Planktonic copepods reacting selectively to hydrodynamic disturbances

In the water column, planktonic copepods encounter small-scale hydrodynamic disturbances generated by fellow zooplankters. Our question is whether or not the copepods can distinguish between hydrodynamic disturbances created by predators, prey, conspecifics and/or mates. We used a Schlieren optical system with a density gradient in the water volume and filmed at 48 frames per second to record the behaviour of copepods during encounters with an artificial hydrodynamic disturbance. We observed the reactions of Cyclops scutifer and Epischura nordenskioldi towards disturbances of different strengths. We also re-examined an earlier report on tandem swimming in C. scutifer while attempting to mate, using novel mathematical tools to analyse possible correlations between the two mates. We conclude that the information within the hydrodynamic disturbances created by swimming zooplankters has enough content for differentiated reactions. We also suggest that the adaptive value of tandem swimming during mating results in offspring capable of executing escape reactions comparable in strength to the disturbances.

Intraspecific morphological variation in a freshwater copepod (Crustacea) in relation to geographic distribution and environment

Intraspecific variation in morphology was studied by means of morphometric analysis in 52 populations of the freshwater planktonic copepod Cyclops scutifer G.O. Sars from Eurasia and North America. The species showed very conservative morphology over wide geographic areas and subspecies could not be established. Variations occurred irregularly, and morphologically very similar forms were found widely separated on different continents. Correlation analysis revealed that many morphological relations were correlated with environmental factors such as depth, temperature, and trophic condition. Variations in body proportions were thus related to environmental factors rather than to geographic distance.

Cyclops canadensis n.sp. and Cyclops scutifer Sars, 1863 (Crustacea: Copepoda) from northern Canada

The occurrence and distribution of Cyclops s.str. species (strenuus subgroup) in Canada and the United States has not yet been reliably studied. In this first attempt, Cyclops scutifer Sars, 1863 is characterized, and a new species, Cyclops canadensis, is established. As no cytological investigations were possible, the species are described on the basis of morphological and morphometrical data. Most probably, the new species, Cyclops canadensis, formerly was identified as Cyclops strenuus.

Zooplankton Species Abundance and Biomass in Relation to Occurrence of Mysis relicta (Malacostraca: Mysidacea)

Year-long comparisons of the occurrence and abundance of zooplankton between two lakes where Mysis relicta is endemic and two where it is not show that in the former, hypolimnetic species Daphnia longiremis, Euhosmina longispina, Cyclops scutifer, and Diaptomus sanguineus are rare. Five other cladocerans were also lower in abundance in hypolimnetic strata and during isothermal conditions. Diaphanosoma spp. were more abundant in the metalimnion of lakes with Mysis; we suggest a competitive release through Mysis predation on Daphnia. Because each pair of lakes is comparable with respect to morphometry and chemistry, nonpredatory influences do not account for the observed distribution of zooplankton. Such species differences are also identical to trends occurring in a canonical analysis of zooplankton abundance for 497 lakes in eastern Ontario and western Quebec and corroborate declines in Cladocera associated with introductions of M. relicta into lakes and reservoirs.