In Search of Big Eye

Neritina labiosa Sowerby, 1836 (figure 1: 1a–e) is a little known species despite being one of the largest freshwater nerites. The shell is rather similar in shape to Neritina asperulata (Récluz, 1843) (figure 1: 2a–e) and to Neritina petitii (Récluz, 1841) (figure 1: 3a–e) but it can be readily distinguished from these and all other species of Neritina by its operculum. On the inner side, in place of the usual peg-shaped lower apophysis, there is a distinctive series of 5 to 8 serrations. No type locality was given with the original description but subsequently Sowerby (1849: 514) referred to this species as being from “Island of Luzon, Philippines”. It has also been recorded from northern Celebes, Amboina, Java and the Solomon Islands (Delsaerdt, 1998). Within the Solomons it appears to have a restricted distribution. Neither Haynes (1993) nor Starmühlner (1976), both of whom surveyed various rivers in the Solomon Islands, found N. labiosa and Delsaerdt only reported it from two rivers, the Ndoma and the Mataniko both of which are in Guadacanal.

In March this year I spent a week in the Solomon Islands with my wife; three days on Malaita Island and the rest of the time in Guadacanal. This gave me an opportunity to check some of the rivers and record the various nerites found in them. The area is something of a hotspot for fresh and brackish water Neritidae with over 20 species present. Most of the rivers I inspected in Malaita proved to be rich in nerites with some species, such as Neritina roissyana (Récluz, 1841) and Clithon corona (Linné, 1758), abundant. I did not, however, find any trace of N. labiosa which I was particularly hoping to see.

In Guadacanal my initial efforts were hampered by heavy rain the day before we arrived which had turned the rivers into brown, raging torrents. Indeed my first attempt to reach the Ndoma River from Honiara was frustrated by an intervening river where the ford on the main road was far too deep for the taxi to cross. Fortunately the river levels soon returned to normal and the water cleared. When I eventually reached the Ndoma a couple of days later the river proved to be rather disappointing compared to those of Malaita; I only found C. corona and Neritina pulligera (Linné, 1767) and there was no sign of N. labiosa. The other river from which Delsaerdt recorded this species, the Mataniko, flows through the capital, Honiara. Like many rivers in large towns it is littered with rubbish and appears, at first sight, to be an unlikely home for molluscs (figure 2).

On closer inspection, however, I found N. roissyana, C. corona and N. auriculata Lamarck, 1816 to be common in the tidal part of the river. Prompted by this I decided to check the river higher up. I found a suitable spot about two kilometres from the mouth with clear water flowing over stones, rocks and submerged logs. Clithon corona, Clithon squarrosus (Récluz, 1842) and Clithon olivaceus (Récluz, 1843) were abundant and several species of Neritina were present including N. asperulata and N. petitii but not N. labiosa. However local villagers confirmed that a nerite answering the description of N. labiosa could be found there; they know it as Big Eye (figure 3).

The following day we were due to fly back to Brisbane in the early afternoon. Encouraged by my wife’s desire to see more of the river, I decided to make a final attempt to find Big Eye. We went further up river before making enquiries. We were told that Big Eyes are delicious to eat and are very popular locally; they are nocturnal and best found by torchlight, being almost impossible to find during the day. This may well be an example of adaptive behaviour; certainly it appears to be the case with N. pulligera which is also eaten locally. Although not uncommon, all the N. pulligera which I found in the Mataniko River were completely hidden from sight on the undersides of submerged trunks of fallen palm trees. In other countries, however, where N. pulligera is not eaten by humans, it is active by day and in Malaysia I have seen considerable numbers crawling over rocks just below the surface. As we were due to leave in a few hours there was no possibility of searching the river at night and that will have to wait for a future visit. However one of the women eventually took pity on me and went off to her village, returning a few minutes later with a handful of empty Big Eye shells. Fortunately one still had its operculum inside and from this I was able to confirm that Big Eye is indeed N. labiosa.

_{figure 2: Mataniko River from the Town Bridge, about 0.5 km from the mouth (tidal).}

_{figure 3: Mataniko River about 3 km from the mouth, where N. labiosa is found.}

figure 1 (left) Neritina from Mataniko River, Honiara, Guadacanal, Solomon Islands.

Key: a. apertural, b. apical, c. abapertural views,
d. operculum outer side, e. inner side.

1a–e: N. labiosa: Height 36.5 mm, width 38 mm. Operculum length 22 mm.

2a–e: N. asperulata: Height 21.5 mm, width 22 mm. Operculum length 11.5 mm.

3a–e: N. petitii: Height 21 mm, width 21.5 mm. Operculum length 12.5 mm.