Interspecific variation in ephippial size between Daphnia galeata and D. pulicaria in Lake Biwa, Japan
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RESEARCH PAPER
Interspecific variation in ephippial size between Daphnia galeata and D. pulicaria in Lake Biwa, Japan Narumi K. Tsugeki1 · Mie N. Honjo2 · Michinobu Kuwae3 Received: 23 December 2019 / Accepted: 27 October 2020 © The Japanese Society of Limnology 2020
Abstract Daphnia, keystone herbivores in lakes, routinely produce immediately hatching eggs; additionally, they also produce resting eggs enveloped by an ephippial case, a thickened carapace that allows population survival under harsh environmental conditions. To examine differences in ephippial morphology between Daphnia species in different subgenera, we conducted microscopic observations and genetic analyses based on the mitochondrial 12S rRNA gene in ephippia from surface sediment in Lake Biwa, Japan. The lengths and heights of ephippia identified as Daphnia galeata Sars (Hyalodaphnia) were less than 0.82 and 0.50 mm, respectively, whereas those of Daphnia pulicaria Forbes (Daphnia) were greater than 0.87 and 0.53 mm, respectively, with the ephippial lengths of the two species differing significantly. The results indicate that D. galeata and D. pulicaria inhabiting Lake Biwa can be distinguished based on ephippium size, with a boundary ephippium length of approximately 0.86 mm. In concordance with this inference, historical data indicated that the length of ephippia recovered from sediment cores did not exceed 0.86 mm prior to the 1980s when D. galeata was the predominant species; however, it exceeded the threshold after 2000, coinciding with the coexistence of D. galeata and D. pulicaria. Keywords Daphnia · Resting egg · Lake Biwa · Ephippial morphology · Egg bank
Introduction Daphnia, which are key herbivorous plankton in lakes and ponds (Carpenter and Kitchell 1993), routinely produce parthenogenetic eggs that develop directly in the brood chamber of the mother. However, they also produce resting Handling Editor:Mingbo Yin. Narumi K. Tsugeki and Mie N. Honjo have contributed equally to this study. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10201-020-00646-8) contains supplementary material, which is available to authorized users. * Narumi K. Tsugeki [email protected]‑u.ac.jp; [email protected] 1
Faculty of Law, Matsuyama University, 4‑2 Bunkyo‑cho, Matsuyama, Ehime 790‑8578, Japan
2
Center for Ecological Research, Kyoto University, Hirano 2‑509‑3, Otsu, Shiga 520‑2113, Japan
3
Center for Marine Environmental Studies, Ehime University, 2‑5 Bunkyo‑cho, Matsuyama 790‑8577, Japan
(dormant) eggs enveloped by a thickened carapace, referred to as an ephippial case, which allows population survival during unfavorable periods. Indeed, such resting eggs are often well preserved in sediments for decades to centuries (Caceres 2012; Frisch et al. 2014), and can be successfully hatched even after several decades when stimulated with appropriate environmental cues (Decaestecker et al. 2004; Frisch et al. 2014; Hairston et al. 1999). Accordingly, resurrection studies based on analyse
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