Molluscs are an important component of the mangrove ecosystem, and the vertical distributions of molluscan species in this ecosystem are primarily dictated by tidal inundation. Thus, sea-level rise (SLR) may have profound effects on mangrove mollusc communities. Here, we used dynamic empirical models, based on measurements of surface elevation change, sediment accretion, and molluscan zonation patterns, to predict changes in molluscan spatial distributions in response to different sea-level rise rates in the mangrove forests of Zhenzhu Bay (Guangxi, China). The change in surface elevation was 4.76–9.61 mm yr⁻¹ during the study period (2016–2020), and the magnitude of surface-elevation change decreased exponentially as original surface elevation increased. Based on our model results, we predicted that mangrove molluscs might successfully adapt to a low rate of SLR (2.00–4.57 mm yr⁻¹) by 2100, with molluscs moving seaward and those in the lower intertidal zones expanding into newly available zones. However, as SLR rate increased (4.57–8.14 mm yr⁻¹), our models predicted that surface elevations would decrease beginning in the high intertidal zones and gradually spread to the low intertidal zones. Finally, at high rates of SLR (8.14–16.00 mm yr⁻¹), surface elevations were predicted to decrease across the elevation gradient, with molluscs moving landward and species in higher intertidal zones blocked by landward barriers. Tidal inundation and the consequent increases in interspecific competition and predation pressure were predicted to threaten the survival of many molluscan groups in higher intertidal zones, especially arboreal and infaunal molluscs at the landward edge of the mangroves, resulting in a substantial reduction in the abundance of original species on the landward edge. Thus, future efforts to conserve mangrove floral and faunal diversity should prioritize species restricted to landward mangrove areas and protect potential species habitats.

Six transects were drawn from the seaward forest edge to the landward forest edge, and elevation was measured in 5–10 m horizontal intervals along each transect using a Global Navigation Satellite System-Real Time Kinematic GPS unit (G970 GNSS RTK, UniStrong Inc.) with a vertical precision of 15 mm (Fig. 1a). Measured elevations were converted to elevations above/below local mean sea level, which was determined based on control points located 3 km from the study area (Fig. 1a). To sample along the entire elevation gradient, a total of 36 sampling sites were established at 25 cm intervals between −15 and 150 cm elevation; there was no sampling site at 135 cm as there was a gap in the mangrove forest at this elevation.From April 2017 to January 2018, molluscs were sampled quarterly at each of the 36 sampling sites during low tide. To collect molluscs as comprehensively as possible, we sampled the arboreal, epifaunal, and infaunal molluscan communities within the mangrove forest at each site. Arboreal molluscs attached to the mangrove trunks, leaves, and prop roots were collected by hand in three randomly placed quadrats (5 × 5 m; 10 m apart) at each sampling site. To collect epifaunal molluscs, five quadrats (1 × 1 m; 5 m apart) were randomly placed at each sampling site, and all epifaunal molluscs on the sediment surfaces within each quadrat were collected. To collect infaunal molluscs, one quadrat (0.25 × 0.25 m) was randomly placed in each epifaunal quadrat. The sediment in each infaunal quadrat was collected to a depth of 30 cm and sieved through a 1 mm mesh to obtain infaunal molluscs. 

From April 2017 to January 2018, molluscs were sampled quarterly at each of the 36 sampling sites during low tide. To collect molluscs as comprehensively as possible, we sampled the arboreal, epifaunal, and infaunal molluscan communities within the mangrove forest at each site. Arboreal molluscs attached to the mangrove trunks, leaves, and prop roots were collected by hand in three randomly placed quadrats (5 × 5 m; 10 m apart) at each sampling site. To collect epifaunal molluscs, five quadrats (1 × 1 m; 5 m apart) were randomly placed at each sampling site, and all epifaunal molluscs on the sediment surfaces within each quadrat were collected. To collect infaunal molluscs, one quadrat (0.25 × 0.25 m) was randomly placed in each epifaunal quadrat. The sediment in each infaunal quadrat was collected to a depth of 30 cm and sieved through a 1 mm mesh to obtain infaunal molluscs. 

Changes in surface elevation were recorded using nine RSET instruments (Cahoon et al., 2002) at 3–12 month intervals between July 2016 and August 2020, comprising a total of eight sets of measurements; the total period assessed was 49 months. The nine RSET benchmarks were established in July 2015 and distributed among the A. corniculatum, K. obovata, and B. gymnorhiza communities along the SJ transect at depths of 5–6 m (Fig. 1b). Each RSET instrument consisted of a deep benchmark and a measuring arm. During RSET installation, a deep benchmark was established at each measurement location by driving stainless steel rods (15 mm in diameter) into the soil profile until refusal and then fixing the rods in place with cement. A machine-notched pipe was connected to the top of the stainless-steel rod to hold the measuring arm. When it was time to take a reading, the measuring arm was attached to the benchmark and levelled. Nine fiberglass pins were inserted into the measuring arm and lowered onto the soil surface, and the distance from the arm to the top of each pin was measured. These nine distance measurements were repeated in four directions, yielding 36 total measurements, which were combined to generate an average reading per sampling site.

Sediment accretion was measured using the marker-horizon method around both the RSET benchmarks and the transect sampling sites. Marker horizons around RSET benchmarks were deployed in July 2016 and were sampled at 3–12 month intervals until December 2018, comprising a total of six sets of measurements; the total period assessed was 29 months. Marker horizons around the transect sampling sites were deployed in July 2017 and were sampled at 3–6 month intervals until August 2019, comprising a total of four sets of measurement; the total period assessed was 25 months. Three feldspar markers (50 cm × 50 cm) were deployed around each sampling site or RSET benchmark. At each sampling time, one cubic soil core was taken at an undisturbed location on each marker horizon, and the depth of the sediment above the marker horizon at three positions in each core was measured to give an average reading per marker horizon. The mean depth across the three cores was used to represent sediment accretion at each sampling site or RSET benchmark.

Abbreviation: Nn, Nassarius nodifer; Ns, Nassarius sinarus; Bz, Batillaria zonalis; Mc, Merisca capsoides; Cc, Cerithidea cingulata; Cf, Clithon faba; Ls, Littoraria scabra; Ny, Nerita yoldi; Bm, Batillaria multiformis; Pp, Patelloida pygmaea; Cd, Cerithidea djadjariensis; Pc, Pinguitellina cycladifomis; La, Laternula anatina; Gc, Glauconome chinensis; Ea, Enigmonia aenigmatica; Ab, Assiminea brevicula; Lp, Littoraria pallescens; Lm, Littoraria melanostoma; Is, Indoaustriella scarlatoi; Iq, Iravadia quadrasi; Os, Onchidium struma; Al, Assiminea latericea; Md, Mainwaringia dantaae; Lo, Laemodonta octanfracta; Lar, Littoraria ardouiniana; Ss, Salinator sanchezi; Isp, Iravadia sp.; Ip, Indoaustriella plicifera; Cl, Cerithidea largillierti; Ts, Terebralia sulcata; Cm, Cerithidea microptera; Pa, Pharella acutidens; Nv, Neritina violacea; Sj, Stenothyra japonica; Ala, Allochroa layardi; Isa, Iracadia sakaguchii; Gco, Geloina coaxans; Co, Cerithidea ornata; Cn, Cassidula nucleus