Metabolic rates of a hypogean and an epigean species of copepod in an alluvial aquifer

Abstract

Summary Reduced metabolic rates of groundwater taxa, compared to those of surface water species, have long been inferred to be an adaptive trait where there is a low and discontinuous food supply and unpredictable shifts between hypoxic and normoxic conditions. However, there have been neither measurements of the respiratory rate of groundwater copepods nor a comparison of rates between closely related groundwater and surface water species. We measured the metabolic rates of two species of Cyclopoida: Cyclopidae, the stygobiotic (hypogean) copepod Diacyclops belgicus and the epigean Eucyclops serrulatus, which co‐occur in the same alluvial aquifer. We expected the metabolic rate of the hypogean to be lower than that of the epigean species, irrespective of the ontogenetic stage, which would be consistent with the hypothesis that there is a generally lower metabolic rate in groundwater species. The metabolic rate of D. belgicus was significantly lower than that of the epigean E. serrulatus irrespective of the ontogenetic stage. We found an allometric relationship between oxygen consumption and body mass for E. serrulatus, an isometric one for D. belgicus juveniles and a rate of oxygen consumption that apparently does not change systematically with body mass for D. belgicus adults. The low metabolic rate of D. belgicus may be advantageous in oligotrophic groundwater habitats, where large fluctuations in oxygen availability occur. However, these physiological adaptations can put hypogean species at risk of replacement by more metabolically active epigean taxa, whenever the availability of organic matter increases, as happens with organic pollution. Moreover, the low metabolic rate of the hypogean species may entail an inability to cope with toxicants, rendering them more sensitive to pollutants. A higher metabolic rate in juvenile D. belgicus compared to that of adults allows copepodids to mature quickly when food is briefly abundant.

Effect of Temperature Rising on the Stygobitic Crustacean Species Diacyclops belgicus: Does Global Warming Affect Groundwater Populations?

“…In order to reduce energetic costs, groundwater ectotherms have evolved metabolic rates that are lower than those of their close epigean relatives [20,[41][42][43][44]. The results of the one and only study on the metabolism of a stygobitic copepod species [26], in which we compared the SRRs of the stygobitic D. belgicus and the epigean Eucyclops serrulatus (belonging to the same family Cyclopidae), were consistent with this statement. The SRRs of E. serrulatus was 7 and 5 fold the SRRs of D. belgicus juveniles and adults, respectively.…”

“…The SRRs of E. serrulatus was 7 and 5 fold the SRRs of D. belgicus juveniles and adults, respectively. Albeit measured with a different device and protocol and under different temperatures, the individual-based measurements of the SRRs of D. belgicus of the present study are of the same order of magnitude (nanograms of O 2 per individual per hour) as those observed in [26]. Details are provided in Table S1.…”

“…Eighteen hours before the trial at 14 • C the copepods were rinsed in standard water (1 L of MILLIPORE MILLI-Q ® (Elix ® , Merck KGaA, Darmstadt, Germany) deionized water remineralized with the following reagent grade chemicals: 0.06 g of MgSO 4 , 0.096 g of NaHCO 3 , 0.004 g of KCl, 0.06 g of CaSO 4 ·2H 2 O; [25]) and kept in a 10-mL glass vial with standard water and in the darkness at the testing temperature. No food was offered during acclimation in order to allow gut-emptying [26,27]. After the acclimation, only the actively swimming individuals were selected for testing.…”

“…The test individuals (juvenile stages, i.e., copepodids) were picked up by a glass pipette under a Leica M80 stereomicroscope at 20× magnification. The pick-up of fecal pellets was carefully avoided so to exclude the overshoot in oxygen consumption due to digestive metabolism [20,26,27]. The respirometric measurements were started on the day 8th for the trials at 14 • C. The whole procedure (collection of new animals from the field, acclimation in bore water and in standard water and testing) was repeated for the trial at 17 • C at the day 9th.…”

“…The camera was connected to a notebook and the pictures were elaborated by LAS EZ vs. 3 software (Leica Microsystems Srl, Wetzlar, Germany) and the free vector graphic software Inkscape (https://inkscape.org/it/). The prosome length (L, mm) and width (W, mm) of each individual were measured using the methods described in our previous studies [26,27]. Dry mass was calculated according to [30]:…”