Introduction

Abstract

Any kind of freshwater aquatic body and some other wet habitats (leaf litter, sphagnum mat, hot springs, hyporheic zones, groundwater, caves, tree holes, etc.) will contain one or more species of freshwater crustacean that belong to Cladocera or Copepoda. The majority of them are planktonic filtrators, an important component of the zooplankton community. Often, they are dominant secondary producers, algae, bacteria, and detrital grazers capable of controlling phytoplankton (transferring more than 50 % of fixed carbon to higher trophic levels), while others are benthic omnivores or carnivorous predators (Gliwicz and Rybak 1976; Scavia 1980; McCauley 1984; Bruce et al. 2006). Both groups are an important component of the trophic food web, especially fish (among them species of economic importance). In addition, their excretions are source of dissolved nutrients (Lehman 1980; Wen and Peters 1994). In lakes, zooplankton excretion of dissolved nutrients can account for up to 52 % and 48 % of the phytoplankton demand for phosphorus (P) and nitrogen (N), respectively (Bruce et al. 2006). There are species with cosmopolitan geographic ranges restricted to one or more continents, but some are endemic species restricted to a single site or relatively small area. Cladocera and Copepoda are small-sized animals mostly around 0.3–6 mm, where the largest species is (Cladocera), measuring around 10 mm in length. Cladocera has two reproduction strategies, namely, parthenogenetic females that just clone themselves asexually and sexually reproducing females and males during autumn or environmental stress, like food shortage, temperature drop, or oxygen depletion. Copepoda have sexual reproduction and distinct sexes. Cladocera and Copepoda are able to survive harsh environmental conditions, by producing ephippia (Cladocera) and by diapause (an obligatory stop in development), dormancy (copepodites or adults), or diapausing (resting) eggs (Copepoda). Copepod resting eggs can rest for centuries in the bottom sediments (egg bank) and are able to hatch when the environmental conditions become optimal (Hairston 1995; Dussart and Defaye 2001; Nevalainen et al. 2011e). We call them time travelers and the science dealing with them resurrection ecology (Kerfoot and Weider 2004; Angeler 2007). Only a few fossils of Cladocera are known. Sparse individuals of Cladocera () were identified from pre-Quaternary fossils found in lake sediments in Mongolia (Smirnov 1992b) and Slovenian relic caves (Moldovan et al. 2011), but plenty of subfossils have been reported in lacustrine sediments of the Pleistocene (Frey 1960). Cladocera remains of European species are presented in the illustrated (photographs) atlas (Szeroczyńska and Sarmaja-Korjonen 2007). Subfossils of Cladocera have been used for a reconstruction of human influence on the environment (lakes) throughout history (Galbarczyk-Gąsiorowska et al. 2009), for a climate change reconstruction (Kamenik et al. 2007), or for Holocene environmental (lake trophic condition, temperature and ice cover, lake water level oscillation) history description (Adamska and Mikulski 1969; Mikulski and Adamska 1972; Mikulski 1978; Szeroczyńska et al. 2007; Nevalainen et al. 2011b).