The windowpane oyster family Placunidae Rafinesque , 1815 with additional description of Placuna quadrangula ( Philipsson , 1788 ) from India

The Bivalvia family Placunidae Rafinesque, 1815 in India is reviewed in this paper based on previous literature and records. Additionally, the species Placuna quadrangula is described from the Indian sub-continent. Being an economically important family in this geographic region, this paper can be regarded as a baseline for further ecological, management and policy-related studies pertaining to Placunidae and other exploited species.


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, the genus Placuna Lightfoot, 1786 has a long documentary history, as it is commercially exploited for pearl production, food, lampshades and shell-craft items (Gallardo et al. 1995). From Indian waters, Placuna placenta (Linnaeus, 1758) (Image 1) is the only species that has been thoroughly studied due to its economic value (Laxmilatha 2015a). Placuna sella (Gmelin, 1791) now P. ephippium (Philipsson, 1788) was first reported from the Andaman & Nicobar Islands, India by Prashad (1932), followed by Gulf of Mannar, southeastern coast of India (Rao & Dey 2000;Venkataraman et al. 2004) and was later revised by Stella (2010) (Image 2) from the Mandapam coast (Adjacent to GOM). Placuna ephippium is also exploited, especially in the Bay of Banate, Philippines and its surrounding areas, and mainly known for its food value (Gallardo 1994). Currently, as per published reports, two species of Placuna, P. ephippium and P. placenta are reported from the Indian coast (Rao 2017) (Image 3). In this study we confirm the presence of a third species from India, P. quadrangula (Philipsson, 1788). This paper further attempts to distinguish all three species from the Indian coast based on morphology.

Materials and Methods
Five valves of P. quadrangula were collected from three different locations along the eastern coast of India (Table 1). The collected dry specimens were cleaned and preserved in zip-lock covers for further assessment. Identification to species level was made based on the characters provided by Lynge (1909) and Huber (2010). Specimens were deposited in the National Zoological Collections (NZC) repository in the Marine Biological Research Centre (MBRC), Zoological Survey of India (ZSI), Chennai. A single specimen of P. placenta deposited at (ZSI/MBRC) was assessed additionally. Measurements of specimens were recorded with Yuri Digital Calliper 200x0.01mm. Study area map was created using QGIS 3.6.3 Noosa.

Systematics
Order Pectinida Gray, 1854 Superfamily Anomioidea Rafinesque, 1815 Family Placunidae Rafinesque, 1815 Genus Placuna Lightfoot, 1786 Placuna is monomyarian, with low umbones, V-shaped crurae and pallial line, often obscured. Valves circular to sub-circular and laterally compressed. All the characters are common in the species observed from the Indian subcontinent, and a detailed species wise description is given below.
Placuna placenta (Linnaeus, 1758) (Image 1) Description: Shell thin, very flat, roughly circular or subcircular in shape, inequivalve, periostracum absent. Inner surface smooth, outer surface lamellate, growth lines present. Transparent when juvenile, turning opaque with age. Lacking radial lines on the external surface. Crurae below the umbones, unequal in size, adductor muscle scar slightly anterior of midline. Pallial line obscure and non-sinuated. Specimen examined has a damaged or broken outline.
Distribution: Extends from Gulf of Aden in the west to Taiwan in the east (Matsukuma 1987;Huber 2010).

Placuna ephippium (Philipsson, 1788)
Description: Shell saddle-shaped with curved dorsal margin without periostracum. Growth lines visible. External colour purple brown/brownish with black shades to large red-purplish-blackish spots on the interior surface. Lacking radial lines on the external surface. Crurae prominent, equal in size and wideset. Single adductor muscle scar in the center of midline; purplish in colour.

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Remarks: The five examples collected from three different localities possess the typical radially rayed pattern.

Discussion
Comparative analysis among the Placuna species from India with some notes on other Indo-pacific species The radial colour patterns originating from the umbones (Image 3) a typical character for P. quadrangula is emphasized in Huber 2010. Although confused with P. ephippium which is larger, the specimen described from Mandapam has no mention of purple muscle scar nor the color predominance in the shell as seen in Huber 2010. Rather it is described as "almost transparent" along with light brownish nature with black patches (Image 3) (see Stella 2010). The transparent nature fits well with P. placenta juvenile as observed in the collections of the Ryukyu Museum (RUMF-ZM-03693) and in Kouri Shell Museum, Okinawa (Rocktim Ramen Das, pers. obs.) but is known to turn white and shiny after maturity. The presence of brown radial rays in the latter from southeastern Asian specimens (see Matsukuma 1987) might be misleading and needs reassessment. P. ephippium lacks such brown radial rays and possesses large red, purplish-black spots on the interior region (Henk Dekker pers. comm.).

Life history of genus Placuna
A review of literature revealed that there is a limited amount of research based on the life history of genus Placuna with exclusive information available only for P. placenta. Adam Young (1980), who did an extensive study on the larval growth and development of P. placenta, revealed that the shells remain inequivalve and transparent from a very early stage. The author also revealed that from fertilized egg to the formation of spat, it takes about 10-11 days and the final sedentary phase is reached when around 600µm in size is reached during which several key morphological changes take place, viz., active foot appearance during larval metamorphosis. Narasimham (1984), who later studied the biology of P. placenta from the eastern coast of India (Kakinada Bay) (Figure 1) mentioned about the biannual spawning strategy of the species and based on the gonadal appearance and morphology divided the maturity into four stages, viz.: active, ripe, partially spawned and spent/resting. Interestingly, a recent observation from the coast of Sonmiani (Balochistan) indicated that P. placenta spawns all-round the year (Parveen et al. 2018) which contradicts the findings of Hornell (1909) and Moses (1939) whose studies were from a not so distant area of Okha, Gujarat, India (~400 km). This probably indicates the local environmental parameters like temperature, salinity and monsoonal characteristics can play an important role at regional scales (Ladja 2002).

Status of genus Placuna in India
Along the coast of India, the windowpane oyster (P. placenta) was initially reported by Hornell (1909a,b). In the 1970s, Narasimham documented its utilization due to its high economic value while Laxmilatha (2015) reviewed the economic value. Though the species is reported from various places of the Indian sub-continent (Table 3), it is commercially exploited only from specific areas along the coastline ( Table 3). Exploitation of the species in areas of Gulf of Kutch was mainly for pharmaceutical purposes (Alagaswami & Narasimham 1973;Narasimham et al. 1993). Presently the exploitation levels are low in the Gulf of Kutch. In Kakinada Bay, P. placenta is regarded as one of the most important bivalve resources, but the stock is under threat due to overexploitation (Rao & Somayajulu 1996;Laxmilatha 2015b). Nauxim Bay in Goa had a minor fishery where the meat was locally consumed (Narasimham et al. 1993). Apart from the above-mentioned locations, the collection of windowpane oyster from the coastal waters of Tamil Nadu mainly for the pearl and shell craft industries. Vellapatti fishing hamlet near Tuticorin is the hub for the utilization of P. placenta in the cosmetic and paint industry. Rameswaram is famous for the windowpane oyster lampshades and mirrors. It is important to highlight that due to such activities, previous densely populated areas of P. placenta are lost (Tripathy & Mukhopadhyay 2015). The saddleshaped oyster P. ephippium collected in Mandapam, Gulf of Mannar is also used in the production of a variety of curios/souvenirs, viz., trays, lampshades (Stella et al. 2010). The third species P. quadrangula, reported here is either invasive or has been overlooked over the past decade. It is important to highlight that Iredale in the scientific reports published related to the GBR expedition refers to a publication from the Bolten Museum dating back to 1798 which mentions Ephippium anomia (synonym P. quadrangula) from Tranquebar (now Tharangambadi, ~230km from Chennai). Further in-depth analysis revealed the information is related to its morphological characters, leaving the information regarding its geography being rather vague (see Iredale 1939;Röding 1906).

Conclusion
Placuna placenta is the only species under the genus that is listed (as Placenta placenta) under Schedule IV of the Indian Wildlife Protection Act, 1972. As mentioned in the act, "No person shall hunt any wild animal specified in Schedules I, II, III, and IV except as provided under section 11 and section 12". Furthermore, Section 11 and 12 allows for hunting only in special cases and with proper documents and permissions from the concerned government authority; however, surreptitious fishing of this species continues even in protected areas, e.g., Kakinada Bay (Coringa Wildlife Sanctuary) (Laxmilatha 2015a,b) apart from other areas as mentioned above. Moreover, the recent news of P. placenta being smuggled to western Asian and South American countries in alternate forms (Ravinesh et al. 2018) further highlights the urgent need to assess the genus. As our study highlights the morphological aspects of this genera, a thorough comparative assessment of the internal organs and the application of molecular methods should provide essential insights. On the other hand, biogeographical assessment of P. quadrangula, its ecology and its implications on P. placenta distributions is urgent. As the Indian Ocean is home to large and unknown malacofauna (see Das et al. 2017), continuous surveys to discover these understudied resources remains imperative.