Estuaries represent transitional zones between rivers and oceans, and provide nursery habitat for myriad fish and benthic invertebrates (Lipcius et al., 2005; Johnson and Eggleston, 2010). Survival and growth can be promoted by the quality of shelter and foraging habitats that these environments provide (Beck et al., 2001; Vermeiren and Sheaves, 2015). The value of estuarine habitat to juveniles varies with factors such as ontogenetic stage, predation risk, prey availability, three-dimensionality of seabed structure, or condition of the habitat itself (Pfirrmann et al., 2023).

Many field and laboratory studies have demonstrated that juvenile crabs tend to associate with structurally complex habitats (Amaral et al., 2009), with seagrass beds, salt marshes, oyster reefs, and gravels having higher juvenile densities than less-structured habitats, and growth–survival ratios in vegetated habitats being higher than in non-vegetated ones (Beck et al., 2001; Polte et al., 2005; Fonseca et al., 2006; Lefcheck et al., 2019). However, highly structured habitats may not always be higher in quality (Taylor and Fehon, 2021), with growth and resource availability for the blue crab Callinectes sapidus (Seitz et al., 2005) and Dungeness crab Cancer magister (Holsman et al., 2006) greater in unstructured intertidal habitats such as mud flats and sand than in adjacent, more structured habitats.

The Chinese mitten crab Eriocheir sinensis, a native freshwater species that is widely distributed throughout China, is traditionally regarded as a culinary delicacy and a high‐valued aquatic product (Wang et al., 2016). Yield increased from 8.4 × 10³ t in 1991 to 8.1 × 10⁵ t in 2021, and its value has increased from an estimated 6.0 × 10⁷ United States Dollars (USD) in 1991 to 1.0 × 10¹⁰ USD in 2021; wild capture of E. sinensis in China was 2.5 × 10⁴ t in 2021 (FAO, 2024). While many studies have examined the ecology and resource dynamics of adults of this species, few have examined those of its juveniles.

Within the Yangtze River system, E. sinensis breeds exclusively within the estuary, wherein mating, spawning, hatching, and early development occur (Du, 2004; Chen and Du, 2017). During winter, females spawn where salt and freshwater mix, where hatched larvae ultimately settle (Chen and Du, 2017). Megalopa and juveniles are transported by the tides into the upper estuarine reaches (Du, 2004), where mortality and growth vary significantly with habitat type and environmental condition (Liu, 2015, 2017). Therefore, Yangtze River estuarine habitat may play an important role in the migration, early development, and supplementation of the wild-capture E. sinensis fisheries resource.

Few studies have investigated juvenile E. sinensis habitat use in Yangtze Estuary (Zhao et al., 2020). We report (1) crab abundance in different habitats, (2) spatiotemporal variation in juvenile abundance and size, and (3) correlations between abundance of juveniles and environmental parameters. An improved understanding of each of these is important for more sustainable management of this species and its habitat.

Juvenile E. sinensis were found between June 2021 and January 2022; no juvenile crabs were found in February 2022. During sampling, 4567 juveniles (mean CL ± standard deviation (SD), 5.5 ± 2.1 mm) of CL 2.3–15.5 mm were collected. These crabs were attributed to stages J1 (1406 ind, 31%), J2 (556, 12%), J3 (1096, 24%), J4 (1019, 22%), and J5 (490, 11%). Juveniles were collected from marsh, and root belt and gravel habitats, but not from mud flats. No megalopa was collected intertidally.

We report the distribution of juvenile E. sinensis in intertidal habitat along the southern branch of the Yangtze Estuary from June 2021 to January 2022. Because the greatest juvenile crab densities occur in root belt and gravel habitats, followed by marshes, and no juveniles were found on mud flats, juvenile E. sinensis appears to prefer more complex habitat. This conclusion is consistent with accounts of E. sinensis mainly occurring in pebble habitat in the intertidal of the Thames Estuary (Gilbey et al., 2007), and in addition to pebbles, in Scirpus sp. and Typha latifolia habitats in the intertidal San Francisco Bay (Rudnick et al., 2003). Subtidally, juveniles mainly frequent densely vegetated shallow waters, with the largest populations found in the waterweed Egeria densa (Rudnick et al., 2003). Crustaceans typically prefer structurally complex habitats. For example, juvenile Callinectes sapidus are significantly more abundant in vegetated habitats than in unstructured mud flat and sand habitats (Hovel and Lipcius, 2001), larval Cancer magister densities are significantly higher in mussel than mud flat habitats (Fernandez et al., 1993), and larval numbers of green crab Carcinus maenas are significantly higher in filamentous algal, diatom, and mussel bed habitats than they are in open sandy areas (Moksnes, 2002).

The quality of refugia regulates populations of crab species (Shervette et al., 2004). Juvenile E. sinensis are abundant in root belt habitats, which may be related to habitat-specific food resource availability (Taylor and Fehon, 2021). Vegetation, rich in organic matter and detritus, supports high densities of benthic in- and epifauna, including important prey for juvenile E. sinensis. Root belt habitat might also offer size-suitable refugia for juveniles. Because juvenile decapod crustaceans benefit from shelter (Stevens and Swiney, 2005), and the structure of root belt and gravel habitats is more complex than that of marshes, they might provide more refugia for juvenile crabs (Shervette et al., 2011). A lack of suitably sized shelter might also explain why no large E. sinensis were collected intertidally (Bromilow and Lipcius, 2017).

Many indoor experiments have reported habitats such as aquatic plants and simulated shelters to significantly increase the developmental rate of juvenile E. sinensis (Liu, 2015). We report juvenile E. sinensis to have higher CL and RGR in root belt and gravel habitat than in marsh habitat from June to August. This complex habitat structure helps juveniles to avoid predators, and to store energy for development. Additionally, because crabs when molting are relatively weak and vulnerable to attack, this complex habitat provides suitable shelter to avoid attacks, thereby increasing molting success. Therefore, although the intertidal wetland in the Yangtze Estuary has been reduced by 36% over the past three decades (Chen et al., 2016) because of frequent large-scale intertidal reclamation, invasion by Spartina, and artificial wetland alteration (Zou et al., 2016), what remains is still important for recruitment and maintenance of E. sinensis populations.

Juvenile E. sinensis density varies over time. The greatest abundances occur in June, when stage J1 and J2 crabs are prevalent. Early juvenile abundance is closely related to spatiotemporal variation in megalopa settlement (Etherington and Eggleston, 2000). Eriocheir sinensis recruitment in Yangtze Estuary occurs in early June, over the space of one week (Li et al., 1997), and crab densities decrease rapidly after June. During recruitment from 2003 to 2013, megalopa catch varied from 3 to 32 t (Wang, 2019). Following recruitment, juveniles migrate into the upper reaches of the Yangtze Estuary. Additionally, the decrease in density of juvenile crabs may correlate with juvenile survivorship, with marine invertebrate mortality often being higher during earlier life stages (Wang and Haywood, 1999). Survival of juvenile crabs often increases with size (Johnson et al., 2008; Hultgren and Stachowicz, 2010). In aquaculture, the survival rate of early juvenile E. sinensis is low (Yang et al., 2018). Liu (2017) reported crabs to have a survival rate of 48% from megalopa to stage J5, and 79% from J5 to J10. Early juvenile crab mortality in the field usually occurs because of predation (Wilson et al., 1990; Thiel and Dernedde, 1994). Large-sized conspecific crabs, intertidal fishes, and migratory birds may also all be predators of juvenile crabs (Hedvall et al., 1998; Lipcius et al., 2005; Choi et al., 2017), with indoor experiments having reported cannibalism to occur in juvenile E. sinensis during periods of food shortage (Zeng et al., 2018). Migratory shorebirds stopping over at the Chongming Wetland also prey extensively on benthic macroinvertebrates (Zhu et al., 2007; Li et al., 2023). Although we might have expected the juvenile population to decline over time because of this migration, densities (mainly comprising stage J3–J5 juveniles) remained relatively stable from July to November of 2021. This may be because of continued juvenile recruitment, because a small number of J1 crabs were collected over the sampling period, or as a function of abnormal growth. In E. sinensis culture, frequent water level changes, high population densities, low dissolved oxygen levels, low vegetation cover, food shortage, and irregular feeding can contribute to abnormal growth (Sun and Wu, 1996), producing small-sized “lazy crabs” that remain in their burrows and neither feed nor move. Lazy crabs are extremely small, usually the size of an early juvenile crab (Sun and Wu, 1996). The semi-diurnal tides in Yangtze Estuary cause frequent changes in tidal height, which may result in lazy crabs. Reductions in intertidal wetland habitat may also lead to high densities of juvenile crabs in the Yangtze Estuary and food shortage, resulting in more lazy crabs.

Juvenile crabs in the upper estuarine reaches are generally larger than those in the lower reaches, consistent with the early migratory life history of the E. sinensis. The spatial and temporal distributions of decapod juveniles are closely related to biotic and abiotic factors (González-Ortegón et al., 2023). We report water temperature to positively, significantly correlate with juvenile E. sinensis density. After December 2021, numbers of juvenile E. sinensis decreased rapidly. It is possible that juvenile E. sinensis migrate to deeper riverbeds, where water temperatures are higher, and that temperature affects their spatial clustering. Consistent with our results, juvenile E. sinensis densities in intertidal zones during winter were also significantly lower than during summer (Gilbey et al., 2007). Low water temperature may hinder crustacean feeding and growth (Frederich et al., 2000) and contribute to juvenile E. sinensis migration during this period.

Understanding juvenile E. sinensis habitat requirements is important for their conservation and management. Understanding the role of estuaries in recruitment requires knowledge of appropriate physicochemical conditions, prey abundance, habitat availability, and interactions with other organisms. Our results from the southern branch of the Yangtze River along Chongming Island indicate that vegetated and gravel habitats should be prioritized for conservation as essential nursery grounds for this species.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

YW: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft. SW: Conceptualization, Data curation, Investigation, Methodology, Writing – review & editing. FZ: Conceptualization, Data curation, Funding acquisition, Investigation, Methodology, Resources, Writing – review & editing. ZQ: Data curation, Investigation, Writing – review & editing. ZG: Data curation, Investigation, Writing – review & editing.