Headshields of ephippial Chydorus piger Sars (Cladocera, Chydoridae) females from northern Finnish Lapland: a long period of gamogenesis?

Abstract

Numerous headshields of ephippial Chydorus piger (currently called Paralona pigra) females were found from the Holocene sediments of Aitajarvi Lake, northern Finnish Lapland (69° 08′ N, 27° 14′ E). Headshields of ephippial females of the genus Chydorus s.l. can be easily identified since their posterior margin is elongated and pointed. According to analysis of fossil cladoceran remains, Chydorus piger lived in the lake throughout the Holocene but was never very common (less than 10% of preserved cladoceran remains). Headshields of ephippial females were found in most samples and they comprised ca 10–30% of all Chydorus piger headshields. No similar phenomenon was met in Chydorus cf. sphaericus which is common up to the most severe arctic conditions. Aitajarvi is situated north of the modern distribution area of Chydorus piger. I suggest that this species reacted to the stress of harsh climatic conditions outside of its distribution area by being gamogenetic during most of the ice-free period. The ephippial headshields could be valuable indicators of severe climatic conditions in pre-Holocene sedimentary environments.