Electra vs Callopora: life histories of two bryozoans with contrasting reproductive strategies in the White Sea

The life histories of two common boreal-Arctic bryozoans – Electra pilosa and Callopora craticula – living on kelps in the White Sea are described for the first time using colony mapping. Colony functional dynamics (sexual, feeding, budding and rejuvenation) was traced seasonally via recording zooidal states. During the ice-free season the population of the zygote spawner E. pilosa was represented by three generations – overwintered (maternal) and two young ones (daughter and granddaughter). The overwintered colonies exhibited two-phased reproduction and the ‘early’ daughter colonies reproduced just once. ‘Late’ colonies, together with the granddaughter generation, remained sterile. Four generations are described for the brooder C. craticula, three of which continuously produced larvae. While the two species co-exist on the same substratum, reproduction in E. pilosa started almost 1 month later (in late June), which could be explained by the insufficient abundance of phytoplankton in early summer to support the start of oogenesis and feeding cyphonautes larvae. The estimated maximal colony lifespan is 13 months in E. pilosa and 15 months in C. craticula. The broad comparison of bryozoan reproductive ecology in the White Sea showed that their life histories are much more diverse, complex and evolutionarily flexible than previously acknowledged.

本研究首次通过群体绘图法，对栖息于白海大型褐藻（kelp）上的两种常见北寒带-北极区苔藓虫（Bryozoa）——Electra pilosa与Callopora craticula——的生活史进行了系统描述。通过记录个员（zooid）状态，本研究对群体的功能动态（包括有性生殖、摄食、出芽与复壮）开展了季节性追踪。在无冰期，受精卵产卵型的E. pilosa种群包含三个世代：越冬亲代（母代）以及两个年轻世代（子代与孙代）。越冬群体呈现两阶段生殖模式，而‘早期’子代群体仅能繁殖一次；‘晚期’群体与孙代群体则始终保持不育。育幼型的C. craticula被记录有四个世代，其中三个世代可持续产生幼虫。尽管两种苔藓虫共享同一栖息基质，但E. pilosa的生殖启动时间几乎晚了一个月（6月末），这一现象可归因于初夏浮游植物（phytoplankton）丰度不足，无法支撑卵子发生以及摄食型帽状幼虫（cyphonautes）的发育需求。据估算，E. pilosa的最长群体寿命为13个月，C. craticula则为15个月。对白海苔藓虫生殖生态学的全面对比研究表明，其生活史远比此前认知的更为多样、复杂且演化可塑性更强。