L A N D SNAILS OF M O T U H O R A ( W H A L E ISLAND) , B A Y OF P L E N T Y , N E W Z E A L A N D

by C .C . Ogle Science Directorate, Department of Conservation, P.O. Box 10420, Wellington, New Zealand.

A B S T R A C T

Fifteen species of land molluscs are listed from Motuhora (Whale Island), 14 of which are indigenous to New Zealand. Their distribution on the island is related to site suitability.

I N T R O D U C T I O N

During a visit to Motuhora in May 1984,1 collected shells of two indigenous species of land snails from under dead wood in pohutukawa (Metrosideros ex-celsa) forest at the island's summit. Because these were the first known collec-tions of land snails from Motuhora (F. Climo, pers. comm.), further collec-tions were made during the visit there by the Offshore Islands Research Group ( O I R G ) , December 28 1985-January 5 1986.

M E T H O D S

Four one-litre samples of litter were collected under different forest remnants on Motuhora and were sorted for land snails. In addition, live snails were in-dividually hand-collected from the bark of standing trees. Other snails were taken from three litter samples which D. Court searched for Arachnida, these litter samples being much larger, of the order of 20-30 litres each. A l l litter was sorted by hand on the island. Specific identifications of the snails were made by Dr. F. Climo of the National Museum, and the specimens are lodged at that museum.

RESULTS

Table 1 lists the land snails identified in each litter sample and summarises the main characteristics of each collection site. In addition to the 12 species of land snails found in the seven litter samples, two live specimens of Flam-mulina perdita were collected from the bark of mahoe (Melicytus ramiflorus) trees on Pa H i l l , and several old, broken shells of Helix aspersa were found below a rock wall near the mouth of Camp Valley. In May 1984, I collected one shell of each of Charopa coma and Phenacohelix giveni from under dead wood below pohutukawa forest near the island's summit (approximately the same location as litter sample D in Table 1).

The highest density of land snails (35 shells per litre of litter) was found in Department of Conservation Scientific Paper No. 10

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Litter Sample and Number of Land Snails Present

A B C D F F G Quantity of litter (litres) 1 1 1 1 20-30 20-30 10-15 Location 70m a.s.l.,

below summit of Pa Hill; old

Maori Pa terrace

10m a.s.l., 4m a.s.l., watercourse on west side of south side of Sulphur Bay;

Pa Hill steep coastal hillside

330m a.s.l., close to main summit; flat

30m a.s.l., upper part of

Camp Valley on east side of

Pa Hill

As for sample E As for sample E, a few metres away

Vegetation Tall pohutu-kawa/sparse

mahoe

Tall pohutu- Tall pohutu-kawa/whau kawa/dense

mamaku

Pohutukawa trees

surrounding mahoe forest

Tall pohutu-kawa/mahoe-

whau (top litter) (Deep litter)

Species RHYT1DIDAE Delos coresia (Gray) 1 1 3 3 1

C H A R O P I D A E Champa montivaga

Suter 2

'Geminoropa' cf. tookiana Dell 1 1

Phenacharopa pseudanguicula

1 rc dale 1 Flammulina chiron

(Gray) 1 1 2

'Flammulina' feredayi (Suter) 1

Allodiscas planulatus 1 (Hutton)

Mocella eta (Pfeiffer) 1 1

Cavellia buccinella (Pfeiffer)

3

PUNCTIDAE 'Laoma' n.sp., cf. poecilosticta

(Pfeiffer) 3 1 20 11 14 29

punctid. n.sp. 29 1

punctid lateumbilicata 1 (Suter)

TOTALS 4 3 3 25 15 20 35

Table 1. Numbers of land snails in litter samples for Motuhora (Whale Island).

sample D, taken from the island's summit. Samples E , F, and G had similar numbers of snails to sample D, but were much larger in volume. The greatest variety of snail species was in litter sample G from forest in the upper part of Camp Valley. Four of the six species of snail in sample G were found in this sample only.

DISCUSSION

Samples E and F were taken only a few metres from sample G, and were most similar to each other in their snail faunas, in terms both of species pre-sent and numbers of each species. Sample E was from the loose, dry, upper litter layer, and sample F comprised the deeper, moist litter from the same site. The upper parts of the floor of Camp Valley appeared to be one of the few permanently moist snail habitats on Motuhora, with many ground ferns and bryophytes, rotting logs, and fallen fronds of tree ferns. By comparison, the island's summit forest seemed dry during our survey. However, the presence of filmy ferns, orchids and other epiphytes on the pohutukawa trees of the summit area (Ogle 1990) indicates that the summit is often shrouded in mist, and dry conditions will prevail there for shorter periods than at lower altitudes on the island. This moistness may be sufficient to sustain greater densities of land snails on the summit than in other areas sampled.

Solem et al. (1981) stated that dry conditions are unfavourable for land snails, and that total annual rainfall is probably of less importance in determining site suitability for snails than is the frequency and duration of dry, stress-creating periods. The nearest rainfall recording station to Motuhora is at Whakatane, 9 km south south-west of the island. Data for the period 1947-80 in Quayle (1984) show that the mean annual rainfall at Whakatane was 1328mm, with the lowest in a single year being 919mm. The driest month was normally January (mean monthly rainfall = 83mm), although the single lowest mon-thly total in the 1947-80 period was 7mm in Apr i l 1958 ( A . M . Quayle, pers. comm.). Quayle (1984) stated that droughts (15 or more days with no rain) and dry periods (15 days or more with less than 1mm of rain on any day) are twice as common in the Bay of Plenty as in Auckland, with an average of one drought and two dry spells each year at most stations. Motuhora is likely to be drier than Whakatane, and of the areas near Auckland discussed by Solem et al. (1981), probably most resembles the Manukau Heads in its rain-fall patterns. As a consequence of its relatively dry and drought-prone climate, Motuhora would be expected to have few species of land snails, even if it were not an island.

Relating Motuhora's snail fauna to the snail habitat categories of Solem et al. (1981) shows that most of Motuhora's snails are characteristic of dry or well-drained habitats. Mocella eta, Cavellia buccinella, punctid n.sp. 29, and punc-tid lateumbilicata, are each stated to be dry litter species; Allodiscusplanulatus, Champa coma, Phenacharopa pseudanguicula, and Phenacohelix giveni are

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species of well-drained sites under stones or rotting logs. Champa montivaga is also a species of open, well-drained litter, less closely associated with rocks or logs than C. coma (F. Climo, pers. comm.). Solem et al. (1981) stated that the ground surface is the habitat of 'Laoma'n. sp. cf. poecilosticta (under well-aerated but moderately-wet litter) and Geminoropa cf. cookiana (under deep, friable litter). Delos coresia is known to inhabit moist sites with large, rotting leaves, especially the bases of mamaku tree ferns (Cyathea medullaris), a local snail habitat type on Motuhora. Flammulina chiron is recorded from similar sites, and under damp logs, and F. perdita is an arboreal species with the abili-ty to seal its shell in dry weather (Solem et al. 1981).

Of the island's known snail fauna only Flammulina'feredayi is characteris-ed by Solem et al. (1981) as an inhabitant of deep, wet litter.

The means by which Motuhora received its land snail fauna are unknown. The 7km stretch of sea between Motuhora and the mainland has a maximum depth of less than 40m. Because sea-level depression during the last Pleistocene glaciation would have maintained a land connection to Motuhora until some 10000-12000 years ago (Hayward 1986), and there has been no devastating volcanic activity on the island since then (B.W. Hayward, pers. comm.), species of land snail may well have survived on Motuhora since its separation as an island.

In addition, some land snails may have reached the island by rafting. Large quantities of logs and other terrestrial vegetation were floating in the sea bet-ween the island and Whakatane as the OIRG party left the island on 5 January 1986. Presumably, this material had come down rivers of the Bay of Plenty region during floods which occurred over two days preceding our departure. Although nothing is known of the time taken by floating debris to reach Motuhora from adjacent rivers, it could be as little as a few hours and hence flood debris offers a possible route for forest-inhabiting snails to have reached the island in the past.

That four of the seven litter samples had one or more land snail species uni-que to that sample strongly suggests that Motuhora has a richer land snail fauna than has been identified by this survey. Sampling in other seasons will un-doubtedly reveal additional species (Solem et al. 1981), a view supported by the fact that although I collected of Champa coma and Phenacohelix giveni in May 1984, these species were not found in January 1986.

ACKNOWLEDGEMENTS

I thank other members of OIRG who assisted in sorting land snails from litter samples, par-ticularly David Court, Ian Southey, and my son, Rhys. I am also grateful to Dr Frank Climo of the National Museum for his identification of specimens from Motuhora, and for additional in-formation on the species found. Ms K. Walker and Dr M . Crawley of the Department of Conser-vation supplied useful criticisms of a draft of this paper.

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REFERENCES

Hayward, B.W. 1986: Origin of the offshore islands of northern New Zealand and their landform development. In: The offshore islands of northern New Zealand (editors A . E . Wright, R.E Beever). Department of Lands and Survey, Wellington, Information Series No. 16: 129-138.

Ogle, C . C . 1990: Changes in the vegetation and vascular flora of Motuhora (Whale Island) 1970-1986. Tane 32: 19-48.

Quayle, A . M . 1984: The climate and weather of the Bay of Plenty region. New Zealand Meteorological Service Miscellaneous Publication 115(1), 2nd edition. Ministry of Transport, Wellington. 56p.

Solem, A.; Climo, F.M.; Roscoe, D.J. 1981. Sympatric species diversity of New Zealand land snails. New Zealand Journal of Zoology 8: 453-485.

Postscript: In 1990, D. J. Roscoe re-examined my Delos collections from Motuhora. He found that the shells were all similar, but that they differ from other known specimens of D. coresia in being of a shrunken form but with a "normal" protoconch; the taxonomic significance of these differences is being considered currently (D. J. Roscoe, pers. comm., July 1990).

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