Introduction

The transitional area separating the sea and terrestrial environment is the intertidal zone or the littoral zone. This zone will be submerged with water at high tide and remains uncovered at low tide. The environmental conditions of littoral zones are frequently changing and challenging, organisms that are adapted to such conditions are found in intertidal zones. Organisms living in an intertidal area are inherently tolerant to a wide range of natural conditions (Halim et al. 2019). The intertidal zone is characterized by extreme changes in salinity, desiccation, temperature, currents, and predators. Hence, many organisms possess special adaptive features to overcome these challenges.

Molluscs are one of the phyla that form an important component of intertidal fauna (Chapman 2012). Molluscs play a significant role in the ecosystem by providing food sources to aquatic animals, migratory birds, and humans (Halim et al. 2019). Molluscs feed on algal matter, other snails, and detritus (Ramesh and Ravichandran 2008). Molluscs form a key link in the food chain and have a high socioeconomic value for coastal fisheries. Coastal Karnataka, comprises sandy and rocky shorelines which are with rich faunal communities with distinctive morphological characters forming the benthic/pelagic realm of the marine ecosystem. The sandy beaches lying adjacent to the rocky stretch includes pebbles, boulders, and small rock submerged in seawater during high tide. The studies on the diversity of molluscs in intertidal habitats of coastal Karnataka are limited (Haragi and Naik, 2012; Tenjing et al. 2018). The factors governing the distribution of molluscs on the coast are not known. Hence, we hypothesize that.

  1. 1.

    The beaches of coastal Karnataka vary in terms of molluscan species richness and abundance.

  2. 2.

    The physicochemical factors have an impact on the diversity of molluscs of the coast.

  3. 3.

    Intertidal habitats of coastal Karnataka provide novel habitats for molluscs.

The objectives of the study are to know the diversity of molluscs in the intertidal habitats of Karnataka and the relationship between environmental, physicochemical parameters on the diversity.

Materials and methods

Study area

Coastal Karnataka lies on the West coast of India, bordered by Goa and Kerala states in the north and south. It includes 3 districts namely Uttara Kannada, Udupi, and Dakshina Kannada. 13 sites of coastal Karnataka were selected for the study based on the type of shore (rocky or sandy) and to have the complete study of all beaches of coastal Karnataka (Fig. 1). The characteristic features of the selected sites are presented in Table 1.

Fig. 1
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Map showing study sites of coastal Karnataka

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Table 1 Characteristics of the sampling sites selected for the study
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The beaches of Karnataka are under the influence of semi-diurnal tides, with the highest high tide 1.66 m and lowest low tide 0.1 m. The average tidal range of coastal Karnataka is 1.58 m during spring tide and 0.72 m during neap tide (Kumar et al. 2012).

Sampling method:

Six intertidal habitats (Someshwar, Panambur, Sasihithlu, Kaup, Kota-Padukere, Gangolli) were visited during low tide every month from February 2017 to January 2018. Beaches of Uttara Kannada district (Murudeshwar, Honnavar, Rabindranath Tagore beach, Majali) were once during each season whereas Om beach of Gokarna, NITK beach, and St. Mary’s islands was visited once during the study, and molluscs were collected by handpicking. Systematic random sampling was followed and a total area of 600 m area was sampled. At each site, three localities which are 300 m apart from each other were selected for sampling (Fig. 2).

Fig. 2
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Google earth images showing intertidal areas of a Someshwar, b Panambur c NITK d Sasihithlu e Kaup f Kota-Padukere g St. Mary’s Islands h Gangolli i Murudeshwar j Honnavar k Om beach l Rabindranath Tagore beach (m) Majali. (Yellow dots represent the sites of molluscan shell collection)

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Based on the type of shore, erratic substratum, and limited time for sampling (two hours) during the low tide, the beaches were classified into 3 zones based on tidal height namely high tidal zone (HTZ), mid-tidal zone (MTZ), and low tidal zone (LTZ). The collection of live molluscan shells was carried out using quadrates (Flores-Rodrigues et al. 2014). The quadrates were placed on the tidal zones over an area parallel with the shoreline. At each study site, samples from nine quadrates of 0.25m2 were taken from each tidal level. A total of 27 quadrats were placed at fixed intervals at selected beaches and samples were collected. In addition, molluscan species found outside the quadrates, within the area of 600 m were also collected. They were brought to the laboratory and preserved in 70% alcohol. Molluscan species were identified using the standard identification keys (Satyamurthy 1952; Ramakrishna and Dey 2007: Apte 2014).

Analysis of abiotic parameters of the environment and physico-chemical parameters of water.

Precipitation data of the study area was obtained from the Giovanni system of NASA GES DISC. Abiotic parameters such as air and water temperature were recorded in the field using a conventional thermometer and values were expressed as degree centigrade (°C), wave frequency was measured as the number of waves reaching the shore per minute. Physico-chemical parameters of water such as salinity, the electrical conductivity of water were analyzed using a Systronics water analyzer (Model 193). pH of the water was analyzed using a digital pH meter (Systronics MK VI) and the results were expressed in pH units.

Abundance was represented as a percentage. Species richness was calculated as the number of species of molluscs found per quadrate. Diversity indices such as the Shannon-Weiner diversity index, Simpson’s index, and Pielou’s evenness index were calculated for each site using PAST 3.0 software. Monthly visits were pooled for every four months and presented as seasons viz. Pre-monsoon (February-May), Monsoon (June–September), Post-monsoon (October-January). Mean and Standard deviation was calculated for data that followed a normal distribution and one-way ANOVA with Bonferroni multiple comparison tests was used to analyze the data between the groups. Kruskal–Wallis test and Mann–Whitney test were applied for the data that showed non-normal distribution. Spearman correlation was performed to find out the relationship between the variables. Data were analyzed using SPSS software 20.

Results:

Diversity of intertidal molluscs of coastal Karnataka

A total of 36 species of molluscs were recorded belonging to 21 families and 30 genera from coastal Karnataka (Table 2). It includes 21 species of gastropods and 15 species of bivalves (Fig. 3a and b). Among the gastropods, the family Muricidae was represented by more number of species (4), whereas in bivalves number of species belonging to the family Veneridae were highest (5) during the study.

Table 2 Distribution of molluscs in the study sites
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Fig. 3
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a Intertidal gastropods of Karnataka coast (a = Abapertural view, b = Apertural view; Scale bar = 5 mm). * 1. Babylonia spirata, 2. Turricula tornata fulminata, 3.Gyrenium natator, 4. Lyncia lynx, 5. Littoraria scabra, 6. Littoraria undulata, 7. Echinolittorina leucostica, 8. Indothias lacera, 9. Purpura bufo, 10. Cellana radiata, 11. Semirincinula tissoti, 12.Oronia konkanensis, 13. Bullia melanoides, 14. Nerita textilis, 15. Nerita oryzarum, 16. Agaronia gibbosa, 17. Oliva oliva, 18.Planaxis sulcatus, 19.Clanculus scabrosus, 20. Turbo bruneus 21.Clypedina notata. b: Intertidal bivalves of Karnataka coast (a = External view, b = Internal view; Scale bar = 5 mm). *1. Donax pulchellus, 2. Donax incarnatus 3. Donax scrotum 4. Mactrinula striatula 5. Mactra luzonica 6. Brachydontes striatulus 7. Perna viridis 8. Saccostrea cucullata 9. Siliqua radiata 10. Abranda myeformis 11. Sunetta solanderii 12. Sunetta scripta 13. Sunetta concinna 14. Paphia textile 15. Dosinia exoleta

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Bullia melanoides was found on all the beaches except Someshwar beach, Om beach, NITK beach. Abranda myeformis was confined to intertidal area of Kota-Padukere. Donax sp were present in all the sandy beaches of coastal Karnataka. Clanculus scabrosus, Babylonia spirata, Mactrinula striatula were the rare species found along the coast. Molluscs such as Gyrenium natator, Saccostrea cucullata, Perna viridis, Cellana radiata, Semirincinula tissoti, Littoraria scabra and Echinolittorina leucostica were collected from NITK beach. Turbo bruneus, Purpura bufo, and Semirincinula tissoti were rare shells at St. Mary’s Islands.

Of the beaches of Uttara Kannada highest number of molluscan species were found in Majali beach (14) and the least species of molluscs (3) were observed in Murudeshwar beach. Turricula tornata fulminata was unique to Majali beach. Dosinia exoleta was restricted to Rabindranath Tagore beach only. Species of the family Neritidae were restricted to Om beach of Uttara Kannada district.

Distribution and seasonal variations in diversity of intertidal molluscs of coastal Karnataka.

The abundance of molluscs was maximum in post-monsoon and minimum in monsoon at all the study sites. However, the intertidal zone of Someshwar showed the abundance of molluscs in pre-monsoon (Fig. 4). Species richness of molluscs was observed post-monsoon in the beaches studied except Kaup and Kota-Padukere. Echinolittorina leucostica was abundant (38%) in the intertidal area of Someshwar whereas Brachydontes striatulas was the rare and unique species to this site. Donax species were more in number (92%) during the study period in Panambur beach. Mactra luzonica, Sunetta solanderii, Sunetta scripta were the rare species at Panambur beach.

Fig.4
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a Molluscan abundance and b species richness in the study sites (Mean ± Standard deviation for three observations)

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Bullia melanoides contributed 47% to the total abundance of molluscs at Sasihithlu beach whereas Sunetta sp, Agaronia gibbosa and Oliva oliva were the least in number (0.21%). Gastropods such as Agaronia gibbosa, oliva oliva and bivalves like Brachydontes striatulus, Perna viridis, Sunetta sp were the rare shells found in Kaup beach. At Kota-Padukere beach, Donax incarnatus and Bullia melanoides were found in all the seasons. Among these Donax pulchellus was abundant (57.22%) on this beach whereas Abranda myeformis, Oliva oliva, Mactra luzonica and Siliqua radiata showed the unusual occurance. Bullia melanoides and Donax incarnatus were found at Gangolli beach and the gastropod Bullia melanoides (88%) were abundant on this beach during the study.

The abundance and species richness of molluscs showed prominent variations in the beaches of Uttara Kannada with the seasons. Molluscan abundance was observed in monsoon and pre-monsoon in Murudeshwar beach (40%), premonsoon at Honnavar beach (98%). The number of molluscan individuals and species was more in post-monsoon in the intertidal area of Rabindranath Tagore beach (60%) and Majali beach (50%).

Species diversity

The diversity indices indicate the ecological status of the habitat. Shannon–Wiener diversity index takes into account species richness and their evenness in an area. The value of 0 indicates less diversity and 1 refers to more diversity. Simpson’s index is a weighted arithmetic mean of proportional abundance species. The value ranges from 0–1, the lesser the value lesser is diversity, and vice versa. Evenness refers to uniformity in species distribution. The evenness index ranges between 0–1, with 0 being no evenness and 1 being complete evenness.

Shannon-Weiner index and Simpson index of molluscs at were maximum in post-monsoon at the littoral zone of Someshwar (H´ = 1.108, 1-D = 0.408, J´ = 0.250), Panambur (H´ = 0.586, 1-D = 0.396, J´ = 0.898) and Gangolli (H´ = 0.603, 1-D = 0.596, J´ = 0.913). Shannon and Simpson diversity indices were highest in post-monsoon (H´ = 0.575, 1-D = 0.372, J´ = 0.592) in the intertidal area of Sasihithlu, and least diversity was observed in monsoon as single species was found in the intertidal area.

Shannon and Simpson diversity indices were observed in the intertidal area of Kaup (H´ = 1.518, 1-D = 0.729, J´ = 0.456) in pre-monsoon and least diversity in monsoon. The distribution of molluscs was more even in pre-monsoon than the other seasons at the Kaup beach. Shannon and Simpson diversity indices were higher in pre-monsoon in the intertidal area of Kota-Padukere (H´ = 1.14, 1-D = 0.490, J´ = 0.347) whereas the least values of diversity indices were observed in post-monsoon. The distribution of species was more uniform in post-monsoon in the intertidal zones of Someshwar, Panambur, Sasihithlu, and Gangolli. However, molluscs were distributed uniformly in pre-monsoon on the shores of Kaup (J´ = 0.456) and Kota-Padukere (J´ = 0.604).

Composition of molluscan families:

The percentage composition of molluscan families at different sites varied with the seasons (Fig. 5). The intertidal habitats differ in the composition of molluscan species. At Someshwar, the dominance of Mytilidae was observed whereas the dominance of Littorinidae was seen in post-monsoon. At the littoral zone of Panambur, the number of individuals of Nassaridae was more in pre-monsoon and they decreased numerically in monsoon and least in post-monsoon. However, Donacids were abundant in post-monsoon., Nassaridae members were dominant and their number declined in monsoon at the sandy shore of Sasihithlu. In this site, members of the family Veneridae were found in monsoon. Molluscs belonging to Donacidae showed the highest occurrence followed by Nassaridae and Olividae at Sasihithlu beach.

Fig. 5
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a-f Percentage composition of molluscan families in the selected sites

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At Kaup beach, the occurrence of Mytilidae members was observed; Nassarids were the least in number in pre-monsoon. The dominance of Littorinidae and Nacellidae was observed in monsoon and pre-monsoon respectively. Donacidae members dominated the shore of Kota-Padukere in pre-monsoon and monsoon but their number declined in post-monsoon. However, the number of Nassaridae members was more in post-monsoon. Nassaridae dominated the intertidal area of Gangolli in all the seasons, were highest in monsoon. Donacidae members were maximum in post-monsoon at Gangolli beach.

Kruskal–Wallis test was applied to check the variations in species diversity with the seasons and between the sites (Table 3). Significant differences were observed between abundance (P = 0.001) and seasons; species richness (P = 0.002) of molluscs and the seasons. There is a significant difference in abundance (P = 0.031) of molluscs with the sites. However, there is no significant difference in species richness overall between the sites.

Table 3 Kruskal–Wallis test for the seasonal variations in species diversity in the intertidal habitats of the coast
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The six sites that were visited on monthly basis were considered for statistical analysis. To find out the significant differences in abundance if any between the seasons and sites, the Mann–Whitney test was carried out. There is a significant difference in species abundance between Someshwar and Sasihithlu (U = 30.5, P = 0.016), Panambur, and Sasihithlu (U = 36, P = 0.037), Someshwar and Gangolli (U = 28, P = 0.011) (Supplementary information 1). There were significant differences in abundance (U = 141, P = 0.002) and species richness (U = 172, P = 0.016) between pre-monsoon and post-monsoon, monsoon, and post-monsoon. However, there were no significant differences in species diversity in pre-monsoon and monsoon.

Table 4 shows the abundance and species richness of molluscs in the intertidal zones during the study. The abundance and species richness of molluscs were highest in the low tide zone followed by the mid-tide zone and least in the high tide zone of the selected sites. This indicates the vertical distribution of molluscs i. e-distribution of molluscs from low tide zone to high tide zone in the intertidal zones of the coast. The abundance of molluscs was maximum in mid-tide zone of Panambur (69.70%) and species richness in the low tide zone of Someshwar (1.69 ± 0.61).

Table 4 Variation in abundance and species richness of molluscs in the three tidal zones of coastal Karnataka (Mean ± Standard Deviation, * ± for 12 observations)
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Abiotic parameters of the environment and physico-chemical parameters of water:

Air temperature ranged from 28.250C-30.250C, 27.620C-29.220C, and 26.500C-28.750C in pre-monsoon, monsoon and post-monsoon in the selected sites. The highest and least values of air temperature were recorded at Kaup in premonsoon (30.25 ± 1.03) and Someshwar in postmonsoon (26.50 ± 0.63). Water temperature varied from 28.750C-30.250C, 27.750C-29.420C, 29.750 C-30.250C in pre-monsoon, monsoon and post-monsoon. The highest values of water temperature were observed in Sasihithlu (30.25 ± 2.06) and least in Panambur (27.75 ± 1.66) in monsoon (Supplementary information 2). The mean values of wave frequency ranged from 6–9 in pre-monsoon, 6–7, in monsoon, and post-monsoon in the selected sites. Highest number of waves were recorded from the intertidal area of Panambur (9.00 ± 2.16) and the least number of waves from Kaup (6.00 ± 0.81).

The pH of water ranged from 8.22–8.47 in pre-monsoon, 7.93–8.15 in monsoon, 7.36–7.94 in post-monsoon in the selected sites. The highest (8.47 ± 0.12) and least values of pH were recoded from Kota-Padukere beach (7.36 ± 0.18) in post-monsoon. The conductivity of water varied from 41.56–45.81, 41.42–57.90, 48.36–51.55 in pre-monsoon, monsoon, and post-monsoon respectively (Supporting information 2). Mean values of conductivity were maximum and minimum in Kaup (57.90 ± 3.70) and Sasihithlu (41.42 ± 8.90) in monsoon. As the electrical conductivity of water increased species richness molluscs increased in the sampling sites. The range of salinity was between 30.84–33.66 in pre-monsoon, 21.7–31.2 in monsoon, and 27.22–30.05 in post-monsoon. The highest (31.2 ± 2.93) and least (21.7 ± 4.71) values of salinity were recorded from Kaup and Sasihithlu in the monsoon. Precipitation was in the range of 0.12–50.15 mm/month.

ANOVA was carried out to test the significant difference in physicochemical parameters in different seasons and the results are given in Table 5. Mean values of pH, conductivity, salinity, water temperature, air temperature, wave frequency overall differed significantly with the seasons (P < 0.05).

Table 5 ANOVA for the seasonal variations in physico-chemical parameters of the coast
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To find the significant differences in physicochemical parameters in the seasons Bonferroni multiple comparison tests were performed (Supporting information 3). There was a significant difference in pH of water in pre-monsoon and post-monsoon (P < 0.05), monsoon, and post-monsoon (P < 0.05). Electrical conductivity showed significant changes in pre-monsoon and monsoon (P < 0.05), monsoon, and post-monsoon (P < 0.05). Salinity varied significantly between pre-monsoon and monsoon (P < 0.05). However, there were no significant variations in salinity from monsoon and post-monsoon. Significant differences in water temperature were observed between the seasons (P < 0.05). The variations in air temperature were statistically significant between pre-monsoon and monsoon (P < 0.05), pre-monsoon, and monsoon (P < 0.05). Wave frequency varied significantly from pre-monsoon to monsoon (P < 0.05).

Correlation analysis

When Spearman correlation was performed, a significant correlation was observed between electrical conductivity and species richness (ρ = 0.277; P < 0.05). Wave frequency correlated significantly (ρ = -0.259, P < 0.05) with the abundance of molluscs (Table 6). However, precipitation, pH, air temperature, water temperature, and salinity did not correlate significantly with species abundance and species richness.

Table 6 Correlation of molluscan abundance and richness with physico-chemical parameters
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4: Discussion:

The present study documented 36 species of molluscs from the intertidal habitats of Karnataka. Of these, 26 species have been reported by Haragi and Naik (2012) from Uttara Kannada coast. Saccostrea cucullata and Semirincinula tissoti were recoded from the Karnataka coast in the present study. Balasaheb et al. (2017) reported these species from the Mumbai coast. Donax sp and Sunetta sp were seen in Panambur and Kota-Padukere beaches of coastal Karnataka. Lokhande et al. (2014) observed the presence of these species in the Ratnagiri coast of India.

Of the beaches of Uttara Kannada, presence of fewer molluscan species in Honnavar beach than the Rabindranath Tagore beach and Majali beaches may be attributed to a steep slope and coarse sand (Barboza et al. 2012). An increase in beach slope decreases the species diversity and coarse sand lessens the growth, burrowing rate, and metabolism of intertidal fauna (De La Huz et al.2002). The least diversity of molluscs in Murudeshwar beach may be due to extensive anthropogenic activities in this intertidal area (D’Souza and Shenoy 2020).

Of the beaches that were visited regularly, the maximum number of species was recorded from the rocky shore of Kaup (11) and the least number of species from Gangolli (2) during the study. This variation in species richness in different sites reveals the suitability of the habitats. Lack of habitat complexity may be the reason for fewer species in the intertidal habitat of Gangolli. Further, this site has been subjected to sand mining for the construction of seawalls. This causes habitat degradation which results in the decrease of molluscan abundance. The species richness of molluscs and their distribution in the rocky shores of Kaup and Someshwar could be related to the presence of suspension feeders and filter feeders such as mussels and oysters on rocky shores. McQuaid et al. (1985) suggested that the mussels and oysters prefer rocky shores for feeding.

The Nassarid, Bullia melanoides was found on the sandy shores of the coast but absent in the rocky shore of Someshwar which is probably correlated to the habitat preference of these gastropods. Species belonging to Olividae and Nassaridae were confined to the sandy shores. The presence of specialized features in these species for sand burying such as the slippery-smooth nature of shells, large, powerful foot, presence of terraced axial ribs, and sutures are advantageous for these molluscs to inhabit sandy shores. These adaptations help in movement through sand efficiently, more importantly, reduce vibrational cues to potential predators, enabling them to bury rapidly (Vermij 2017). Pandey et al. (2018) suggested that Nassarids prefer soft sandy substratum and are distributed on shores with fine sand and less beach slope. Thus, it is evident that habitat heterogeneity plays a major role in the distribution of molluscan species.

The abundance of Littorinids was noted from the rocky shores which may be owing to the presence of suitable microhabitats. Chapman (1994) showed that microhabitat variables as the slope of the rock surface, the presence of pits and shallow pools, and barnacles determine the distribution of molluscs. Further, the abundance of littorinids is possibly accounted for the environmental conditions of the intertidal zones. Veras et al. (2013) showed that the environmental components in the intertidal zones help in the formation of microhabitats that contribute to the establishment and survival of molluscs. Such environments affect the number of gametes formed, released, and also the survival of recruits (Yennawar and Tudu 2014). The abundance of littorinids in rocky shores may be correlated to the smaller shell size and higher breeding potential of molluscs during monsoon and their emergence in post-monsoon when the environment becomes suitable. Hornell (1949) suggested that species having smaller shell size and breeding potential can develop in optimum environmental conditions.

Donax species were abundant in the sandy shores studied. The presence of fine sand may be a reason for their abundance in the sandy beaches. Tenjing et al. (2018) recorded donacids from intertidal zones of Panambur and Kota-Padukere and showed that sand in these shores allows high water retention and helps these species to burrow in these habitats. The abundance and species richness of molluscs were observed post-monsoon in most of the study sites. This may be by the virtue of recruitment of juveniles in post-monsoon and favourable environmental factors during post-monsoon and pre-monsoon. The abundance of molluscs in post-monsoon has also been reported for the Indian coasts (Lokhande et al. 2014; Paul et al. 2014). Reduced salinity and unfavourable environmental conditions prevailing along the coast are known to decrease species diversity. Hartnoll (1976) showed that the beaches of Karnataka remain subjected to greater wave action and erosion which lessens the stability of the substratum minimizing the abundance of molluscs. Besides, the rocky shores of coastal Karnataka get covered by algal mat in the monsoon. Flores-Rodríguez et al. (2012) stated that algal mat reduces space for the gastropods, causes their movement towards the regions with habitable space. The decline of the diversity of molluscs from post-monsoon to monsoon was observed during the study which is a consequence of the seasonal variation in beach stability and ecological conditions of the coast (D’Souza and Shenoy 2020).

During the study, significant differences in species abundance at the six sites were observed. It may be because of the variation like the habitats and biotic interactions in the study sites. This variation in the habitats is possibly be related to the geomorphological and physical conditions of the study sites. Wilson (1991) proposed that the geomorphological and physical conditions of these sites have superior effects on mollusc distribution in these intertidal habitats. The distribution of organisms along the coast depends on the competition for food and space in the intertidal areas.

The low tide zone of the coast was found to encompass more species and individuals of molluscs owing to the presence of optimum conditions for their survival. Molluscs require low temperature, less desiccation, and heterogeneous habitats for the survival of molluscan species (Jaiswar and Kulkarni 2001). Menge and Sutherland (1987) related differences in species richness to the higher desiccation in the high tide zones. The shores of Karnataka experience higher stress levels because of increased temperature at low tide. Verdun-Castello et al. (2013) proposed that these stresses are due to the changes in temperature and desiccation increase with elevation as a result of continuous aerial exposure. Lower littoral zones remain exposed to air for a shorter period which would offer more suitable conditions for a range of different organisms (Scrosati et al. 2011).

Echinolittorina leucostica is the common and abundant species found in the rocky littoral zones of Karnataka. Kulkarni and Jaiswar (2000) stated that this herbivore feeds on diatoms, blue-green algae, and detritus that cover the rocks. The food availability may be responsible for the abundance of this species on the rocky shores. Moreover, intertidal molluscs exhibit behavioural isolation wherein they withdraw their shells to withstand continuous exposure to the air (Marshall et al. 2015).

Bullia melanoides and Donax incarnatus were spread throughout the shores. The variations in current speed and its direction brought about by tides distributes these organisms in the intertidal zones. Molluscs utilize these changes by vertical and/or horizontal migration synchronous with the tidal cycle. This behaviour is known to allocate the species to new habitats within the intertidal zone (Gibson 2003).

The species diversity varied with the seasons between the sites. It ranged between 0.000–1.518 in sites over the seasons, evidencing the dynamic nature of the intertidal habitats of the coast. The fluctuations in the environmental conditions are known to control the species diversity (Field et al.1982). Species diversity was more in post-monsoon in the intertidal zones of Someshwar, Panambur, Sasihithlu, and Gangolli. The presence of suitable substratum and the favourable environmental conditions in these beaches may be accountable to the species diversity of molluscs. However, the diversity of molluscs in the intertidal zones of Kaup and Kota-Padukere was highest in pre-monsoon (Fig. 3). This may be attributed to the predominance of site-specific conditions such as changes in beach profile, number of waves and, erosion/accretion in the shores in pre-monsoon. Further, the ability of molluscs to tolerate environmental stress in pre-monsoon may be responsible for the higher diversity in these sites. Verdun-Castello et al. (2013) made similar observations from beaches of North Atlantic Europe.

The spatial distribution of molluscs varied between shores during the study, possibly owing to the combination of morphological, physiological, and behavioural adaptabilities of molluscs. Ali et al. (2019) from the Karachi coast related molluscan distribution to their behaviour and adaptation to the intertidal habitats. Significant differences in the abundance of molluscs at the selected sites were observed (Table 3) which may be ascribed to the variation like the habitats, biotic interactions, and physical conditions of the beaches that have superior effects on molluscs distribution in these environments.

The absence of molluscs on the shores of Sasihithlu and Gangolli during pre-monsoon indicates that the diversity in sandy shores is determined by the presence of food, physical factors, and productivity of the surf zone. These observations are in line with the study of Carcedo et al. (2014) from beaches of Argentina.

The distribution of molluscs in the selected littoral areas varied during the study. The distribution of species has been revealed to depend upon the type of substrate and the habitat heterogeneity (Fuxue et al. 1994; El-Komi 1996). Species richness and abundance of molluscs in intertidal habitats are greatly affected by changes in the physicochemical characteristics of water (Garg et al. 2009; Udayantha and Munasinghe 2009; Sharma et al. 2013). In the present study, electrical conductivity correlated significantly with species richness. Chandrasekera and Hettiarachchi (2011) noted that the species richness increased with an increase in conductivity on the Sri-Lankan coast. In the present study, it was evident that electrical conductivity is the important physicochemical parameter that is responsible for the spatial distribution of molluscan species on the coast. A significant negative correlation of wave frequency with molluscan abundance was observed which indicates that reduced wave-action increases the molluscan assemblages. In the present study, it was evident that the exposure of molluscs to the waves determines their richness and abundance in the littoral zone. Similar observations are made by Rahman and Barkati (2012) from the beaches of Karachi.

Iqbal et al. (2018) suggested that anthropogenic activities are also known to decrease the biodiversity in an area. Industries such as Mangalore Refineries and Petrochemicals Limited, Mangalore Chemicals and Fertilizers Limited, and Baden Aniline Soda Factory are situated beside coastal zone release effluents to the sea (D’Souza and Shenoy 2018). Construction of sea walls parallel to the shoreline was observed at Sasihithlu and Gangolli. Sea wall construction involves sand removal from the intertidal zones that leads to habitat displacement (Ravinesh and Kumar 2013).

Conclusion

Intertidal habitats of Karnataka provide novel habitats for molluscan species. The overall observations on the seasonal changes in the number of intertidal molluscs show that their diversity is influenced by tides, wave action, the electrical conductivity of water, heterogeneous habitat, and the presence of suitable ecological niches. The beaches of coastal Karnataka very concerning species richness and abundance of molluscs. This information on the diversity of molluscs of coastal Karnataka provides fundamental ecological knowledge about their dynamics on the coast.