''Cerithidea decollata'',

common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...

the truncated mangrove snail, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

sea snail Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the ...

, a marine

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Potamididae.

Description

Adults have a thick, approximately 3 cm long shell, with 5

whorls A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral d ...

and around 20 axial ribs on each whirl. A distinguishing characteristic of adults is the broken-off tip of the shell, although this feature can be difficult to discern in some individuals.

Ecology

This species is common in coastal mangrove forests, particularly near ''

Avicennia marina ''Avicennia marina'', commonly known as grey mangrove or white mangrove, is a species of mangrove tree classified in the plant family Acanthaceae (formerly in the Verbenaceae or Avicenniaceae). As with other mangroves, it occurs in the intertid ...

'' trees, in western part of the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

Kenya ) , national_anthem = "Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi , ...

Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...

Mozambique Mozambique (), officially the Republic of Mozambique ( pt, Moçambique or , ; ny, Mozambiki; sw, Msumbiji; ts, Muzambhiki), is a country located in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi ...

,Branch G. M. Griffiths C. L. Branch M. L. & Beckley L. E. (2000). ''Two Oceans: A guide to the marine life of southern Africa.'' 5th impression, David Philip, Cape Town,

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

and

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

. Juveniles are seldom seen and therefore little is known about their ecology. The adults feed on small organic particles (

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commun ...

) and

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the orde ...

es that are brought in with the

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...

. Their

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

is a gently sloping

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

with two high and two low tides each day. There are large differences in the duration of flooding and sea level between the two high tides each day, between

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...

s and between places nearer to or further from the shoreline.

Tree-climbing behaviour

When the water recedes, the snails feed scattered on the ground. Then, one or two hours before the incoming tide, they start climbing on tree trunks and gather in groups of up to several dozen specimens, waiting above the water level until the sea recedes again. This behaviour probably makes it possible for them to avoid the unfavorable

physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

effects of submersion, or possibly makes it easier to escape from marine

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

such as crabs. Similar or reverse tree-climbing strategy is employed by other related species. The strategy of this particular species is unusual, because the tides are very unpredictable in this environment. The higher and lower semidiurnal tides vary in amplitude, and during neap tide, one or both high tides each day are not high enough to reach the grounds where the snails feed. The snails start climbing when the shoreline is still dozens of meters away and an hour or two before the water floods their feeding grounds. They invariably stop at a point twenty to seventy centimeters above the future water level and wait there for the tide. If the incoming high tide is too low to reach their feeding grounds, they remain on the ground until an hour before the next high tide will be high enough (especially the animals that live further inland where the shore is a bit higher and therefore more seldom flooded). It has been found out that the individuals "measure" their height by detecting the amount of energy used for climbing: when artificially loaded, the snails climbed proportionally lower, whereas they climbed higher if the tree trunk was replaced with a smoother surface or if the researchers raised the starting platform. However, it is still a puzzle how the animals are able to predict the water level so far in advance. The difference in body weight caused by the fluctuation of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

that also causes the tide is probably too low for detection by an organism this small. Chemical cues, such as

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

released from the ground, and acoustic cues, such as infrasound caused by the waves, are probably unreliable indicators of the water level as well, because of the local weather's influence. Every high tide is similar in amplitude to the one before the last, but the snail migration is – statistically speaking – better adjusted to the following high tide than to the one before the last. The underlying process is probably regulated by an internal clock, which can be "confused" by carrying an individual to a lower or a higher part of the coastline. In this case, the animal continues to climb as it would in its original surrounding for several more days or until it gets submerged, then the mechanism "resets" and it is again able to predict the oncoming high tide reliably. Since the cue used by these animals to predict the level of the incoming high tide is still a complete mystery, researchers jest that these snails can foresee the future.

References

External links

*
''Cerithidea decollata'' - identification guide
. A field guide to Kenyan mangroves {{Taxonbar, from=Q3167747 Potamididae Molluscs of Africa Gastropods described in 1758 Taxa named by Carl Linnaeus