Cowries are mostly found in tropical, shallow water (< 30m). Most are omnivores that are widely distributed throughout the ocean (Burgess, 1985). There are approximately 220 recent living species and more than 500 extinct species of cowries (Kay, 1996).

The Purpuradusta gracilis is defined by having a bluish shell, with white spotted base, stained with brown to purple spots on the anterior and posterior canals (Burgess, 1985). Further, they can be distinguished by having prominent dark brown terminal spots, and a dark brown dorsal blotch on the dorsal surface of the shell (Burgess, 1985).
This species is commonly found in tropical shallow water oceans, with a size range of 30.4 mm to 9.0 mm (Burgess, 1985), and all individuals of Purpuradusta are recognised as small shelled cowry species (Meyer, 2002). 

Taxon details of Purpuradustagracilis (WoRMS Mollusca, 2013).

        Ventral surface of Purpuradusta gracilis (Photo by: Viveca Lim)

Siphon

The siphon is situated at the anterior end of the shell. It is of the same colour as the mantle (orange- pink), with colour fading to white away from the body. Widely- spaced white processes are located at the tip of the siphon (Burgess, 1985).

The siphon function as a respiratory organ, by drawing water into the shell and over the gills for gaseous exchange to take place (Walls, 1975).

Foot

The foot of Purpuradusta gracilis is broad and of a lighter coloration as compared to the mantle, with the ventral surface almost white (Burgess, 1985). Similar to the mantle, the foot of the cowry has several important functions:

A pair of tentacles are located at the anterior end, which bear a pair of well- developed eyes at the base of the tentacles. Tentacles of Purpuradusta gracilis are long, thick, blunt and tapered, with an orange- pink colouration (Burgess, 1985).

The eye of P. gracilis is considered to be rather complex within the class Gastropoda, with the presence of a lens, cornea, vitreous body and the retina (Gibson, 1984). The retina is composed of a mixture of both photoreceptor and pigmented cells, and nuclei of both cells can be differentiated by the shape, with both containing melanin, giving rise to the dark colouration (Gibson, 1984). The function of the eyes of P. gracilis are most probably solely built for the purpose of light and dark detection (research project).

Purpuradusta gracilis vision: A comparison study between P. gracilis and I. obsoleta

by Viveca Lim.

The aim of this project is to examine the structure of the eye of Purpuradusta gracilis by using Ilyanassa obsoleta as comparison, to determine the possible functions of the eye of P. gracilis.

Histology images of the eye of Purpuradusta gracilis was compared with the eye of a marine gastropod, Ilyanassa obsoleta (Gibson, 1984). Similar to I. obsolete, a pair of cephalic eyes are found in the P.gracilis, situating at the base of the tentacles (Gibson, 1984). The eye is spherical in shape and hollow in the middle, and within it sits the hard crystalline lens (fig. 1).

 Figure 2. Eye structure of P. gracilis (left) and I . obsoleta (right). Magnification of the eye of P. gracilis, showing the lens, vitreous body, retina and cornea. Magnification of the eye of I. obsoleta, showing the lens (L), optic cavity/ vitreous body (OC), retina (R) and cornea (C). (Image of P. gracilis eye by: Viveca Lim; Image of I. obsoleta adapted from: Gibson 1984).



Result shows that the eyes of P. gracilis is highly complex within gastropods, consisting of a closed vitreous body, a lens, cornea and retina (Gibson, 1984). When compared to the eye of I. obsoleta, the complexity of the eye is highly similar based on composition and structure, consisting of the vitreous body, lens, cornea, retina and neuropile. The neuropile is situated right at the back of the retina which contains the accessory neurons (fig. 2).

It was observed that the lens of P. gracilis is larger than that of the I. obsoleta, with lesser space between the lens and the retina (the vitreous body) (fig. 2). To enable images to form, vitreous body must be present to separate the retina and the lens (Zieger and Meyer- Rochow, 2008). Both the receptive and not receptive regions of the lens are surrounded by the vitreous body (Zieger and Meyer- Rochow, 2008). Larger vitreous body volume suggests that the animal has better visual abilities, capable of forming sharp images (Zieger and Meyer-Rochow, 2008). On the other hand, the lack or reduced space between the lens and retina suggests that the eyes are not constructed to focus images (Zieger and Meyer- Rochow, 2008). It was observed that the vitreous body volume in P. gracilis is relatively lesser than that of the I. obsoleta, as shown by the lens being situated closer to the retina (fig. 2). Thus, this suggests that P. gracilis has poor vision and the eyes are not constructed to focus images. The eyes of cowries are most likely built solely for the purpose of detecting changes in light levels.

Figure 3. Higher magnification of the eye of P. gracilis (left) and I. obsoletea (right) through the retina. P. gracilis eye, Photoreceptor cell nuclei and pigmented cell nuclei (rod shape) can be distinguished by their shape. I. obsoleta eye is bleached by potassium permanganate. Photoreceptor cell (PR) nuclei (circular) and pigmented cell (PC) nuclei (columnar) can be distinguished by their shape. Accessory neurons (AN) are situated behind the retina. (Image of I. obsolete eye adapted from: Gibson, 1984)



Further, it was observed that the retina of P. gracilis is not as pigmented as compared to that of I. obsolete (fig. 3), suggesting that I. obsoleta is a species of gastropod that is adapted to brightly lit environments (Gibson, 1984). This is true as they are found to inhabit intertidal muddy areas, in which they are constantly being exposed to high light intensity, hence, the retina needs to be deeply pigmented to protect the eyes from excessive sunlight. On the other hand, many species of cowries are nocturnal and negatively phototactic where they hide under rocks and crevices during the day. Although there are currently no literatures about the phototropic behaviour in P. gracilis, it was noted that the individual that was experimented on, demonstrated negative phototropic behaviour by seeking regions sheltered away from light, suggesting their preference for sheltered habitats with low light levels. Therefore, this suggests why the lens of P. gracilis is not as pigmented as compared to the intertidal gastropod species.

Factors affecting shell thickness

Calcium carbonate crystals form the shell of most molluscs, including the shell of cowries (Walls, 1975). The crossed lamellar arrangement of crystals, aligning in different directions over hundreds of layers, contributes to the increased strength of the cowry shell (Walls, 1975). This ensures that a mechanical damage on one layer of the shell, will be dissipated evenly throughout each layer (Walls, 1975).
 

 Sectioned cowry shells from different species (Adapted from: shells-of-aquarius.com) 

Gastropods shell thickness is dependent on several environmental factors (Foin, 1989):

1. Latitude and temperature

• Predation can reduce rate of growth (Foin, 1989)
• Predator abundance is higher in low latitudes (Irie, 2005)
• Higher pressure for the cowries to develop stronger shells to protect themselves against predation (Irie, 2005)
• Hence, harder shells at lower latitudes due to high predatory pressure (Irie, 2005)

• Cowries that are constantly exposed to high wave actions develop thicker lateral calluses (Foin, 1989)
• Fully extended mantle protecting the shell from abrasion observed in species inhabiting areas of strong wave energy (Foin, 1989)

The sex of Purpuradusta gracilis can be differentiated based on colour pattern of shell, with females being larger and more abundant than males (Webber 1977; Osorio et al.1999). Further, the male reproductive organ (penis) is located at the back of the right tentacle (Webber 1977; Osorio et al. 1999).

Mating behaviour in Barycypraea teulerei, a species of cowry (Image from: http://www.cypraea.info)

To copulate the female, the male extend and insert its penis into the genital aperture of the female, which is located at the posterior end. The female cowry is able to keep the active sperm in the seminal receptacle for days before fertilisation of eggs occurs (Walls, 1975). After copulation, the female cowry will choose an ideal location to lay eggs. The fertilised eggs are deposited in egg capsules which are attached to the substrate, and development begins (Moretzsohn, 2003). 

Although the final stage of development is mainly genetically controlled, resource quality and availability, and habitat quality are important factors that determine rate of growth of cowries (Walls, 1975).

Cyphoma gibbosum is a mesogastropod from the superfamily Cypraeacea and from the family Ovulidae, which is very closely related to cowries (Cypraeidae) (Ghiselin and Wilson, 1966).

Structures in the mantle cavity of Cyphoma gibbosum (Ghiselin and Wilson, 1966).

Posterior to the gill lies the hypobranchial gland (Ghiselin and Wilson, 1966). It is involved in secreting white mucous, and is also a site for chemical interaction and biosynthesis (Laffy et al., 2013).

Osphradium

The osphradium is a sensory organ that monitors the entry of chemicals and sediments in the water into the mantle cavity ((Ruppert et al., 2003). When high chemical and sediment load is detected, the ciliary on the osphradium will start beating, with water flowing over the gills to remove and prevent uptake of unwanted substances (Ruppert et al., 2003).

Magnification of gill of P.gracilis (Image by: Viveca Lim)

To enable efficient gaseous exchange to occur using as little energy as possible, the lamellae are positioned perpendicular to the water current, allowing water to move directly through the mantle cavity at a fast and efficient rate (Ghiselin and Wilson, 1966).

Radula and Odontophore

The radula and odontophore are unique feeding structures found in gastropods. Due to its distinctive structure, they are crucial in providing information for phylogenetic classification and comparative morphology, and phylogeny controls the diversification of radula and other related structures (Katsuno and Sasaki, 2008). 

In cowries, the radula is a long, strip- like membrane with many rows of small, hard, chitinous backwardly pointing teeth (Walls, 1975). The radula is located at the anterior part of the mouth and is able to extend from the mouth by using a complex range of muscles, to move back and forth across the odontophore, creating a grinding action (Walls, 1975).

As a result of high grinding activities, teeth are worn away quickly, however, only 10% of the teeth are used each time, and new teeth are continuously formed to replace old damaged ones (Walls, 1975). 

Torsion and partial detorsion

Process of torsion in gastropods (WSU Tri-Cities, 2001)

Tortion

Tortion results in a 180° counter-clockwise twisting of visceral mass, shell and mantle cavity, leading to the anterio- dorso movement of the mantle cavity and the organs, above the head; posterio- dorso movement of stomach; and anterio- ventro movement of the mouth and anus (stage 1 to 5 in image above) (Ruppert et al., 2003). Torsion is a defining characteristic that occurs in all gastropods (Ruppert et al., 2003).

Partial detorsion

Partial detorsion has been demonstrated in many derived gastropods. Detorsion of approximately 90°, results in the shifting of the anus, genital opening and gill position, posteriorly and to the right side of the body (Ghiselin and Wilson, 1966; Ruppert et. al, 2003). Partial detorsion of P. gracilis results in the shifting of organs back to stage 3 (in diagram above).

The most significant features classifying the cowries are their chronological and anatomical characteristics (Kay, 1990). Evidence suggests that cowries evolved in the Upper Jurassic period about 140 million years ago, in regions of shallow tropical waters of the ocean of Tethys (Kay, 1990). In the Upper Cretaceous period, diversification and evolution of cowries began (Kay, 1990).

The first Cypraeidae (Zitellia) found was fairly similar to the cowries today, with presence of apertural structure, lateral margins that are rounded, and labial teeth (Walls, 1975). Cowries today have retained most of its forms from the early Tertiary period, with most of species existing since the late Miocene (Kay, 1990). Evidence also suggests that cowries might have evolved from the Columbellinidae, which is an extinct group of gastropods, which might have evolved from Strombacea (conchs and strombs) and Tonnacea (triton and frog shells) (Walls, 1975).

A fossil cowry from the Eocene period, Gisortia gisortiana
(Image from: http://odin.pagesperso-orange.fr/tec/collection.htm)

Evidence suggests that cowries from the genus Gisortia (image above), was highly abundant during the lower Tertiary period in the ancient Tethys Ocean. Shell of this species was thick and heavy and they were massive in size (> 35 cm) (Walls, 1975).  

Genus Purpuradusta has been proven to be a monophyletic group. There are 2 main subclades within the genus Purpuradusta; with the first subclade consisting of P.serrulifer, P. minoridens, P. microdon and P. hammondae; and the second subclade consisting of P. gracilis and P. fimbriata (Meyer, 2003).

P. gracilis is differentiated into 2 groups based on haplotypes; P. gracilis gracilis is found in the Indo- West Pacific Ocean, inhabiting the central Indian Ocean and Andaman Sea, while the P. gracilis notata is a special group occupying the West Indian Ocean (Meyer, 2003).