Cerithidea decollata
Encyclopedia
Cerithidea decollata, common name
the truncated mangrove snail, is a species
of sea snail
, a marine gastropod mollusk in the family
Potamididae
.
, with 5 whorls
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.
forests, particularly near Avicennia marina
trees, in western part of the Indian Ocean
- Kenya
, Tanzania
, Mozambique, South Africa and Madagascar
.
Juveniles are seldom seen and therefore little is known about their ecology. The adults feed on small organic particles (detritus
) and seagrass
es that are brought in with the tide
. Their habitat
is a gently sloping intertidal zone
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
s and between places nearer to or further from the shoreline.
effects of submersion, or possibly makes it easier to escape from marine predators
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 that also causes the tide is probably too low for detection by an organism this small. Chemical cues
, such as hydrogen sulfide
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.
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
the truncated mangrove snail, is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....
, a marine gastropod mollusk in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Potamididae
Potamididae
Potamididae, common name potamidids , are a family of small to large brackish water snails living in mangroves, gastropod molluscs of the clade Sorbeoconcha. Traditionally, potamidids and batillariids have been confused with each other for having similar shells and living in a similar environment...
.
Description
Adults have a thick, approximately 3 cm long shellGastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...
, with 5 whorls
Whorl (mollusc)
A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...
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 mangroveMangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...
forests, particularly near Avicennia marina
Avicennia marina
Avicennia marina, commonly known as grey mangrove or white mangrove, is a species of mangrove tree classified in the plant family Acanthaceae...
trees, in western part of the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
- Kenya
Kenya
Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...
, Tanzania
Tanzania
The United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...
, Mozambique, South Africa and Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
.
Juveniles are seldom seen and therefore little is known about their ecology. The adults feed on small organic particles (detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
) and seagrass
Seagrass
Seagrasses are flowering plants from one of four plant families , all in the order Alismatales , which grow in marine, fully saline environments.-Ecology:...
es that are brought in with the tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
. Their habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
is a gently sloping intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
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
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
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 physiologicalPhysiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
effects of submersion, or possibly makes it easier to escape from marine predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
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 that also causes the tide is probably too low for detection by an organism this small. Chemical cues
Sensory cue
A sensory cue is a statistic or signal that can be extracted from the sensory input by a perceiver, that indicates the state of some property of the world that the perceiver is interested in perceiving....
, such as hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
released from the ground, and acoustic cues, such as infrasound
Infrasound
Infrasound is sound that is lower in frequency than 20 Hz or cycles per second, the "normal" limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high...
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.
External links
- Cerithidea decollata - identification guide. A field guide to Kenyan mangroves