The family Pinnidae Leach, 1819, comprises of three genera Atrina, Pinna and Streptopinna. They are semi-infaunal, relatively large bivalves, characterised by a thin, wedge-shaped shell (Audino & Marian, 2020; Beer & Southgate, 2006). In some species, Pinnidae can reach lengths > 50cm (Rangel et al., 2016). They are commonly found partially buried in gravel, sand or mud substrates, or attached by thick, silky byssus threads to hard substrates by their anterior region (Figure 1) (Audino & Marian, 2020; Printrakoon et al., 2018). The body of Pinnidae species is often vertically positioned in the sediment whilst above the sediment, their enlarged posterior region is exposed (Audino & Marian, 2020). Pinnidae have two adductor muscles, with the ‘callus’ (i.e. the largest posterior muscle), commercially harvested for human consumption (Rangel et al., 2016). In the Indo-Pacific, Pinnidae are cultured or harvested from wild populations as a prized delicacy (Beer & Southgate, 2006). The family Pinnidae are considered to be ecologically and economically significant.

The described species, Pinna bicolor, belongs to genus Pinna, and can be distinguished from the other genera by a nacreous layer divided by a sulcus into dorsal and ventral lobes on the valve’s interior (Lemer et al., 2014). Pinna bicolor has a wide geographic distribution in littoral sand and mud habitats in the Indo-pacific (Beer & Southgate, 2006; Wu & Shin, 1998). In Australia, Pinna bicolor can found along both tropical and temperate coasts (Beer & Southgate, 2006). The species is long-lived, they can attain a size over 50cm and can grow relatively fast (Beer & Southgate, 2006). These features of the Pinna bicolor, along with the economic value of their meat (i.e. the ‘callus’) results in heavy exploitation by fisherman (Beer & Southgate, 2006).

Common Names

The family, Pinnidae Leach, 1819, has many common names and as a result, can often be mistaken for other families or orders. Most species within the family, on first observation, are often referred to as pen shells, razor clams, fan mussels, razor shells and fan shells (Atlas of Living Australia, 2021).

The shells of fan mussel, Pinna bicolor, resemble an inequilateral triangle with a rounded posterior margin that is attenuated to the anterior margin (Allen, 2010; Idris et al., 2009). The species has two adductor muscles and can attach to substrate by fine byssal thread (Audino & Marian, 2020). Pinna bicolor has an internal nacreous layer that is divided into a dorsal and a central lobe by a sulcus (Printrakoon et al., 2018). The internal nacreous layer exhibits a slow growth relative to the rest of the shell and consequently the primary part of the shell is comprised of a calcite outer layer (Cuif et al., 2020). In order to distinguish species, the location of the posterior adductor muscle scar with reference to the margins of the nacreous layer is ascertained (Lemer et al., 2014). Generally, the posterior adductor muscle scar is positioned on the dorsal lobe and in the nacreous layer (Lemer et al., 2014). It is one of the primary taxonomic characters applied to distinguish Pinnidae species. According to Idris et al. (2011), the mean total length of sampled P. bicolor can reach 22.11cm whilst the width was found to average at 9.36cm. The species is generally translucent, and specimens can range in colour, from being described: as yellowish to light horn to dark brownish purple (Idris et al., 2009). The nacreous layer of Pinna bicolor is iridescent and has an average length of 13.32cm (Idris et al., 2009).

Habitat

The razor clam Pinna bicolor has an extensive geographic distribution in the Indo-Pacific (Rosewater, 1961; Wu & Shin, 1998). Pinna bicolor can generally be found partially buried in muddy sand and reef flats (Wu & Shin, 1998). The species is usually found in shallow waters, approximately 2 to 6 metres deep (Wu & Shin, 1998). In Australia, Pinna bicolor is distributed along tropical and temperate coasts of Australia (Beer & Southgate, 2006). The species is a conspicuous and a relatively large contribution to the fauna of the gulf floors in South Australia (Butler & Brewster, 1979). In addition to muddy sand and reef flats, Pinna bicolor can also be found in shallow seagrass habitats (Macreadie et al., 2014). The species can occur in dense clumps of 8 per m² (Butler and Brewster, 1979). As a result, professional and amateur fisherman alike can collect individuals of the species with ease (Butler and Brewster, 1979).

Feeding

As aforementioned, Pinna bicolor, partially buries its sharper edge into the sediment whilst its posterior extension of the shell, mantle and gills are exposed above the sediment to facilitate filter feeding (Sudatta et al., 2020; Wu and Shin, 1998). The burrowed edge anchors itself using byssal threads; ensuring the individual holds steady in currents (Davenport et al., 2011). This is particularly useful as some Pinna bicolor individuals are found with the posterior end orientated with the water current as to facilitate filter feeding (Davenport et al., 2011).

Characteristic of many of its Bilvalvia relatives, Pinna bicolor survives by filter feeding, particularly suspension feeding (Newell, 2004). The posterior extension of Pinna bicolor projects into the water column and via their ciliated gills food particles are obtained (Newell, 2004). P. bicolor filters suspended particles from the water column and leaves undigested remains to be ejected as mucus-bound feces which then sink to the sediment surface (Newell, 2004). Pinna bicolor generally feed upon detritus, zooplankton, bacteria, and phytoplankton (Davenport, 2011). According to Davenport (2011), it is likely that smaller individuals of Pinna bicolor inhale particulate material with higher organic matter contents than larger shells. This is hypothesised to be due the idea that smaller shells are generally closer to where the detritus settles and is thus enriched by fungal and bacterial activity (Davenport, 2011). In addition, the family Pinnidae have distinct rejection channels that move from the labial palps though the mantle surface, to the point between gills and mantle, close to the exhalant current (Davenport, 2011). These likely contribute to particle selection.

Many Bivalvia individuals do not filter at a constant rate nor continuously; however, some bivalves with limited food adapt continuous filtering. In a study by Butler et al., (1993), Pinna bicolor individuals were observed open and actively pumping throughout both day and night; with the exception being during low tide or when disturbed (Butler et al., 1993).

Predation

A study by Wu & Shin (1998) discovered that mortality of P. bicolor is primarily a result of predation by fish and crabs. The study suggests 95% of natural mortality in the species is likely linked to fish/crab predation (Wu and Shin, 1998). An additional study focusing on predation on Pinna bicolor named fish as their major predators (Beer and Southgate, 2006). In their research, fish were recorded biting through the exposed umbonal portion of the shells to remove flesh (Beer and Southgate, 2006). Predators of Pinna bicolor can also include predation by starfish, octopus, and gastropods (Butler and Brewster, 1979).  Predation is often distinguished by an empty shell. In regard to specific predators, predation by naticid gastropods can be recognised by a tapering bore hole, muricid gastropods are identified by a non-tapering bore hole whilst predation by fish/crabs results in broken shells (Beer and Southgate, 2006).

In Pinna bicolor, highest mortality is prominent in the small-size pen shell class and correspondingly, lowest mortality in the large-size class (Wu and Shin, 1998). The small-size pen shell class is prone to a multitude of predators that take advantage after small Pinna bicolor settle whereas large shells are confined to C. calamaria, the largest starfish, and large fish (Butler and Brewster, 1979). Evidence also suggests that within the first 12 months of growth, the natural population of P.bicolor is controlled by its predators and rate of mortality (Wu and Shin, 1998).

In addition, species of Pinnidae can regenerate damaged shells and mantles thus making them more likely to survive attacks (Wu and Shin, 1998). This is suggested to reduce natural mortality (Wu and Shin, 1998).

Disease

Pinna nobilis, a much-studied species within the Pinna genus, suffered large decreases in populations worldwide due to a mass mortality event provoked by Haplosporidian protozoan parasite (Panarese et al., 2019). Although Pinna bicolor has not suffered similarly, it is still prone to many of the same dangers and potential diseases (Carella et al., 2019). For marine invertebrates, disease is often associated with increased water temperature, host reduced immune competence and growth, and pathogen distribution and virulence (Carella et al., 2019). With the ever-growing impacts of climate change, the sea surface temperature is increasing. In addition, the effects of climate change can also affect the distribution of water-pathogens due to shifts in precipitation (Wu et al., 2016).

The ecological importance of Pinna bicolor

Pinna bicolor, alike many Pinnidae species, provide significant ecological and economic benefits for their habitats. Pinnidae are known to host a myriad of commensal organisms that enjoy food supplies in the ejected particles trapped by the bivalve mucus and find refuge in the mantle cavity (Audino and Marian, 2020). In a study recording epifauna of Pinna bicolor in South Australia (Ward & Young, 1984), it was found that the epifauna of P. bicolor contained 72 taxa, a large portion of species were identified as gastropod molluscs, sponges, bivalve molluscs, bryozoans, chitons and barnacles (Ward and Young, 1984). Not only is Pinna bicolor itself an ecologically important species due to its contribution to biodeposition, it also hosts ecologically significant species.

Pinna bicolor are suspension-feeding bivalves that contribute to biodeposition through their rejected mucus-bound feces sinking to the sediment surface (Newell, 2004). This biodeposition is vital to regulating water column processes in coastal systems in which bivalves are abundant and during active feeding seasons (Newell, 2004). Bivalves, such as Pinna bicolor, within these regulated conditions can subsequently reduce the dominance of phytoplankton production by exerting top down grazer control (Newell, 2004). In addition, these top down effects can also reduce turbidity and thereby increase the level of light that reaches the sediment surface (Newell, 2004). Subsequently, this can extend the depth at which ecologically important benthic plants may grow (Newell, 2004).

Furthermore, there are situations where P. bicolor can wield bottom up nutrient control on phytoplankton production by altering the nutrient regeneration processes within the sediment (Newell, 2004). In addition, the biodeposits produced by the bivalves can, in certain conditions, permanently remove N from the sediments as N2 gas (Newell, 2004). This could potentially have useful applications in curbing anthropogenic N and P input to eutrophied aquatic systems (Newell, 2004). This potential application may be more effective when considering Bivalvia as a whole, instead of the one species, Pinna bicolor.

​Season of spawning
According to Roberts (1984), gonad histology evidence suggests seasonal reproductive cycles with spawning occurring in autumn.

Duration of the planktonic phase 
​The larvae of Pinna bicolor are telepanic; meaning they have an extended planktonic existence. The planktotrophic bivalve larval period is estimated to typically last 3 to 4 weeks and according to Butler (1987), the larvae are predicted to settle from late December to late February. 

Natural History 

The life history of Pinna bicolor is comprised of a planktonic larvae stage and a sedentary adult life stage (Stirling et al., 2018). The extended planktonic stage ensures the veliger (free-swimming) larva to travel long distances before settling into the sedentary adult life stage (Stirling et al., 2018). Efficient Pinna larva settle into the sediment and metamorphism begins as to lead the larvae into the adult life stage (Allen, 2010). The adult shell develops by the extended mantle at the posterior and ventral margins of the larval shell (Allen, 2010). Until the adult shell is three times more than the length of the larva, the hind gut remains within the confines of the larval shell (Allen, 2010). During this stage, the velum of the larvae erodes off as fine cellular debris (Allen, 2010). Metamorphosis continues until the gill axis extends posteriorly towards the mouth and the gills function in a manner described for adult Pinna bicolor (Allen, 2010).

Lifespan & Growth

Pinna bicolor can live up to 18 years (Wu and Shin, 1998). Their growth is rapid, and individuals of the species may attain a shell length of approximately 300mm in one year (Beer and Southgate, 2006). The species can attain a maximum size of over 50cm (Beer and Southgate, 2006).

​

The family Pinnidae has unique morphological features, adapted to environments rich with suspended particles (Audino & Marian, 2020). Specifically, the pallial organ and the waste canal, are two specialised structures that have provoked the interest of morphologists (Audino & Marian, 2020). 

The pallial organ is comprised of a distal, swollen head and a proximal stalk which it utilises to clean the suprabranchial cavity, where it is located (Audino & Marian, 2020). The waste canal, on the other hand, is considered an important adaptation for cleansing the mantle cavity (Audino & Marian, 2020). These two adaptations are considered very important to the Pinnidae family as its species rely on feeding upon suspended particles which can fill their mantle cavity with surplus sediment and in turn obstruct their ctenidial filaments (Audino & Marian, 2020).

There is lack of literature regarding anatomy and physiology of Pinna therefore its anatomy and physiology will be summarised through similar points as made in a review of a Pinnidae species as referenced in Grave (1907)

• The venous system lacks the sinus venosus
• In transversing the kidney, the blood passes through a closed capillary system.
• The blood, after entering the gills, must pass through a capillary system before re-emerging
• The species has two adductor muscles 
• It has an internal nacreous layer that is divided into a dorsal and a central lobe by a sulcus (Printrakoon et al., 2018).
• The waste canal is formed by two parallel folds (Audino & Marian, 2020)
• They possess giant kidneys capable of producing large quantities of metal-sequestering nephroliths (Reid and Brand, 1989)

​Reproduction

This species is believed to be gonochoristic (Roberts, 1984), meaning the male and female reproductive organs are found in different individuals.

Simply put, reproduction occurs by an annual mass spawning event in which male individuals release sperm into the water and the female draws the sperm through her siphons for fertilisation to occur. The larvae are developed inside the female’s shell and once released, the larvae in veliger form will travel until settling along the sea floor to prepare for their adult sedentary phase.

In a recorded reproductive cycle of Pinna bicolor (Butler, 1987), the species has a single, summer peak with gametogenesis occurring during late winter and spring. The species is considered to be a brief breeder, with the season in which males and females are ready to mate being relatively brief. After males experience a peak in gonad development, spawning generally occurs during the warmest months (this is usually December, January and February). Factors such as temperature and food can critically affect the duration of gametogenesis and spawning: temperature and food.

The bivalve gonad system, present in Pinna bicolor, consists of a system of tubules that can be differentiated into ducts and follicles (Tranter, 1958). The follicles within the gonad is where the germ cells are produced (Tranter, 1958). The sperm cells travel to the vestibule via the primary genital follicles; however, it is ciliary action which likely causes spawning (Tranter, 1958). There is a long tract of cilia at the gonopore itself that assists with spawning (Tranter, 1958). Gametogenesis in Pinna bicolor includes spermatogenesis and oogenesis. Spermatogenesis is where stem cells and spermatogonia exist in in the follicle wall and gradually fill up to develop spermatocytes (Khamdan, 1998). In spermatogenesis, the spermatozoa continue to develop and increase in number, preparing for the spawning event. (Khamdan, 1998) The mass spawning event occurs when a gap between the mass of spermatozoa and the follicular wall presents itself (Khamdan, 1998).  The oogenesis sees the active development of ovary where the oocytes attached to the follicle wall undergo cytoplasmic maturity (Khamdan, 1998). Maturity is indicated by the ova changing shape significantly and relieving pressure from the follicle wall (Khamdan, 1998). This indicates it is time for spawning (Khamdan, 1998). The male and female events present in gametogenesis are synchronised in Pinna bicolor (Khamdan, 1998).

​Local Distribution

In Australia, Pinna bicolor occur along temperate and tropic coasts of Australia (Figure 1). The species is patchily common, yet still relatively common, in South Australia and south-west Western Australia (Coleman and Cuff, 1943). It is quite common along the coasts of Queensland and New South Wales. However, it is much less likely to occur in Victoria and there is a lack of sightings for Tasmania (Coleman and Cuff, 1943). Locally, Pinna bicolor has been observed in Townsville, Queensland, Australia on mixed sand cobble flats (Idris et al., 2009).

Global Distribution

Pinna bicolor is widely distributed in the tropical and subtropical Indo-west Pacific (Butler and Brewster, 1979). Many publications have involved Pacific Pinna bicolor specimens from Japan to Australia (Lemer et al., 2014).

Evolution

Pinna bicolor belong to the Mollusca phylum, which likely evolved more than 500 million years ago (Lemer et al., 2014). By the early Cambrian, the phylum had evolved hard outer mineralized shells composed of calcium carbonate (Lemer et al., 2014). In regard to Pinnidae, the elongated triangular outline of the bivalves is an adaptation to their sedentary life in which they maintain a vertical position partially buried in sandy sediments by strong byssal threads (Chinzei, Savazzi and Seilacher, 1982a). 

A highly cited revision of Pinnidae taxonomy identified 55 species distributed within the three genera: Pinna, Atrina and Streptopinna (Lemer et al., 2014). It is hypothesised that Atrina appeared and diversified first during the Carboniferous period as opposed to the genus Pinna, which likely appeared in the Jurassic period (Lemer et al., 2014). In this period, it likely developed a divided nacreous layer - the defining taxonomic feature of Pinna (Lemer et al., 2014). Lastly, Streptopinna appeared most recently during the late Tertiary. This revision identifies three genera whereas a recent review utilising molecular data suggests Streptopinna is a subgenus of Pinna (Lemer et al., 2014). However, at present, Streptopinna is recognised as genera of Pinnidae. 

Systemics 

kingdom     ANIMALIA

            phylum     MOLLUSCA

                        class     BIVALVIA

                                    subclass     PTERIOMORPHIA

                                                order     OSTREIDA

                                                            superfamily     PINNOIDEA

                                                                        family      PINNIDAE

                                                                                    genus     Pinna

                                                                                                species     Pinna bicolor

 

Presently, Pinna bicolor is not a vulnerable species, however, it is exploited by fisherman due to its economic value (Wu & Shin, 1998). It is hypothesised that severe exploitation of natural populations of P. bicolor may result in drastic reductions in populations as P. bicolor has a limited reproductive investment (Wu & Shin, 1998). In Australia, the species is exploited for bait and food, in addition to being removed from recreational waterways due to their sharp edges (Macreadie et al., 2014). In Lake Macquarie, eradication of Pinna clams was suggested from seagrass meadows to protect citizens (Macreadie et al., 2014). Furthermore, population numbers of Pinna bicolor seem to decline when near an urban settlement (Butler and Brewster, 1979). This is likely due to the collection of Pinna bicolor, made easy by their preference for shallow habitats, by fisherman (Butler and Brewster, 1979).