''Corculum cardissa'', the heart cockle, 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 ...

of marine

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

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 ...

Cardiidae A cockle is an edible marine bivalve mollusc. Although many small edible bivalves are loosely called cockles, true cockles are species in the family Cardiidae. MolluscaBase eds. (2022). MolluscaBase. Cardiidae Lamarck, 1809. Accessed through: W ...

. It is found in the

Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

region. It has a

symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

relationship with

dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

s (

zooxanthella Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus '' Sym ...

e), which live within its tissues.

Description

The two

valves A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

of ''Corculum cardissa'' are unequal in size and often asymmetric. Their shape is very variable but viewed from above, the outline is roughly heart shaped which gives the molluscs their common name. Viewed from the side the shape bears a resemblance to the shell of a cockle (''

Cerastoderma ''Cerastoderma'' is a genus of marine bivalves in the family Cardiidae. It includes the common cockle ''Cerastoderma edule.'' Fossil records This genus is known in the fossil records from the Paleocene to the Quaternary (age range: from 58.7 t ...

'' spp.). In some specimens the posterior valve is nearly flat or has a slight hump. In others it is more rounded. The boundary of the valves is usually flat but is sometimes somewhat

sinuous Sinuosity, sinuosity index, or sinuosity coefficient of a continuously differentiable curve having at least one inflection point is the ratio of the curvilinear length (along the curve) and the Euclidean distance (straight line) between the ...

. Smaller shells tend to be elongated; larger shells are more rounded and the growth rings can be seen clearly. The shell is thin and translucent, particularly the upper side. There is an intricate mosaic pattern of more and less transparent white regions. The lower valve has a mainly white surface with a few transparent regions. The

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s and mantle, especially the lower

siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...

, are dark brown owing to the presence of microscopic algae. The outer surface of the mantle also contains granules of reddish, purple and blue pigment.

Habitat

''Corculum cardissa'' is often found lying on a surface of sand among coral debris and broken shells. It usually lies horizontally in a hollow it excavates and its top is often covered with

filamentous The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

algae and muddy deposits.

Biology

''Corculum cardissa'' is a

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

. The shell gapes slightly at the ventral end and two siphons are protruded. Water is drawn in through one and expelled through the other and

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

and

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 ...

are extracted. At the same time, water passes over the gills where oxygen is absorbed. ''Corculum cardissa'' is a

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

Eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

are laid and the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e develop with great rapidity. Within 24 hours of fertilisation, the

veliger A veliger is the planktonic larva of many kinds of sea snails and freshwater snails, as well as most bivalve molluscs (clams) and tusk shells. Description The veliger is the characteristic larva of the gastropod, bivalve and scaphopod ...

larvae have been observed to develop two valves and be swimming on the surface of the substrate. A day later, they had undergone

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

and had settled on the bottom as juveniles, miniature versions of the adult bivalves.

Ecology

''Corculum cardissa'' and some other members of the family

live in symbiosis with dinoflagellates in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellul ...

''. These are found in the mantle, gills and the liver. It was originally thought that the photosynthetic

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

were in the

haemocoel The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, the fluid between the cells. It has since been found however that, in response to the presence of ''Symbiodinium'', a tertiary series of tubules develop from the walls of the digestive system and ramify through the tissues. The algae are found in these and are separated from the blood cells in the

haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

by a layer one cell thick. This is analogous to the tertiary tubular system found in ''

Tridacna ''Tridacna'' is a genus of large saltwater clams, marine bivalve molluscs in the subfamily Tridacninae, the giant clams. They have heavy shells, fluted with 4 to 6 folds. The mantle is brightly coloured. They inhabit shallow waters of coral re ...

'' and first described by Mansour in 1946, but subsequently overlooked by other researchers. The algae enter the system through the mouth. The translucent shell and the bivalve's habit of lying on the surface both encourage photosynthesis in the algae. The mollusc benefits from the metabolites produced and the alga benefits from a safe environment in which to live.

References

{{Taxonbar, from=Q2423505 Cardiidae Molluscs of the Indian Ocean Molluscs of the Pacific Ocean Molluscs described in 1758 Taxa named by Carl Linnaeus