Twenty sites located in the Cameroon coastline were chosen according to mangrove scope in six localities, the importance of human activities in ecosystem and variability of the stands (Figure 1). The sites were distributed as follows: two sites in Limbe, three in Tiko, seven in Douala, three sites in Mouanko, three sites

in Kribi and Two in Londji (Table 1). Human activities within the mangrove habitats are the excessive harvest of mangrove trees for firewood, charcoal, clearing of mangrove areas for agricultural purposes coupled with the rapid urbanization of the adjacent towns and pollution [1] [17] [18] [19], which have led to a gradual degeneration of these ecosystems.

Two types of climate dominated the study area. In the northern and central parts, climate belongs to a particular equatorial regime called Cameroonian regime, characterized by a long rainy season (March-November) and a short dry season (December-February) with an annual average temperature of about 26.7˚C [17]. Climate in the southern region is of a typical equatorial regime with four seasons (two rainy seasons and two dry seasons well individualized), marked by high and stable temperatures of about 28.7˚C. Heavy annual rainfalls are observed from more than 5000 mm in Limbe area and decreased to about 3000 mm in Kribi area. The tidal regime along the coastline reaches 3 m in the Cameroon Estuary, 1.2 m in the mouth of Nyong and 1.5 m in Kribi [20].

These three coastal regions are also distinguishable in terms of mangrove area (small patches in the South and large units in the Cameroon estuary and Rio del Rey estuary which gathered a great part of mangroves in Cameroon) and anthropogenic pressure [21]. Soils are gray or black muds, of silty, sandy or clay texture, and deriving from fluvial sediments [22]. The flora has consisted of seven

indigenous or true mangrove species (Acrostichum aureum L., Avicennia germinans (L.) Stearn, Conocarpus erectus L., Laguncularia racemosa (L.) Gaertn. F, Rhizophora harrisonii Leechman, R. mangle L. and R. racemosa GFW Meyer) with the last species being the most dominant [1]. These species live in association with several others considered like associates. The faunal component includes vertebrates, such as birds, reptiles and fish, and a wide range of invertebrates, mainly crabs and molluscs which constitute the bulk of benthic diversity [8] [16] [22] [23].

To estimate degradation, human activities that occurred in mangrove in different sites were noticed. Anthropogenic disturbance within each site, associated with transect line, were valued and patch scoring the degree of degradation according to four (04) categorical variables:

· Vegetation (V): Evidence of logging on mangrove species and presence of invasive species.

· Trampling (T): The presence of human footprints.

· Waste (W): The presence of solid wastes on the forest floor and entangled on the roots.

· Structure (St): The evidence of human modifications of mangrove hydrology and/or topography.

According to the local situation, each variable was scored from 0 to 5 where:

· 0 = absence of damage;

· 1 = very little damages;

· 2 = little damages;

· 3 = average damages;

· 4 = high damages;

· 5 = very high damages.

The score of each categorical variable was combined and summarized into Anthropogenic Disturbance Index (ADI). According to [24], values for each variable were average each local/site context and summarized as follows: V + T + W + St = ADI. Therefore, the ADI ranged from 0 to 20 which characterizes the mangrove degradation level. Three mangrove degradation levels (low, medium or moderate and high) were recognized, considering the value of ADI in any site:

· 0 < ADI < 6 equivalent to low degradation;

· 7 < ADI < 12 equivalent to moderate degradation;

· ADI > 12 equivalent to high degradation.

Molluscs were caught by hand and counted within 1 × 1 m² plots placed at three zones established using a straight line transect of 100 m perpendicular to the water shoreline in the mangrove vegetated area as mentioned by [25]. Arboreal species were also harvested by hand from roots and stems of mangrove trees [26]. To quantify abundance of molluscs, the transect is carefully looked through to search specimens that were not collected in the plots. Given the variability of each site, the distance between two transects was about 100 m. Individuals were sorted out by eye, identified, when possible, in the field and counted. Fully unidentified collected molluscs were washed and preserved in 70% alcohol then were taken to laboratory for further identification using available data [14] [15] [16] and molluscs database (https://www.molluscabase.org/) websites.

Depending on the presence/absence of each species and the number of its individuals in the different sites, the following annotations have been retained:

· − (absent) = no individual of the species was found;

· + (low recorded) = species is present and its number of individuals is less than or equal to 5;

· ++ (medium recorded) = species is present and its number of individuals is between 6 and 50;

· +++ (high recorded) = species is present and its number of individuals is greater than 50.

The occurrence of the sample species was calculated as following [27]:

F = P i P × 100

where F represent the frequency of occurrence (%), Pi and P are the sample sites where species i is present and the total number of sites surveyed respectively.

The result obtained were used to qualified the species as rare (0 - 15%), occasional (15% - 25%), common (25% - 50%), constant (50% - 75%) or ubiquitous (75% - 100%) [28].

The abundance and species richness (Margalef index) of different sites and relative density of each species were assessed, likewise, diversity index of Shannon-Weaver (H’) and the evenness index of Pielou (J’) of each site were calculated using PAST v 3.05” software. ADI, mean and standard deviation of data were assessed using Microsoft Office Excel 2019.

Between different categorical variables choose to assess degradation, vegetation was highly affected in Apollo beach, Mbomè, Bois des Singes, Essenguè, Pont du Wouri, Village, Mpalla and Nziou. Trampling was well marked in down beach, Bois des Singes, Bonendalè, Essenguè, Village and Nziou. Concerning dumping of wastes in mangrove ecosystems, Bois des Singes was the most affected. The mangrove structure was highly altered at Pont du Wouri and Appolo Beach sites (Table 2).

An assessment of the mangrove degradation, based on the scores of the variables affected by human activities is given in Figure 2 below. Among the 20 sites sampled, depending on the damage caused by human activities, the anthropogenic disturbance index (ADI) was higher in the Bois des Singes site (15) and lower in Londji I (3). This index permitted to distinguish four (04) slightly degraded sites (Mbiako, Londji I, Yoyo I and Yoyo II), nine (09) moderately degraded (Down beach, Londji II, Mabeta, Avion beach, Mbomè, Bonamouang, Bonamoussadi, Mpalla and Mpolongwe) and seven (07) highly degraded (Apollo beach, Bois des Singes, Bonendalè, Essenguè, Pont du Wouri, Village and Nziou).

A total of 34 species of molluscs were collected during this study. They comprised two classes: Bivalvia (4 species) and Gastropoda (30 species). The list of different species collected is shown in Table 3. Pachymelania fusca showed higher occurrence percentage (100%) found to be high recorded in the different assemblages, while most species with low occurrence percentage (5%) showed

low numbers of individuals in the sites where they were presents. However, Potadoma liricincta is medium recorded in Bonamouang although with a low percentage of occurrence. The bivalve Crassostrea gasar is medium recorded or high recorded in the sites where it was collected (medium recorded at Down beach; Apollo beach and Avion beach; high recorded at Mbomè, Londji II and Mpalla.

Figure 3 shows distribution of occurrence among these species. Six species are rare (Cyrenoidarosea, Potadomaliricincta, UD1, UD2, UD3 and UD4), fourteen species are uncommon or occasionals (Egeria radiata, Iphegenia rostrata, Littorina sp., Melanopsis sp., Murex sp., Neritilia rubida, Neritina adansoniana, N. lineolata, N. senegalensis, Pachymelania byronensis, P. quadriseriata, Theodoxus sp., Melanopsis pergracilis and UD5), nine species are commons (Crassostrea gasar, Achatina achatina, Littorina angulifera, Neritina glabrata, Pachymelania granifera, P. mutans, Pachymelania sp., Thais callifera and Melanoides tuberculata), two are constants (Pachymelania aurita and Theodoxux niloticus) and three species are ubiquitous (Pachymelania fusca, Tympanotonus fuscatus and Tympanotonus radula).

Following classification of mangrove based on anthropogenic disturbance index (ADI), abundance of molluscs, species richness and diversity index given are the means of cumulative sample values in different sampling sites. Globally, slightly differences were observed in abundance of molluscs, species richness and Shannon-Weaver index among degradation levels of mangrove stands (Figure 4). The abundance of molluscs and species richness were higher (respectively D = 172.25 ± 73.09 Ind·m² and R = 1.29 ± 0.56) in areas with a low degradation level. Areas with high degradation level presented low values of abundance (D =

100.57 ± 62.84 Ind·m⁻²), of species richness (R = 1.09 ± 0.15) and of Shannon-Weaver index (H’ = 2.30 ± 0.31). The high value of mean Shannon-Weaver diversity index was found in moderate degradation level (H’ = 2.64 ± 0.40).