Identifying century-old long-spined Daphnia: species replacement in a mountain lake characterised by paleogenetic methods

摘要

Mountain lakes often harbour morphologically or genetically unique populations of zooplankton species, including cladocerans. Daphnia lacustris Sars, predominantly found in Fennoscandia but also known from two Central European lakes in the Tatra Mountains, is one of such taxa. This Daphnia species often forms morphotypes with extremely long tailspines. Historical literature from a century ago documented similar morphs from another lake in the Tatra mountain range, presently inhabited by the phenotypically very different D. galeata. Using a paleogenetic approach (partial sequencing of the mitochondrial gene for 12S rRNA from preserved ephippial eggs in the lake sediment), we tested the hypothesis that Daphnia species composition changed in the lake due to anthropogenic disturbances, and that long-spined morphs were actually another relict population of currently extinct D. lacustris. Ephippia with extremely long spines were successfully retrieved from sediment cores. Despite being morphologically very well preserved, intact eggs were found in less than 2% of analysed ephippia. Genetic analyses, benefiting in most cases from amplification of short 12S fragments using internal primers, proved that long-spined ephippia belonged to D. longispina, which apparently coexisted with D. galeata in the mid-twentieth century. Our results confirm that paleogenetic methods are useful for studying the recent population structures of zooplankton species forming dormant egg banks but lacking reliably identifiable remains in sediments, and show that the extreme development of tailspines in mountain-lake Daphnia is associated with as-yet unclear environmental factors rather than taxonomic status.