Population features of the pea crab Dissodactylus crinitichelis Moreira, 1901 (Decapoda, Brachyura, Pinoteridade) associated with the sand-dollar Encope emarginata (Echinodermata, Echinoidea) in the southeastern Brazilian coast

Abstract Crabs of the Dissodactylus genus are commonly found in symbiosis with other marine invertebrate organisms. This study describes the structure of the population of the Pinnotheridae crab Dissodactylus crinitichelis living in symbiosis with the sand dollar Encope emarginata on a temporal scale and over the size class distribution, sex-ratio, recruitment defined as the specimens of the two first size classes, and breeding period. Specimens were collected in sand bottoms at the infralittoral zone at Flamengo Beach in Ubatuba, in the northeastern coast of São Paulo state, using free diving performed by one diver from August 2017 to July 2018, for 30 minutes per month. 337 juvenile, 51 adult females, and 38 adult males were analyzed, comprising 12 ovigerous females. There was registered recruitment, in almost all months. The sex ratio was 1:1.3, with more females than males in the population. In March, there was a significant recruitment period with 337 juveniles in one sample. The maximum carapace width was 6.6mm in males and 7.65mm in females. The representative number of specimens in the first size class may provide information that D. crinitichelis presents some levels of spatial segregation in the population in this area and should be monitored for preservation.

摘要 隐豆蟹属（Dissodactylus）的蟹类通常与其他海洋无脊椎动物形成共生关系。本研究从时间尺度、体型分级分布、性比、以最初两个体型等级个体定义的补充群体，以及繁殖期等维度，系统记述了与沙币海胆（Encope emarginata）共生的豆蟹科（Pinnotheridae）隐毛螯隐豆蟹（Dissodactylus crinitichelis）的种群结构。样本采集工作于2017年8月至2018年7月间开展，由一名潜水员采用自由潜水方式，在巴西圣保罗州东北海岸乌巴图巴市弗拉门戈海滩的潮下带沙质底质中进行，每月采样时长为30分钟。本次研究共分析337只幼体、51只成年雌性个体与38只成年雄性个体，其中包含12只抱卵雌性个体。研究几乎在所有月份均记录到补充群体的出现。该种群的性比为1:1.3，雌性个体数量显著多于雄性。在3月的一个采样样本中，出现了显著的补充群体事件，共记录337只幼体。雄性个体的最大甲宽为6.6mm，雌性个体的最大甲宽为7.65mm。首个体型等级中较高的个体占比表明，该区域的隐毛螯隐豆蟹（D. crinitichelis）种群存在一定程度的空间隔离，需对其开展监测以实现有效保护。