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Research on Gastropods
In this in-depth blog (thesis) shows the very detail of Gastropods as a species existing on this planet.Review of Related Literature
Gastropods are diverse species that are predominantly found in marine habitats such as rocky shorelines, tide pools and mangrove-laden areas. Gastropods – which include aquatic snails, slugs and limpets are members of the macroinvertebrates (The Living Marine Resources of the Western Central Pacific, 2002).
General Characteristics
Gastropods are sluggish animals that belong to the class Gastropoda, which is considered as the largest group in the phylum Mollusca. The organisms constituting this class are abundant for they are found in all major habitats, marine, terrestrial and freshwater (Gong, 2003). This class is entirely composed of snails, varying mostly in morphology and shell structure (Denning and Russel, 2006). Harley and Miller (2007) described gastropods as members of the group of invertebrates which is highly distinguished from other groups by having a soft segmented body.
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| Gastropods are mollusks with different varieties of shells. |
In view of the fact that the gastropods dwell in the marine ecosystem, these organisms are much more abundant in the lower intertidal or lower littoral area of the sea (Esqueda, et al., 1999). The lower intertidal zone is usually submerged underwater and is only exposed during low tide. Several species of gastropods are more abundant in this zone for the reason that more interaction takes place among organisms. Thus, this zone is considered to have a high diversity of organisms. Relative to this, gastropods are also abundant in rocky areas. More often than not, desiccation and wave action occurs at the littoral zones, gastropods cling directly to rocks and hide underneath the seaweeds which are situated in tidal pools so as to avoid themselves from desiccation and wave action (Young, 2012).
Importance of Gastropods
Intertidal gastropods play a significant role in the environment. These organisms aid in the decomposition of dead plants and animals into substances that plants can make use of to manufacture new organic compounds (Williams, 1980; Beesley, 2008; Davis & Christidis, 1997; Ingram et al. 1997). In addition, gastropods are of great help in maintaining the ecological stability. These animals have developed wide-ranging modes of feeding from gastropods that inhabit the shallow waters feeding on microscopic benthic algae or large seaweeds and kelp. Other species, such as the Lepsiella scobina, and the species from the genus Tonna, prey on other organisms by using their radula to scrape off food from bivalves, gastropods or echinoderms (Morton, 2005). Gastropods, specifically species from the genus Cominella, feed on dead fish and invertebrates or discarded food scraps (Stewart and Walsby, n.d.). Given that some species of gastropods are herbivores and carnivores, they play a significant role in averting the exceedance of plant growth (primary producers) and controlling the population size of the herbivores (Solem, 2015).
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| A snail like this one is an example of a gastropod. |
Gastropods are good biological indicators for water quality or pollution on the basis of their tolerance against extremes of physico-chemical components of water (Harman, 1974; Edmondson et al. 2010; Druart et al. 2011). Ganesh et al. (2014) stated that due to the gastropods’ imposex capability, ability to accumulate toxins, spatial distribution and size, they serve as bioindicators that determine the effect of marine pollution. The presence of polluted ecosystem may cause a change in animal population, presence of toxins in animal tissues, and the apparentness of deformities in animal population.
Diversity of Gastropods
There were several studies conducted concerning the diversity of gastropods in the marine ecosystem worldwide due to its capability as environment indicators (Ganesh, et al. 2014).
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| [Diagram] Parts of a gastropod mollusk. |
In Cuastecomate Bay, Jalisco in Mexico, Esqueda, et al. (1994) conducted a study on vertical distribution and abundance of gastropods and bivalves. In this study, 6,643 molluscs were collected and it was found out that gastropods dominated the samples since they have obtained a total of 6,272 individuals (44 species) and the bivalves they have obtained were only 371 individuals (5 species). Quadrants, with a measurement of 0.75 m2 were used and was placed along replicate transect lines, which are 10 m long in the supralittoral and mesolittoral (upper, middle and lower intertidal) zones of their study sites. It was also known that the abundance of gastropods decreased from the supralittoral to the lower tidal zones while the number of species increased in the same direction. Similarly, the number of species of bivalves also increased from the supralittoral to the lower intertidal zone; the abundance of individuals was higher at the middle intertidal zone. Furthermore, they have learned that surf exposure is probably the most important factor determining shore populations and influencing their distribution.
Along the rocky intertidal area of Veraval, Gujarat, India, Ganesh, et al. (2014) conducted a study in relation to gastropods and their distribution. In this study, random sampling was done. The samples were collected using the square transects of 1m2 (100 x 100 cm.) which was laid randomly in a line up to water line during low tide. They found out that Cerithium morus and Turbo coronatus showed dominance along the study location. The species diversity (H’) values ranged between 0.99 to 1.21. The species richness (D) values ranged between 0.90 to 0.97 and species evenness (E) values having range between 0.62 to 0.75 individuals.
In the intertidal habitats of the Ratnagari Coast Maharashtra in India, Kurhe, et al. (2014) studied the vertical distribution, species diversity, ecological preferences, and intra-specific shell and abundance of gastropod molluscs. They have collected the gastropods through hand picking methods. They had their sampling sites fixed and they had the survey completed from the north to south part of their selected areas. They chose their locations in order to get a variety of different habitats which were either sheltered or unsheltered from the open coast to demonstrate the zonation of littoral animals. Each area was examined for species composition, distribution, population density and abundance of the species. Profile was drawn of the shore from the low to high water marks occurred approximately one meter above the level of high water marks. In this study, 127 species were identified representing 11 orders and 20 families of gastropod molluscs. Cerithium morus, C. rubus, Planaxis similis, P. niger, P. sulcatus and P. acutus, were the most abundant gastropod molluscs in all the localities. They have found out that the abundance of gastropods decreased from the supralittoral to the lower tidal zones, while the number of species increased in the same direction, the abundance of individuals was higher at the middle intertidal zone. Moreover, they also found out that there is no similarity in zonation among rocky, sandy and muddy habitats, and species distribution in the intertidal areas showed differences in zonation. Accordingly, the species showed vertical separation in the intertidal segregation according to a range of increasing frequency of water movement in pools and on boulders. The patterns of vertical distribution of gastropods in various exposed rocky shore habitats of Ratnagiri coast are vertically diverse and abundant. The density of the individuals was determined at different levels on intertidal rock walls, in pools and on boulders and in association with biotic and abiotic community where they find optimum living conditions.
Trivedi and Vachhrajani (2013) conducted a study entitled, “Study of intertidal distribution of Cerithium scabridum, Philippi, 1848 (Mollusca, Gastropoda) along the coastal Saurashtra, Gujarat, India”. Three different sites in the coastal area of Saurashtra were utilized as study areas namely: Sutrapada, Dhamlej and Kodinar. Accordingly, the intertidal area is mostly rocky and the upper portion is sandy. The intertidal zone’s exposure area ranges from 60 to 150 meters and its width also depends on the tide cycle. In all study sites, an evident zonation pattern was observed. As for the results, there was a variation on the mean seawater temperature on the different study areas of the site, with Dahamlej having the highest temperature of (32.45 ± 1.62) during summer and Sutrapada having the lowest temperature of (28.46 ± 0.78) during winter. As for the mean sea water pH, it did not change among the different stations in different seasons. The highest pH observed was at Kodinar (8.36) during the monsoon season while the lowest pH was observed at Dhamlej (7.77) still during the monsoon season. On the other hand, the mean water salinity was reported to change in the different stations – in Kodinar, the mean water salinity ranged from 39.63% in summer to 30.1% in monsoon. This could be due to the combination of freshwater in sea water during the monsoon season. Furthermore, it was found out that the highest abundance of the species of Cerithium scabridum was on the upper intertidal zone since this species of gastropods prefer to live in tide pools which are located at the upper and middle intertidal zone. This species of gastropods are known to live on a variety of habitats but their preference is in the shallow marine ecosystem. Moreover, majority of its species have a wide geographic distribution and the Indo-Pacific Marine Province provides support for 68% of all Cerithium species. Since the rocky area of the Saurashtra’s coast is mostly covered by tide pools, it then provides an excellent habitat for gastropods to thrive on. A total of 42 living species was reported to be found under the genus Cerithium. In addition to that, accordingly, Cerithium scabridum is found out to be one of the most common gastropod species used as a source of food by different carnivore species while hermit crabs use its empty shells as shelter.
In 2012, a study on the “Checklist of benthonic marine invertebrates from Malaga Bay (Isla Palmas and Los Negritos), Colombian Pacific” was conducted by Lozano-Cortes and his colleagues. Here, the researchers aimed to produce a preliminary checklist of marine macroinvertebrates that inhabit the two localities of Malaga Bay which are the Isla Palma and Los Negritos. This is due to the fact that there is incomplete information about animal biodiversity on the intertidal and shallow subtidal marine ecosystems on their research site. Since mollusks and crustaceans are the most common animals there, they were the ones chosen to be sampled. Details regarding the poorly known taxonomic groups of the samples were scrutinized in order to be able to provide new and useful information for the conservation and protection of Malaga Bay. The samples were collected through rigorous examination of animals concealed in cracks and even under stones, at each site – both in the subtidal and shallow subtidal. In this study, scuba and snorkeling was used and the sampling was done during low tide. Samplings were performed with a maximum depth of 6m for Isla Palma and 12m for Los Negritos. Upon collecting the samples, they were first kept underwater prior to sorting them to their respective taxonomic groups. Moreover, freshwater and magnesium chloride was gradually applied to the samples in order for them to be anesthetized. Then, the samples were fixed in 10% formalin or 70% alcohol. Most animals lose coloration upon being in contact with fixatives. Due to this, photos of most live or recently fixed animals were taken in order to be able to have a record of natural coloration of the animals. Afterwards, after being fixed, the samples were already stored and identified to the lowest possible taxonomic level. All in all, 128 species (64 families) of marine invertebrates were collected. 64.1% of the samples collected were from the Isla Palma whereas 40.6% was found in Los Negritos and only 7.8% was shared between the two sites (Lozano-Cortes et al. 2012).
In the Brazilian coast, Luz de Sousa, et al. (2010) intended to present a taxonomic list of gastropods and bivalves (which includes the first reference inventory of the mollusk fauna of the Brazilian coast) of the marine bivalves and gastropods at 70 archeological sites of shellmound type which are distributed to numerous municipalities of the state of Rio de Janeiro. Out of the 70 sites, 44% had a complete malacological list published and verified whereas 37% presented a partial list and 19% did not have any publication. Since it aims to reconstruct scenarios of past mollusk diversity and to reconstitute ecological history in an evolutionary perspective, this study was considered significant. The archeological materials were located and sorted – shell matter was separated from the sediments with the use of a 0.5 mm brush whereas the malacological materials were sorted by following a process – selection of the samples, cleaning the samples with a brush and washing the shells then observing them during and after doing so. As for the bivalves that were not open, accordingly, they were preserved for other studies. Afterwards, shells were identified at the lowest possible taxonomic level and labeled. At the end of the study, it was found out that shell mounds indicate stability of the mollusk biodiversity on the Rio de Janeiro coast. The taxonomic list obtained may be used as a reference inventory for research studies that are in relation to biological invasions, biogeography and the like due to the fact that it aids on the determination of natural expansion of species from time to time, pattern and dispersion rates, and also in defining whether a particular species is exotic or native.
In Mexico, particularly at the Gulf of California, Angulo-Campillo, et al. (2011) conducted a study on holoplanktonic mollusks. Accordingly, 246 plankton samples were obtained with the use of Bongo nets with the measurement of 505-μm. Then, the collected samples were preserved and fixed in 96% ethanol in order to avoid shell deterioration. As a result of this study, 25,404 organisms were identified, grouped in five orders, four suborders, fifteen families, twenty-eight genera and sixty-two species.
Ourives et al. (2011) conducted a study on gastropods in a marine ecosystem particularly the Camamu Bay in Brazil. The study aimed to assess the composition of the gastropod mollusks in order to add more information about their presence in the study site. In their data collection, the samples were collected on unconsolidated substrate with the use of a trawling boat with one or more parallel trawls that are horizontally kept apart by otter boards. The trawling lasted for about 10 minutes in each collection points. The sampling was carried out between July 2003 and September 2006, in eight fixed points – accordingly, this time span was sufficient to provide the representative specimens of the estuarine malacofauna. Furthermore, 3,193 individuals were collected within the time frame. These individuals comprise 94 species and are distributed into 46 families of gastropods.
Locally, there were some studies conducted on the diversity of gastropods in marine waters. In the adjoining protected bays in Puerto Princesa City, Palawan, Dolorosa and Picardal (2014) aimed to obtain baseline information on species richness of gastropods and bivalves in two protected bays - Turtle and Binunsalian in Puerto Princesa City, Philippines before the establishment of the proposed mega resort facilities. In this study, a total of 108 species were recorded, (19 bivalves and 89 gastropods). Moreover, it was found that Turtle Bay had very low visibility, low coral cover, substantial signs of ecosystem disturbances and shift from coral to algal communities. Although Binunsalian Bay had clearer waters and relatively high coral cover, associated fish and macrobenthic invertebrates were of low or no commercial values.
In the selected sites of Tubbataha Reef National Marine Park, Palawan, Dolorosa and Schoppe (2006) conducted a study on the diversity of focal benthic mollusks. Accordingly, seven pre-established stations with survey sites at 5 and 10 m depth and one intertidal area were assessed using 150 m permanent belt transects. Focal benthic mollusks were found one meter to the left and right of transects were identified and counted. A total of 19 species belonging to eight families were recorded, of which 15 species are univalves. In the intertidal area a total of 12 species were noted, 13 species at the shallow (5 m) and five species at deeper (10 m) areas. Among the subtidal stations, the highest number of individuals was noted at a shallow reef flat (station VI). In terms of density, the intertidal area had the highest, followed by the shallow and the deep.
The Philippines, being a haven of a rich diversity of marine organisms, posed a tremendous challenge to researchers and the existing security threat in some areas of the archipelago had led to a dearth of information with regard to the diversity of organisms especially the islands located in the province of Sulu. With this, in the Island of Hadji Panglima Tahil, Sulu, Jamasali, et al. (2013) assessed, described and identified some economically important molluscs in the island of Hadji Panglima Tahil, in the province of Sulu, Philippines. In this study, 18 molluscs (marine bivalves & gastropods) species were found and identified in the island. For their sampling procedure, the researchers took digital images of the samples, descriptions of each samples were made and the samples were identified with the aid of a personnel from the Marine Science Museum of MSU-Iligan Institute of Technology, College of Science and Mathematics, Department of Biological Sciences. The species found were considered as a great source of food and ornaments.
Grande Island, Subic Bay is an excellent site for studying spatial distributions on a local scale because of its topographic heterogeneity. Arce, et al. (2010) aimed to provide a systematic survey and spatial distribution of shoreline malacofauna of the island using a total of 956 continuous 2 x 2 m plots. Substrate types were classified as sand, sand-pebble, rock-sand, rock-pebble, pebblerock-coral rubble, boulder, and wood. One hundred mollusc species were identified from 44,465 individuals with 54 additional individuals still unresolved. Molluscs were found to be distributed according to the substrate type (composition and particle size) and are independent of the total area sampled. Substrates that are loose or unstable tend to have only few species while those that are compact have more species. Only bivalves were found in the sand substrate. Planaxids, nerites, trochids, and siphonarids were the dominant groups in substrates with rocks, pebbles, and coral rubble. Nudibranchs only occurred in rocky substrates. Nacellids and littorinids predominate particularly in boulders. Littorinids were also numerous in woody substrates. Juvenile trochids and planaxids were most abundant in regions with calm water while Cellana species and adult trochids were particular in areas exposed to strong currents. The morphological adaptations of different species enabled them to occur in specific habitat types.
Influence of the Physicochemical Parameters to the Diversity of Gastropods
The diversity of gastropods are influenced changes in their habitat – one of which are the physicochemical parameters. In the study conducted by Trivedi and Vachhrajani (2013), regression and correlation analysis between sea water temperature and abundance of a particular gastropod species - C. scabridum, was made. A significant correlation was shown at all study sites and it was found out that seawater temperature plays a crucial role in the distribution of the gastropod species. In one of the study sites (Dhamlej), a significant correlation between all the abiotic factors and abundance of the species was observed. Dhamlej had the highest record of temperature and it implied a healthier habitat for the C. scabridum species compared to the other sites. Depending on the species of gastropods, temperature could either be a cause of their fluctuation in a particular area, or their abundance. Moreover, sea water quality, which is affected by contamination of different pollutants in sea water, also affects the distribution, density and abundance of molluscan species. Regions where water is prone to drying up or freezing contain fewer species of gastropods (Bishop et al. 2002). Aside from that, Zhao (2013) investigated how major environmental factors such as temperature and salinity affected the survival, development, behavior and physiology of two dominant subtidal scavenging gastropods: N. siquijorensis and N. conoidalis. Temperature and salinity, as well as their interactions, had significant effects since the highest survival and fastest development of both species occurred at 30°C whereas at lower temperature and salinity levels, both species showed higher mortalities. Moreover, temperature, salinity and their interaction also effects the oxygen consumption, clearance rate and swimming behavior of the two species. Significantly lower clearance rates and oxygen consumption rates were observed at lower salinities and temperatures. There are certain gastropod species that are tolerant to temperature and salinity changes and in this case, the N. siquijorensis showed more tolerance compared to N. conoidalis. Moreover, increased levels of dissolved oxygen implies an increase in the size of gastropods (McClain, 2001).
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| [Diagram] Gastropod Family Tree. |
Another factor which may affect oxygen consumption is pH. A study showed that the distribution of snails relates to the pH of their environment. In a study on the effects of temperature and pH to the oxygen consumption on gastropods, particularly on Viviparus contectoides, it was observed that the Viviparus contectoides follows the expected relationship of increasing oxygen consumption with increasing ambient temperature as seen in other studies. Metabolic rate is dependent on levels of the organisms’ activity. Moreover, in intermediate temperature ranges (22-27°C), oxygen consumption was proven to be significantly higher for males than for females – implying that it is energetically more advantageous to have a lower metabolic rate and that female gastropods are energetically more efficient at these temperatures. Therefore, metabolic demands on the female would be greater during periods of reproduction. Aside from that, it was also found out that the rate of oxygen consumption is related to pH due to the increasing and decreasing levels of metabolic activity at different pH values. The trough pH (8.1) is more favourable compared to other levels of pH. At peak pH levels (7.1 and 8.9) they are still able to thrive but it is accordingly more stressful for them to do so (Buckingham and Freed, 1975). With that, it is proven that pH, temperature, salinity and dissolved oxygen indeed influence the abundance of gastropods in a particular area greatly.
Littoral Zones
The littoral zone, also referred to as the intertidal zone is the area between tide marks and is found worldwide along coastlines. This complex marine ecosystem is abundant in nutrients and oxygen. Moreover, it is also a home for numerous varieties of organisms (Duarte, 2000). This zone is inhabited only by small, relatively uncomplicated organisms. The littoral zone is actually separated into three subzones namely upper littoral zone, middle littoral zone and lower littoral zone (Young, 2012).
Young (2012) described the upper littoral zone as an area that is usually dry but it is sprayed with salt water during high tides and flooded only during high tides and/or occurrence of storms. In this area, there is little vegetation and organisms that can be found here are barnacles, isopods, lichens, lice, periwinkles and whelks.
The middle littoral zone is covered and uncovered with salt water from the tides. Compared to the other subzones, temperatures in this subzone is less extreme since the exposure to sunlight is shorter and lesser, which implies that salinity is higher here than in ocean levels. In terms of marine vegetation, this subzone has a higher population than the upper littoral zone (Young, 2012).
As for the lower littoral zone, it is usually underwater and is only exposed if the tide is unusually low. In terms of its vegetation compared to the other subzones, this area has the greatest marine vegetation and has a great biodiversity. Organisms in this area, and even marine vegetation, can grow larger compared to the ones on the other subzones since more energy is present. Moreover, the water is shallow which allows greater exposure to sunlight for substantial photosynthetic activity on the vegetation and normal salinity levels (Young, 2012).
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