Variation in the body growth parameters of the ghost crab Ocypode quadrata from morphodynamically distinct sandy beaches

Abstract Investigations of patterns of distribution of sandy beach macrofauna show that dissipative beaches tend to concentrate higher richness and biomass at the intertidal zone, as well as better fitness and larger sizes of a species, than reflective beaches. For supralittoral crustaceans, these trends are inverted, as predicted by the habitat safety hypothesis (HSH). For ghost-crabs, however, higher population abundance has been observed on dissipative beaches, but their life-history traits and the response to different beach morphologies were not well defined. Here, variation in crab size and growth was evaluated over a one year period using indirect measures from three low-impact beaches on the northern coast of São Paulo State, southeastern Brazil. These beaches represent the three basic types of morphodynamics: dissipative (Costa beach), intermediate (Felix beach) and reflective (Puruba beach). At each site, five 2m wide transects were randomly selected each month, and assessed across the crabs' entire area of occurrence. The fit of the von Bertalanffy body growth model was compared among sites. For all three beaches, a main mode due to the settlement of juveniles (mm) was identified in August and September. In the following summer, according to growth estimates, the settlement group would be approximately the same size as the main mode (mm) that was recorded during the previous summer. Specifically, the growth parameters estimated were: Costa (dissipative) L∞= 46.36mm, K= 0.90year-1 and t0= -0.19; Félix (intermediate) L∞= 57.95mm, K= 0.87year-1 and t0= -0.12; Puruba (reflective) L∞= 53.01mm, K= 1.11year-1 and t0= -0.07. The body growth adjustments were similar between Puruba and Felix, but distinct from Costa. Growth performance indexes ranged from 3.29 (Costa) to 3.46 (Félix) and 3.49 (Puruba), indicating both the homogeneity of this parameter among distinct beaches and close agreement between the intermediate and reflective beaches. This study suggests that variations in the development of ghost crabs concur with those observed for other supralittoral, crustaceans and the habitat safety hypothesis, with higher individual fitness for reflective beaches.

摘要 针对沙滩大型底栖动物（sandy beach macrofauna）分布模式的研究表明，相较于反射型海滩（reflective beaches），消散型海滩（dissipative beaches）在潮间带（intertidal zone）往往拥有更高的物种丰富度与生物量，且物种个体适合度更佳、体型更大。而根据栖息地安全假说（habitat safety hypothesis, HSH）的预测，潮上带甲壳动物（supralittoral crustaceans）的相关趋势恰好相反。但针对沙蟹（ghost-crabs）的研究发现，消散型海滩上的种群丰度更高，不过其生活史特征以及对不同海滩地貌形态的响应机制尚未得到明确阐释。本研究以巴西东南部圣保罗州北部海岸的3处低干扰海滩为研究对象，通过间接测定方法开展了为期1年的沙蟹体型与生长变异研究。这3处海滩分别对应海滩动力地貌学的3种基本类型：消散型（科斯塔海滩/Costa beach）、过渡型（费利克斯海滩/Felix beach）与反射型（普鲁巴海滩/Puruba beach）。每月在每个研究点位随机选取5条宽度为2米的样带，对沙蟹的全部栖息分布区域进行调查。对不同点位的冯·贝塔朗菲个体生长模型（von Bertalanffy body growth model）拟合效果进行了比较。在3处海滩中，均于8月和9月检测到由幼体补充形成的主要体型频数峰（单位：毫米/mm）。根据生长估算结果，次年夏季的补充群体体型将与前一个夏季记录的主要体型频数峰（单位：毫米/mm）大致相当。具体而言，估算得到的生长参数如下：科斯塔海滩（消散型）：渐近体长L∞=46.36mm、生长速率常数K=0.90年⁻¹、理论生长起点t0=-0.19；费利克斯海滩（过渡型）：L∞=57.95mm、K=0.87年⁻¹、t0=-0.12；普鲁巴海滩（反射型）：L∞=53.01mm、K=1.11年⁻¹、t0=-0.07。普鲁巴与费利克斯海滩的生长模型拟合效果相近，但与科斯塔海滩存在显著差异。生长性能指数范围为3.29（科斯塔海滩）至3.46（费利克斯海滩）与3.49（普鲁巴海滩），表明该参数在不同海滩间具有同质性，且过渡型与反射型海滩的该指数结果高度一致。本研究表明，沙蟹的生长发育变异与其他潮上带甲壳动物的相关规律以及栖息地安全假说相符，即反射型海滩上的沙蟹个体适合度更高。