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Fragum unedo (Linnaeus, 1758)
The Strawberry Heart Cockle

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Heterodonta
Order: Veneroida
Superfamily: Cardioidea
Family: Cardiidae
Genus: Fragum
Species: F. unedo

Zaine Morrick 2016


The Strawberry Heart Cockle, Fragum unedo (Linnaeus, 1758), is a marine bivalvian mollusc within the family Cardiidae. They are commonly found buried, slightly, on sandy flats, within intertidal and shallow subtidal zones (Habitat). F. unedo is distributed throughout the Indo-Pacific region, and can commonly be found on the eastern shores of Australia (Biogeographic Distribution).

Ecology, life history, anatomy, evolution and conservation of F. unedo will be discussed in the species page.

F. unedo specimens were collected 100m off shore, north-east of the Morten Bay Research Station (M.B.R.S.) (Fig. 1). The specimens were then stored within the University of Queensland Aquariums while observational studies were completed.
Figure 1

Physical Description

Physical Morphology

The Strawberry Heart Cockle is a mollusc belonging to the class Bivalvia (Phylogeny). The two domed valves are equivalve, valves that mirror each other in both size and shape. However, each valve is asymmetrical. The valves are usually white or cream in colour, with strawberry-red scales (Fig. 2). When in a still environment two siphons are able to be observed, an inhalant siphon (Respiration and Feeding), used to pump water and organic particles into the bivalve, and an exhalant siphon (Excretion), used to expel filter water back into the water column.

F. unedo
individuals usually grow to lengths of 40mm, but when conditions are good they are able to grow to maximum lengths of 65mm.

Figure 2

The Shell

Chen, Peng and Wu (2007) observed the structure of a F. unedo shell with a scanning electronic microscope. They found that the shell was a bioceramic composite. Made up of long and thin aragonite, crystal form of calcium carbonate CaCO3, sheets and layers of collagen protein. The aragonite sheets were found to have a herringbone structure, which gives the shells added strength (Chen, Peng and Wu 2007). 

The physical description of Fragum unedo shells are described using the three collected specimens.

 Shell Shape:
The general shell shape of F. unedo,
Lateral view, the shell is of rhomboidal shaped (Fig. 3).
The posterior margin, has a relatively long flat edge, which lies parallel to the surface when they burrow (Habitat).
The anterior margin, curves down from the umbo and eventually back upward to meet with the posterior margin.
Dorsal view, the prosogyrate umbo aids to form the major characteristic heart shape (Fig. 4).
The umbo/beak is the oldest part of an individuals’ shell, cockles have a high umbonal ridge which aids in forming the heart shape.

Valves Structure (Fig. 5):
Each valve usually has,
Between 25-27, relatively broad radial ribs with narrow interstices.
Ribs are structured with irregular concentric scales, which are anteriorly nodulous and posteriorly obsolete.

Hinges (Fig. 6):
The shells have a short heterodont, cyclodont hinge, and an external parivincular ligament.
The hinge of each valve usually has two cardinal teeth (Fig. 7):
Those on the left valve are of unequal size (the anterior tooth is the larger)
In the right valve, the cardinals are fused to some extent.
The lateral teeth are distant from the cardinals (Fig. 7).

Figure 3
Figure 4
Figure 5
Figure 6
Figure 7



Strawberry heart cockles are an entirely infaunal marine species of bivalves (Rudman 1998). Most individuals occupy soft substrates, ranging between muds and slightly course sand, habitats like coastal beaches, intertidal mudflats as well as sandy inner reef zones (Kawaguti 1983; Terufumi et al. 1994; Rudman 1998). These molluscs, like many other cardiids, live as solitary individuals. They burrow below the substrate, maintaining a relatively vertical orientation, burying themselves until the relatively flattened posterior edge of their shells sit just below the substrate, lying parallel to the surface (Kawaguti 1983; Rudman 1998). This allows their short siphons and filter feeding apparatus to slightly protrude through the thin layer of substrate into the waters above. Strawberry heart cockles have been found residing in waters, between the low tide mark (0m), up to maximum depths of 60m (Poutiers 1998).

Environmental Conditions

The strawberry heart cockle, is known to be able to live in environments that lie within the following parameter ranges:
   Minimum Maximum
Temperature range (°C) 25.491 26.803
Dissolved Oxygen (mL/L) 4.666 4.696
Oxygen Saturation (%) 99.79 101.57
Salinity (PPS) 34.975 35.070
Nitrate (umol/L) 0.090 0.232
Phosphate (umol/L) 0.131 0.172
Silicate (umol/L) 1.005 3.928
             (table information collected from E.o.L. 2012)

Zooxanthellae Association

The posterior mantle edges of F. unedo, are lined with hypertrophied tentacles, which are positioned ventrally to the inhalant siphon (Fig. 8). The tentacles form fan like lobes (Fig. 9). The lobes continue along the posterior mantle margin, dissipating into broad fleshy striped muscle (Terufumi et al. 1994) (Fig. 10). F. unedo shares a symbiotic relationship with the micro-algae, zooxanthellae. The zooxanthellae live in the molluscs mantle and exposed soft tissues, the tentacles (Kawaguti 1983; E.o.L. 2012). The mollusc extends these tentacles through the thin layer of sediment, out over the substratum surface exposing the tentacles directly to a light source, photo-symbiotic bivalves that display this behaviour are termed heliophilous, sun loving, (Terufumi et al. 1994) in order to maximise the amount of sunlight available for photosynthesis (E.o.L. 2012). Terufumi et al. (1994) found that a large proportion of F. unedo the individuals showed a negative reaction to shade, draing their posterior mantle edges and tentacles back when a shadow passed over them.
Figure 8
Figure 9
Figure 10

Life History and Behaviour

Sedentary Adult

Like many other cardiids, F. unedo individuals spend most of their adult lives as a sedentary (Soo & Todd 2014), suspension feeder (Respiration and Feeding). However, F. unedo individuals are not stuck in a single place for their entire lives, as they have the capability to locomote and burrow into sediments, using a muscular foot (Locomotion). The suspension feeding lifestyle imposes constraints upon the bivalves’ lifestyle, generally restricting it to aquatic environments (Habitat). However, during seasonal changes in tidal levels (Underwood 1981; Tsimplis & Woodworth 1994), expose the intertidal reef zones. Like other intertidal bivalves, F. undeo have developed the ability to survive without access to water for periods of time (up to 8h at times). They accomplish this via aerial respiration, which is able to take place across the exposed posterior mantle region.


Members of the Cardiidae family, like many other marine bivalves are gonochoristic, meaning individuals are either male or female. By means of broadcast spawning they are able to reproduce sexually (Hickman et al. 2011), utilizing processes of external fertilisation (Ruppert, Fox and Barnes 2004; Wilbur and Yonge 1964).

No research has been conducted into the seasonal spawning patterns of F. unedo individuals. However, research conducted by Kandeel et al. (2013) on Cerastoderma glaucum, a cardiid cockle, has found that spawning events maybe be dependent on temperature and lunar cycles. C. glaucum are known to have an annual pattern of four spawning events (Kandeel et al. 2013), it is possible that F. unedo may follow a similar trend.

Development and Larvae

No research has been conducted into embryonic or larval development. The most educated assumption, is that they have similar embryonic and larval development to most other heterodont bivalves.

After the egg has been externally fertilized, the embryo undergoes the process of spiral cleavage (Hickman et al. 2011; Wilbur and Yonge 1964). Upon completion of embryonic cleavage, the embryo develops into a free swimming trochophore larvae (Ruppert, Fox and Barnes 2004). This simple planktonic larval form, is dispersal utilizing ocean currents. 

The trochophore larvae develops into the, longer lived, veliger larvae, unique to the molluscan phyla. During the veliger stage the mantle, shell and foot of the mollusc begin to develop (Hickman et al. 2011). After successful feeding and dispersal, of the veliger larvae in the pelagic zone, the veliger larval form is lost as the bivalve develops into their adult forms (Sedentary Adults).


Savazzi (1985) states that members of the Cardiidae family, display a variety of locomotory patterns that utilize the molluscs well developed L-shaped foot (Fig. 11). These include, burrowing, jumping, ploughing and emerging (Fig. 12).

To loosen the sediment, cardiids eject water into the substrate at the beginning of each burrowing sequence (Wilbur and Yonge 1964) allowing for easier burrowing. The burrowing process consists of a small number of burrowing sequences (Savazzi 1985) in which the shell rocks forwards and backwards (Fig. 13), while being pulled downward by the foot (Ruppert, Fox and Barnes 2004).

Jumping involves the rapid extension of the muscular foot, propelling the shell backwards. It is not uncommon for several jumps to occur in response (Savazzi 1985).

Ploughing, horizontal movement through the sediment, is the process of repeated burrowing sequences, with the hinge line horizontal and the foot pointing forward (Savazzi 1985).

Laboratory tests completed by Savazzi (1985) found that movement involved in emerging from substrate occurred in a single movement, displacing the cardiid forward. Emerging was often a result of insufficient oxygenation (Savazzi 1985).

Figure 11
Figure 12
Figure 13

Anatomy and Physiology

Respiration and Feeding

Like most other molluscs, F. unedo respire through the use of gas exchange which primarily occurs via the gills. Additionally, when exposed from water, aerial respiration is possible through oxygen diffusion through the mantle (Kawaguti 1983).

Most heterodont bivalves are suspension feeders (Fig. 14), pumping water and plankton in through an inhalant siphon (Ruppert, Fox and Barnes 2004). Large ctenidia are used to filter water and food particles. Within ctenidia, water is moved using specialised cilia, that create current, causing the oxygenated and plankton filled waters to pump past the thin gill filaments of the demibranchs (Wilbur and Yonge 1964). This allows nutrients and gasses to diffuse across the cells. The filtered water is then pumped out through the exhalent siphon (Ruppert, Fox and Barnes 2004).

Figure 14


It is assumed that, F. unedo have a typical bivalve open circulatory system. With a three chambered heart, consisting of a single ventricle fed by a pair of lateral atria (Rudman 1998), situated in a pericardial chamber. The heart is used to pump recently oxygenated blood, oxygenated haemoglobin is then transported to the rest of the body through blood (Ruppert, Fox and Barnes 2004; Hickman et al. 2011). While veins return de-oxygenated blood back to the gills and eventually the heart.


It is assumed that, the excretory system of F. unedo is similar to many other bivalves. Made up of a complex, were the heart and kidney have become a single organ. The excretory system is separated from the circulatory system by the pericardium (Wilbur and Yonge 1964). The pericardium contains pericardial glands, which aid in the nutrient filtration process. The system contains two nephridium, that act as pseudo-kidneys and concentrate waste products (Rudman 1998). Each nephridium empties into the exhalant siphon chamber via their respective nephridiopore (Ruppert, Fox and Barnes 2004).

Nervous System

It is assumed that, F. unedo have a similar nervous system to other bivalves. Where a reduction of the head has resulted in limited cephalisation. Instead of having a single large brain, bivalves have four separated ganglia (Wilbur and Yonge 1964):

Cerebropleural ganglia, aids in the coordination of the muscular foot and the adductor muscle. Pedal ganglia, provide motor control over the anterior pedal retractor muscles, byssal retractor muscles and the muscular foot. Visceral ganglia, are responsible for the mantle, siphons, gills, heart, nephridia and gonads. Siphonal ganglia, receives sensory information obtained by the siphons (Ruppert, Fox and Barnes 2004).

As the head is located internally within the bivalves’ shell, it is unable to collect sensory information, and this task has been passed onto the foot and siphons (Rudman 1998).

Biogeographic Distribution

Fragum unedo is commonly found in the Indo-Pacific region. Prior literature shows that specimens have been found in Sri Lanka, southern Japan, the Pacific Islands as well as the northern and eastern coasts of Australia. (12.87N,93E, 41.23N,142.55E, 35.9S,159.08E) (E.o.L. 2012).
Figure 15

Evolution and Systematics


Evolutionally bivalves undergone a complete loss of their radula, and developed two shells (valves) that are bilaterally symmetrical, with their hinge as the axial point. Balvalia can be grouped into three major clades based on their morphological characteristics (Ruppert, Fox and Barnes 2004):

Protobrachs, representative of ancestral bivalves, they mostly deposit feeders with relatively small and simple diagonally orientated gills.

Lamellibranchs, consists of most extant bivalves, they are filter feeders with complex filibranch gills.

Septibranchs, gills have been replaced with a horizontal septum, they are the only bivalves that do not feed off particles, but instead they use their stomach to envelop small organisms.

Fragum unedo fall into the Lamellibrach clade, but have also shown additional evolutions processes, where they have developed a symbiotic relationship with photosynthetic microbes, zooxanthellae (Zooxanthellae Association).


Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Heterodonta
Order: Veneroida
Superfamily: Cardioidea
Family: Cardiidae
Genus: Fragum
Species: F. unedo

Common Name:
Strawberry Heart Cockle

Conservation and Threats

Human Use and Conservation

Prior work completed by Wright and Ricardi (2014), found that Fragum unedo, was an important economic subsistence species for Aboriginal communities in the western Torres Strait, before modern times.

As of yet the F. unedo has not yet been assessed for the IUCN Red List, and the current status of the species in unknown. However, like most other animals with a calcium carbonate, strawberry cockles are becoming more and more disadvantaged as the oceans become more acidic.


As F. unedo burrows just below the surface (Habitat) they are still susceptible to predation. In northern Australian cockle-bed, starfish and rays are important ecological predators of cardiids (Rudman 1998). The locomotory pattern of jumping (Locomotion) is used as a means to escape from predatory species, displacing the cardiid a few centimetres from their prior location.


Atlas of Living Australia. Distribution Map (n.d.) Retrieved from a2af40dc5edd

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Encyclopedia of Life 2012, “Fragum unedo”, Details, viewed 18 May 2016,

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