Habitat selection and behavior characteristics of Chinese mitten crab (Eriocheir sinensis)

The transition from planktonic larva to benthic juvenile is a critical period for decapod crustaceans. If the habitat was suitable, the post-larval individuals will quickly settle down. In addition, individuals in different development stages may choose different habitats. Chinese mitten crab (Eriocheir sinensis) is an important economic species in China. The juvenile E. sinensis mainly live in the intertidal zone of the Yangtze Estuary, but there is no systematic study on its habitat selection patterns. Therefore, in order to explore the habitat preference of E. sinensis larvae, simulation experiments of four habitats common on Yangtze Estuary, namely Scirpus mariqueter, mud bottom, consolidated silt and gravel, were conducted in the laboratory. In addition, behavior observation and quantitative analysis of the megalopa, juvenile of E. sinensis (at stages Ⅰ, Ⅲ and Ⅴ) were carried out in different habitats. The results showed that there were significant differences in the distributions of the four stages in the four habitats (Kruskal-Wallis test, P < 0.05). The distribution proportions of megalopa (73.7%±6.8%), stage Ⅰ juvenile (78.2%±8.8%), stage Ⅲ juvenile (53.6%±8.0%) and stage Ⅴ juvenile (54.8%±6.8%) in the S. mariqueter habitat were significantly higher than other habitats (P < 0.05). The proportions of stage Ⅲ and Ⅴ juvenile in mud and consolidated silt were significantly higher than other stages (P<0.05). Juveniles had swimming, cling, epibiosis and burying behaviors in S. mariqueter habitat. Megalopa and stage Ⅰ juvenile showed cling behavior, accounting for 82.1%±5.1% and 83.3%±8.7% of the group, respectively. Only megalopa showed swimming behavior (9.0%±3.7%). For stage Ⅲ, epibiosis (44.2%±17.9%) and burying rate (36.5%±16.7%) were significantly increased (P<0.05). In addition, since it is difficult to observe the stage Ⅴ juvenile, the behavior experiment of stage Ⅴ juvenile showed no results. Figure 4 shows the cumulative percentage of molting and mortality rates of juveniles within 5 days. Feeding and habitat had significant effects on the molting rate (feeding: F=166.48, P<0.05, habitat: F=198.09, P<0.05), and the interaction between them was significant (F=21.84, P<0.05). The juveniles were found to show significantly higher molting rate in S. mariqueter habitat (feed: 86.7%±6.9%; not feed: 81.1%±5.1%) compared to the mud and empty basin (P<0.05) regardless of being fed or not. Habitat had significant effect on mortality (F=27.16, P<0.05), while feeding had and interaction didn’t (F=0.22, P>0.05 and F=1.72, P>0.05). By pairwise comparison, in feeding group, the juveniles were found to show significantly higher mortality in mud (24.4%±1.9%) compared to the S. triqueter (13.3%±3.3%) and empty basin (14.4%±1.9%). In the non-feeding group, the juveniles had in significantly higher mortality in mud (26.6%±5.7%) compared to the empty basin (17.7%±1.9%) and S. triqueter (10.0%±3.3%). The results showed that the larvae of E. sinensis showed preference for vegetation habitats such as S. mariqueter, which may be related to the shelter provided by vegetation. The results of this study indicate the importance of vegetation habitat for the larval stage of E. sinensis, enrich the early life history data, and provide basic reference for resource conservation and habitat restoration.