Captive Rearing and Biology of the Endangered Giant Land Snails Placostylus Ambagiosus and P
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Captive rearing and biology of the endangered giant land snails Placostylus ambagiosus and P. hongii (Pulmonata: Bulimulidae) I.A.N. Stringer and E.A. Grant DOC RESEARCH & DEVELOPMENT SERIES 279 Published by Science & Technical Publishing Department of Conservation PO Box 10420, The Terrace Wellington 6143, New Zealand DOC Research & Development Series is a published record of scientific research carried out, or advice given, by Department of Conservation staff or external contractors funded by DOC. It comprises reports and short communications that are peer-reviewed. Individual contributions to the series are first released on the departmental website in pdf form. Hardcopy is printed, bound, and distributed at regular intervals. Titles are also listed in our catalogue on the website, refer www.doc.govt.nz under Publications, then Science & technical. © Copyright September 2007, New Zealand Department of Conservation ISSN 1176–8886 (hardcopy) ISSN 1177–9306 (web PDF) ISBN 978–0–478–14287–7 (hardcopy) ISBN 978–0–478–14288–4 (web PDF) This report was prepared for publication by Science & Technical Publishing; editing and layout by Sue Hallas. Publication was approved by the Chief Scientist (Research, Development & Improvement Division), Department of Conservation, Wellington, New Zealand. In the interest of forest conservation, we support paperless electronic publishing. When printing, recycled paper is used wherever possible. CONTENTS Abstract 5 1. Introduction 6 1.1 The study animals 6 1.2 Conservation and previous attempts at captive breeding 7 2. Methods 9 2.1 Source of specimens 9 2.2 Identification of individual snails 9 2.3 Rearing 10 2.3.1 Rearing conditions 10 2.3.2 Rearing containers 10 2.3.3 Measurements 11 2.3.4 Food and maintenance 12 2.4 Testing numbers of eggs laid in substrates of differing particle size 12 2.5 Analysis 13 3. Results 13 3.1 Rearing Placostylus ambagiosus 13 3.1.1 First attempt at maintaining snails in captivity 13 3.1.2 The second attempt at rearing snails in captivity 15 3.1.3 Eggs 17 3.1.4 Egg laying and incubation period 19 3.1.5 Substrate particle size preference for oviposition 19 3.1.6 Snails that developed in captivity 20 3.2 Rearing Placostylus hongii 22 3.2.1 Snails collected from the Poor Knight Islands 22 3.2.2 Snails collected from the Mokohinau Islands 23 3.2.3 Eggs 23 3.2.4 Egg laying and incubation period 24 3.2.5 Substrate particle size preference for oviposition 25 3.2.6 Snails that hatched in captivity (Poor Knights Island origin) 25 4. Discussion 27 4.1 Rearing conditions 27 4.1.1 Choice of rearing conditions 27 4.1.2 Effects of rearing conditions on Placostylus ambagiosus paraspiritus 27 4.1.3 Effects of rearing conditions on Placostylus hongii 28 4.2 Eggs and incubation period 28 4.3 Growth 29 4.4 Lifespan 31 5. Conclusions 31 6. Recommendations 32 7. Acknowledgements 33 8. References 33 Appendix 1 Change in mass and mass:length ratio for individual Placostylus ambagiosus paraspiritus kept in captivity (collected near Cape Maria van Diemen, 19 October 1992) 36 Captive rearing and biology of the endangered giant land snails Placostylus ambagiosus and P. hongii (Pulmonata: Bulimulidae) I.A.N. Stringer1 and E.A. Grant 1 Research, Development and Improvement Division, Department of Conservation, PO Box 10420, Wellington 6143, New Zealand Email: [email protected] Abstract Rearing of the land snails Placostylus ambagiosus paraspiritus, P. a. michiei and P. hongii in captivity was investigated as a conservation management option for establishing new populations of these threatened animals. Snails were kept in the laboratory with mostly karaka, Corynocarpus laevigatus, leaves as food. Snails were exposed to differing frequencies of handling, crowding conditions and given different substrates in which to lay eggs. Less frequent disturbance resulted in better survival in P. ambagiosus. Both P. ambagiosus and P. hongii laid clutches of 1–84 similarly sized eggs buried up to 2 cm deep, but the species appeared to have different substrate size preferences. Placostylus ambagiosus kept singly or in pairs grew larger than those in the wild. Crowding reduced growth rate, increased mortality and resulted in smaller adults. Captive-bred adult P. ambagiosus lived for 3.3–11.4 y: one adult P. hongii lived for 2.6 y and another lived for 4.6 y before being translocated onto an island. Both Placostylus ambagiosus and P. hongii can be successfully reared in captivity, in the laboratory, and large numbers of snails can potentially be produced, albeit with substantial costs. Although potentially risky for augmenting existing populations, these protocols may enable the establishment of new populations of these snails. Keywords: growth rate, lifespan, egg, incubation, oviposition preference, Placostylus, land snail © September 2007, New Zealand Department of Conservation. This paper may be cited as: Stringer, I.A.N.; Grant, E.A. 2007: Captive rearing and biology of the endangered giant land snails Placostylus ambagiosus and P. hongii (Pulmonata: Bulimulidae). DOC Research & Development Series 279. Department of Conservation, Wellington. 36 p. DOC Research & Development Series 279 5 1. Introduction Two of the three species of Placostylus land snails (Pulmonata: Bulimulidae) found in New Zealand, P. ambagiosus and P. hongii, survive tenuously on the mainland because of habitat loss, habitat modification by introduced mammals and predation by mammals. They now occur mostly in small scattered remnant populations which are vulnerable to extirpation from accidental fire or predation (Powell 1979; Parrish et al. 1995; Brook 2002). The risk of such accidental extinction can be reduced by increasing the number of populations through translocation, but removing sufficient individuals to ensure that translocations succeed may be detrimental to small source populations. One solution is to remove some snails and rear them in captivity in sufficient numbers to establish new colonies (e.g. Coote et al. 2004). We do not advocate using captive-bred snails to supplement an existing population unless there is no other option because of the possibility that a novel pathogen could be contracted in captivity and introduced to the wild population (Cunningham & Daszak 1998). No method for rearing these species of Placostylus has been published despite four previous attempts, and few details of the methods used previously survive. We therefore investigated captive rearing these snails a fifth time. We used P. hongii from a large population on Aorangi Island, Poor Knights Islands, P. a. paraspiritus from near Cape Maria van Diemen, and P. a. michiei from Surville Cliffs near North Cape. Both populations of P. ambagiosus numbered several thousand individuals when snails were removed for captive breeding (Parrish et al. 1995). 1.1 The stUDY animals Both species of Placostylus have large (maximum length 60–95 mm) heavily calcified shells with a brown periostracum. Growth is determinate and the shell stops elongating when the aperture edge splays outward and thickens to form a distinctive lip or varix. The main predators are rats (Rattus norvegicus, R. rattus), mice (Mus musculus), birds (probably Turdus philomelos) and pigs (Sus scrofa). Rodents and birds prey on juvenile snails but the varix or the weight of the adult snail confers some protection to adults against them. Pigs prey on large juvenile and adult snails (Parrish et al. 1995). Placostylus hongii is now classified as ‘range restricted’ by Hitchmough et al. (2007) because many thousands occur on the Poor Knights Islands whereas elsewhere, only remnant populations survive—along the east coast of the mainland from Whangaroa to Whangarei and on some nearby islands. Placostylus ambagiosus occurs in small scattered populations at the end of the Aupouri Peninsula and on Motuopao Island off Cape Maria van Diemen. Six of the eight described subspecies of P. ambagiosus and a further eight populations that have not been described as subspecies are classified as ‘nationally critical’, the highest level of threat, by Hitchmough et al. (2007). Three subspecies, including P. a. paraspiritus and P. a. michiei, are classified as ‘nationally endangered’, the 6 Stringer & Grant—Captive rearing and biology of Placostylus land snails second highest level of threat, by Hitchmough et al. (2007). There are now perhaps 2000 P. a. michiei whereas the other populations range from < 50 to probably < 500 individuals (Brook 2002). In addition, a few P. a. pandora survive on Motutakupu Island, in the Cavalli Island group, after they were translocated there by the now defunct New Zealand Wildlife Service in 1984 (Parrish 1989). 1 . 2 C onser V ation and pre V io U S attempts at captiVE breeding Attempts to protect some of the populations of both Placostylus ambagiosus and P. hongii included intermittent rodent poisoning of the sites where they occurred since September 1982 (New Zealand Wildlife Service file WIL 33/2/6) and September 1985 (Wildlife Service file WIL 33/5/35), respectively, and by fencing off nine areas where snails were present (each < 1 ha) at the end of the Aupouri Peninsula between 1990 and 2000. These exclosures were designed to exclude pigs and other livestock to protect a nucleus of snails that could re-colonise the surrounding areas should snails in them be killed. Both P. ambagiosus and P. hongii have previously been kept in captivity on four different occasions. The first was by N. Douglas who kept at least three P. a. keenorum in captivity at his home in Waiuku. This was before Placostylus species were protected by the Wildlife Act 1953 so no permit or documentation was required. However, Penniket (1981) reported that one adult kept by N. Douglas survived for 3.5 y, and there are two references in a file of the New Zealand Wildlife Service (file 33/5/37) that two juveniles kept by N.