Rethinking resting eggs decapsulating

Abstract Aim Temperature and light have been recognized as important factors for ephippia hatching in temperate and tropical freshwater systems. Oddly some authors suggested that decapsulation of resting eggs would be a pro when it comes to ex situ hatching studies, exposing those eggs to a greater amount of light. This study aimed to compare the difference in the hatching rate between resting eggs decapsulated and intact ephippia of Daphnia laevis, a zooplankton that occurs at lakes, in tropical freshwaters (Cladocera). Methods The ephippia used in this work were collected at the sediment of a reservoir, in Belo Horizonte city (Minas Gerais, Brazil). We set up the laboratory experiment with two distinct groups: intact ephippia and decapsulated resting eggs. For that, we manually decapsulated 120 ephippia and kept 120 others intact (six replicas with 20 ephippia each), then incubated them all with culture water at 22ºC (12h photoperiod) for 30 days with daily monitoring. Results The results showed that decapsulation influenced negatively the hatching success, as the intact ephippia had a hatching rate of 22%, while those decapsulated only had 6%. In addition, Daphnia hatchlings were observed for intact ephippia group up to the twenty-seventh day, while for the decapsulated the last hatching occurred on the tenth day. Decapsulated eggs are subject to high exposure to light and it may jeopardize the embryo development. Conclusions In this context, we suggest that removing the protective capsule from the eggs needs to be done with caution, since in some species this can damage the resting eggs, which alters the viability and compromise the accuracy of the hatching rates studies.

摘要
温度与光照是温带及热带淡水生态系统中卵鞍（ephippia）孵化的关键调控因子。令人意外的是，已有学者提出，在异位（ex situ）孵化实验中对休眠卵进行脱囊处理可带来益处，因该操作能使卵暴露于更多光照下。本研究以热带淡水湖泊中的浮游动物光滑水蚤（Daphnia laevis，枝角目（Cladocera））为研究对象，旨在比较脱囊休眠卵与完整卵鞍的孵化率差异。

实验方法：本实验所用卵鞍采集自巴西米纳斯吉拉斯州贝洛奥里藏特市某水库的沉积物中。本实验设置两组处理：完整卵鞍组与脱囊休眠卵组。具体操作如下：手动对120枚卵鞍进行脱囊处理，另保留120枚卵鞍不作处理（每组设置6个重复，每个重复含20枚卵鞍）；随后将所有样本置于22℃、光照周期12小时的培养水中孵化30天，每日进行观测记录。

结果：结果显示，脱囊处理对孵化成功率存在负面影响：完整卵鞍组的孵化率为22%，而脱囊组仅为6%。此外，完整卵鞍组的水蚤幼体孵化可持续至第27天，而脱囊组的最后一次幼体孵化发生在第10天。脱囊后的卵易受到强光照射，这可能会损害胚胎发育。

结论：基于上述结果，我们建议对卵的保护性囊膜进行脱囊处理时需谨慎：对于部分物种而言，脱囊操作可能会损伤休眠卵，进而影响卵的活力，降低孵化率实验结果的准确性。