Journal of Insect Physiology 47 2001 95–109 www.elsevier.comlocatejinsphys
Critical thermal limits, temperature tolerance and water balance of a sub-Antarctic kelp fly, Paractora dreuxi Diptera: Helcomyzidae
C. Jaco Klok , Steven L. Chown
Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa Received 11 April 2000
Abstract
Paractora dreuxi displays distinct ontogenetic differences in thermal tolerance and water balance. Larvae are moderately freeze tolerant. Mean larval onset of chill coma was
25.1 °
C, and onset of heat stupor was 35.5 °
C. Larval supercooling point SCP was 23.3
° C with 100 recovery, although mortality was high below
24 °
C. Starvation caused SCP depression in the larvae. Adults were significantly less tolerant, with critical thermal limits of
22.7 and 30.2 °
C, no survival below the SCP 29.6
° C, and no
change in SCP with starvation. Moderate freeze tolerance in the larvae supports the contention that this strategy is common in insects from southern, oceanic islands. Fly larvae survived desiccation in dry air for 30 h, and are thus less desiccation tolerant
than most other sub-Antarctic insect larvae. Water loss rates of the adults were significantly lower than those of the larvae. Lipid metabolism did not contribute significantly to water replacement in larvae, which replaced lost body water by drinking fresh water,
but not sea water. Kelp fly larvae had excellent haemolymph osmoregulatory abilities. Current climate change has led to increased temperatures and decreased rainfall on Marion Island. These changes are likely to have significant effects on P. dreuxi, and pro-
nounced physiological regulation in larvae suggests that they will be most susceptible to such change.
2000 Elsevier Science
Ltd. All rights reserved.
Keywords: Ontogenetic differences; CT
Min
; CT
Max
; Freeze tolerance; Osmoregulation; Marion Island; Climate change
1. Introduction
The majority of studies concerning temperature and water relations of arthropods in the sub- and maritime
Antarctic regions has concerned strictly terrestrial spec- ies. With the exception of a few studies of semi-aquatic
flies, beetles and copepods Chown and Van Drimmelen, 1992; Chown, 1993; Convey and Block, 1996; Daven-
port and MacAlister, 1996; Davenport et al., 1997, little emphasis has been given to environmental tolerances of
arthropods living in shoreline or semi-aquatic habitats see Block, 1984; Klok and Chown 1997, 1998; Sinclair,
1999. Nonetheless, these habitats, and particularly the shore zones, are characterized by high arthropod diver-
sity Bellido, 1981; Trave´, 1981; Chown, 1990; Marshall et al., 1999, of which a variety of species are major
Corresponding author. Tel.: +
2712-420-3236; fax: +
2712-342- 3136.
E-mail address: cjklokzoology.up.ac.za C.J. Klok.
0022-191000 - see front matter
2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 1 9 1 0 0 0 0 0 0 8 7 - 1
contributors to ecosystem functioning Tre´hen et al., 1985; Crafford and Scholtz, 1987; Ha¨nel and Chown,
1998. In consequence, there is little information avail- able on the way in which these ecologically important
arthropods cope with their often variable environments, and how their physiological responses differ to those of
arthropods from terrestrial environments in the region.
Such comparisons are of considerable interest from three perspectives. First, the ways in which terrestrial
and aquatic species cope with their respective environ- ments are usually quite different e.g. Lee and Denlinger,
1991; Hadley, 1994, yet many insect species make the transition from one environment to the other as they
develop. Although marked ontogenetic differences in physiology have been documented in some insect spec-
ies Morrissey and Baust, 1976; Tre´hen and Vernon, 1986; Vernon, 1986; Vernon and Vannier 1986, 1996,
the subtleties of such differences remain to be thor- oughly explored see Spicer and Gaston, 1999 for
discussion. Second, it has been mooted repeatedly that the cold hardiness strategy adopted by most mid-latitude
96 C.J. Klok, S.L. Chown Journal of Insect Physiology 47 2001 95–109
southern hemisphere insect species is one of freeze toler- ance e.g. Klok and Chown, 1997; Van der Merwe et
al., 1997. This is thought to be a consequence of the oceanic nature of the region, which leads to generally
moist habitats and, as a result, a high risk of freezing due to external inoculation by ice crystals. Nonetheless,
data with which to test this idea are limited, and it has not been well-explored in species where adult and larval
habitats might differ see Addo-Bediako et al., 2000; Convey and Block, 1996 for exceptions. Third, differ-
ences in the physiological tolerances of terrestrial and aquatic species are not only likely to influence the range
of habitats they can occupy, and hence, ultimately differ- ences in their geographic ranges Chown and Gaston,
1999, but they may also influence the extent to which changes in the abiotic environment will differentially
influence these species. The latter is of particular con- cern in Antarctic and sub-Antarctic environments where
climates are changing rapidly Smith et al., 1996; Berg- strom and Chown, 1999.
In this study we examine thermal tolerances and water balance in adults and larvae of Paractora dreuxi Se´guy
Diptera: Helcomyzidae, a brachypterous kelp fly that frequents kelp deposits in the littoral habitats on the
coastlines of the Prince Edward and Crozet islands in the sub-Antarctic Crafford, 1984; Tre´hen et al., 1985;
Crafford and Scholtz, 1987. The associations of P. dreuxi with kelp deposits differ markedly between the
larvae and the adults. The larvae are less mobile than adults and are mostly confined to kelp, or the substrate
below the kelp, where they either burrow into the fronds, feed between the fronds, or, in the later instars, feed on
the underlying detritus Crafford, 1984; Crafford and Scholtz, 1987. Here they are subject to substantial fluc-
tuations in temperature and water availability. In con- trast, the shorter-lived adults, though brachypterous, are
highly mobile as a consequence of their search for ovi- position and feeding sites Crafford, 1984; Tre´hen et al.,
1985. However, they often take refuge, at least on Marion Island Prince Edward group, between the boul-
ders below the kelp-substrate interface either when pred- ators are present, or when weather conditions are
unfavourable to them. Using flies collected at Marion Island, we test the hypothesis that stage-related differ-
ences in association of P. dreuxi with stranded kelp will result in differences in their environmental tolerances.
We also examine the idea that these differences will affect the likely impact of climate change especially
declining rainfall on the stages.
2. Material and methods
