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Fiji Freshwater Shells

Fiji's freshwater snails and their shells (sici) are usually less than 25 mm long. Nevertheless, they with shrimps and prawns, are the most abundant invertebrates in fast flowing streams in Fiji and other Pacific Islands. The two main families of freshwater snails in the Fiji Islands are the round shelled Neritidae with 22 species and the tall turret shaped Thiaridae with 10 species. Both families have an operculum, a calcareous plate that is able to shut the snail inside the shell and make it safe from small predators. The neritid limpets of the genus Septaria have a reduced operculum that is embedded in the muscles of the foot. This makes them able to cling tightly to a rock face or a boulder in very swift flowing water.The Neritidae are most abundant within a few kilometres from the sea as they are generally amphidromous . Their eggs are laid in egg cases on stones or on the shells of other snails . The eggs hatch as swimming larvae, called veligers, that need brackish water to settle on the bottom and change into snails. The larvae on escaping from the egg case are washed downstream towards the sea. They can survive for weeks in sea water and may be carried by sea currents to another stream or even another island. As a result many species have been distributed throughout South East Asia and the Pacific islands. Once the larvae settle on the stream bottom, the tiny snails migrate upstream. Most do not travel more than 10 km but one species, Neritina pulligera (tabia, takau) is found in pools more than 50 km upstream on Viti Levu. They grow to 30mm long and are collected by Fijian villagers for food.
Reproduction is very different in the Thiaridae. These snails with tall shells that in some species reach a height of 100 mm, may live well inland in calmer water. Nearly all the snails are female and their eggs develop without being fertilised. The eggs are shed from the ovary into the mantle cavity where the embryos grow and are voided into the river when the snail is fully developed at about 3mm high.
The snails of both families have paired eyes and tentacles for sensing their surroundings. They are herbivorous and feed on the algal and bacterial film on the stones or mud on the bottom of the stream or river in which they live. They move over the bottom by contracting and relaxing the muscles of their foot while the teeth of their ribbon -like radula scrap their food from the surface of the stones.
Fiji has 12 endemic freshwater snail (Gastropoda) species. These are the thiarid, Fijidoma maculate ($1.20) stamp, the little Hydrobidiae Fluviopupa which has recently been divided into 10 species and the shell-less gastropod Acochlidium fijiense. This last animal, instead of having a shell, is covered in shiny spicules.
The main threat to freshwater snails is excessive sedimentation caused by logging of forests, road making, mining and the cultivation of steep hillsides. Silt and grit is washed into the streams and deposited on the bottom where if covers the snail's food. The snails will only return to the stream or river if and when the silt is washed away.

First Day Cover envelope: Septaria bougainvillei (Recluz, 1841) This limpet species is endemic to Fiji, New Caledonia and the island of Tanna (Vanuatu).

Larger Clithon have found on stones and rocks in the swift streams of all the high islands of Fiji. Until recently this shell was identified as Septaria porcellana but this species is found further north in Asia, South-East Asia, Solomon Islands,Vanuatu and parts of New Caledonia.

Fiji has four other species of Septaria. Three of them are also
40c Clithon diadema (Broderip, 1832).

This species is known from South East Asia, New Guinea, Solomon Islands, Vanuatu, Samoa and Fiji. It lives on stones near the mouth of streams, often in brackish water. The shells have rows of sharp long spines. The spines are thought to protect the snails from predators which are more common near the mouth of streams than further inland shorter spines as the spines get worn as the snail ages.

90c Neritina variegata (Lesson, 1831).

This species is found in South-East Asia, Solomon Islands, Vanuatu, Fiji, Samoa, Guam, Truk and Ponepe. It lives in streams on stones and boulders from a few metres from the sea to about 5 km inland. The shell is yellow or olive with black markings but it is often completely covered with a black encrustation. The operculum is
black and lacks the orange-red horn border found in other Neritina species. It is abundant in fast flowing streams.

$ 1.20  Fijidoma maculata
(Mousson, 1865). This species is endemic to Fiji and has been found only in inland Viti Levu in the Wailoa, Wainimala and upper Wainibuka and Ba rivers. It is not tall like other thiarids but is round like a neritid due to an enlarged body whorl. Its shell is adapted to living in swift flowing rivers. It reproduces by parthenogenesis and no male have been found. When the young are released from the brood pouch, they are 1.0 -1.6 mm high and are entirely brown with no dark red markings. They grow to about 14 mm high.

$2 Neritina squamaepicta (Recluz, 1843) .

This shell has a sharp point at the corner of its columella area made by an elevated fold. The columella has a bright red splash at its upper edge. It is found on stones not far from the sea in South-East Asia, Solomon Islands, Vanuatu, New Caledonia and Fiji. Only a few are present at any one site in Fijian streams, but quite a large population was found in slow flowing water in a shallow pool on Efate, Vanuatu.

$1.20	$2.00
40c	90c

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