillan and Sinclair2011), and chill coma recovery are key fitness attribute in these flightless insects. The critical thermal limits of these insects determine the thermal window in which they can feed, breed and escape from predators. In addition, critical thermal minimum and recovery from chill coma vary both within and between species (Castañeda et al. Reference Castañeda, Lardies and Bozinovic2005, Jumbam et al. Reference Jumbam, Terblanche, Deere, Somers and Chown2008). Here we report the critical thermal limits of three native sub-Antarctic insect species: Amblystogenium pacificum (Putzeys) (Coleoptera: Carabidae), endemic to Iles Crozet, Anatalanta aptera Eaton (Diptera: Sphaeroceridae) from Crozet, Kerguelen and Heard islands and Calycopteryx moseleyi Eaton (Diptera: Micropezidae) from Kerguelen and Heard islands. The thermal biology of C. moseleyi and A. pacificum has not yet been investigated and we hypothesize that the geographical distribution of these three insect species should result in a distinctive thermal performance.
Material and methods
Imagoes of Amblystogenium pacificum (n = 24; fresh mass = 14.4 ± 2.3 mg; mean ± SD) were collected in fellfield at Ile de la Possession (Iles Crozet, 46°25′S, 51°51′E, altitude: 110 m). Imagoes of Anatalanta aptera (n = 13; fresh mass = 8.4 ± 2.3 mg) and Calycopteryx moseleyi (n = 23; fresh mass = 20.8 ± 4.7 mg) were collected in coastal areas at Point Suzanne, Iles Kerguelen (49°21′S, 70°13′E, altitude: 5 m). All specimens were sampled in December 2007 and were kept at 4°C (R.H. of 70 ± 5%), with a 15 h/9 h light/dark cycle, for one week before being used for the experiments. Critical thermal minimum (CTmin) was measured using a method described by Lalouette et al. (Reference Lalouette, Vernon, Amat and Renault2010). Briefly, individual insects were placed in a 35 × 5 cm double-jacketed glass column, and the temperature was reduced from 4°C to the CTmin at 0.5°C min-1. The CTmin was the temperature at which each beetle lost coordinated muscle function. Individuals were immediately transferred to a Petri dish at 4 ± 1°C, and the duration needed to regain complete motor function (co-ordinated movements of the legs, T) was recorded. Each individual was weighed using a Sartorius M4 microbalance (d = 0.1 mg). Adults of A. aptera and C. moseleyi were sexed just after CTmin was measured. Kruskal-Wallis tests (H test) were performed to compare CTmin and the time necessary to recover from chill coma among species and genders. Mann-Whitney post hoc tests with Bonferroni correction were conducted. Statistical analyses were carried out using MINITAB Statistical Software Release 13 (MINITAB, Penn State University, USA).
Results
There was no correlation among FM, CTmin and the duration of recovery (P > 0.05). There were no significant differences between genders for CTmin and T in either A. aptera or C. moseleyi and the genders were thus pooled in subsequent analyses. Fresh mass (FM) and CTmin differed significantly among the three species (H2 = 49.15, P < 0.001; H2 = 30.73, P < 0.001 respectively) (Fig. 1a). CTmin fell to -6.1 ± 0.7°C in C. moseleyi and differed significantly from both A. aptera and from the beetle endemic to the Iles Crozet A. pacificum having a CTmin in the order of -4.2°C (W = 339.5, P < 0.001; W = 497.5, P < 0.001, respectively) (Fig. 1a). The duration of the recovery from chill coma did not differ significantly between the three species (H2 = 1.59, P > 0.05) (Fig. 1b).
Fig. 1 a. Critical thermal minima (CTmin), and b. recovery from chill coma (duration needed to recover walking activities with coordinated movements of the legs) in adult Calycopteryx moseleyi, Anatalanta aptera and Amblystogenium pacificum. Boxes represent quartiles, black dots represent mean values, lines within the box represent the medians, and whiskers represent the extreme values. Distinct letters indicate significant differences (P < 0.05).
Discussion
The activity threshold has rarely been investigated in sub-Antarctic insects (Klok & Chown Reference Klok and Chown1997, Reference Klok and Chown2003, Slabber & Chown Reference Slabber and Chown2005, Lalouette et al. Reference Lalouette, Vernon, Amat and Renault2010). In the present work, CTmin ranged from -4 to -6°C and may be an important feature for yearly active insects that are regularly subjected to negative temperatures at the Southern Ocean islands (see Lebouvier et al. Reference Lebouvier, Laparie, Hullé, Marais, Cozic, Lalouette, Vernon, Candresse, Frenot and Renault2011 for more information on the meteorological conditions). Our results are consistent with the current geographical distribution of these three species: whereas A. pacificum is endemic from Iles Crozet (Davies et al. Reference Davies, Bouvet and Vernon2007) and was thus characterized by the most northern geographical distribution, C. moseleyi is restricted to locations with the higher latitudes in the Southern Indian Ocean - Kerguelen and Heard islands (53°06′S, 73°31′00′′E) (Harrison Reference Harrison1970) - and A. aptera occur at Crozet, Kerguelen and Heard islands (Harrison Reference Harrison1970). Moreover, despite the differences among species for the CTmin, similar duration of recovery after the flies entered chill coma were found. This finding suggests that similar cold damages were experienced by the adults of C. moseleyi despite their lower CTmin and/or that they are better able to recover from chill coma than the two other studied species.
Matching of the species’ critical thermal limits to the microclimates of their environments has already been demonstrated in two sub-Antarctic spiders from Marion Island (Jumbam et al. Reference Jumbam, Terblanche, Deere, Somers and Chown2008), and at a larger geographical scale in populations of a given species sampled along latitudinal gradients (Castañeda et al. Reference Castañeda, Lardies and Bozinovic2005). As Lalouette et al. (Reference Lalouette, Vernon, Amat and Renault2010) recently found a higher thermal tolerance when ageing in adults of A. aptera, further studies should examine if age-related differences could also occur in both C. moseleyi and A. pacificum.
Acknowledgements
We thank P. Vernon, C.M Williams and an anonymous referee for helpful comments. This research was supported by the “Institut Polaire Francais” (IPEV 136) and the “Agence Nationale de la Recherche” (ANR-07-VULN-004, EVINCE).
