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A pitfall trap for sampling the mesovoid shallow substratum (MSS) fauna
Heriberto López and Pedro Oromí
Departamento de Biología Animal, Universidad de La Laguna. 38206 La Laguna, Tenerife, Canary Islands, Spain. Email: (HL) [email protected]; (PO) [email protected] Key Words: Canary Islands, MSS, subterranean fauna, new trap.
Studies on subterranean biology carried out in the Canary Islands by our group of researchers since the 1980ʼs have demonstrated the existence of a diverse terrestrial hypogean fauna7,8,9. Most of these studies have been in lava tubes9, but after the discovery of mesovoid shallow substratum (MSS) habitat on these islands10, this environment was included in our studies to evaluate the richness of this substratum and the connectivity of lava tube caves and the MSS, as has been demonstrated in continental areas2. On the islands where lava tubes are rare or absent (Gran Canaria and La Gomera) the MSS may be the main terrestrial hypogean habitat. In earlier studies4,5,6 we used the traditional method of Juberthie3, which consists digging a hole, placing a pitfall in the hole and covering it with the removed soil. We observed that when the same traps were used repeatedly, there was considerable disturbance to the MSS resulting from collapse of the substratum and modification of the environment. For repeated sampling as many times as desired in a single MSS location, we began to modify the pitfall trap design, partially based on other authorsʼ ideas 1,11. After frequent trials beginning in 2000, we have developed a simple and economical MSS pitfall trap which is easy to construct and to install. The system is permanent and produces a minimal disturbance on the habitat once it has stabilized. The main component of the trap is a 75 cm long PVC pipe with an inside diameter of 11 cm. Many small holes (5-7 mm in diameter) are drilled along its surface, except for an 8 cm band at the bottom and a 10 cm band at the top (see part B in Fig. 1). The pipe dimensions and the distribution of the holes can vary according to particular requirements of each study, but the diameter of holes must be small enough to avoid the entrance of non-hypogean fauna, particularly small vertebrates (mice, shrews, small reptiles, etc). A plastic tray 8 cm high and 11 cm in diameter (a width that fits perfectly inside the pipe, see Fig. 2A) is fitted with a nylon cord as a handle. The nylon cord is slightly longer than the pipe, and is tied to two points on the opposite lips of the tray with small, hard plastic tubes (labelled pt in Fig. 2A). This tray can be made from the bottom of any plastic bottle with the appropriate diameter. An 8 cm high, 3 cm diameter plastic bait container (labelled bc in Fig. 2A) is fixed to the center of the tray with a nut,
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bolt and washers (Figure 2B). The nut, bolt and washers are covered with silicone to prevent leakage of fluid and rusting. The tray will contain the preservative liquid (propylene glycol, labelled pg in Fig. 2A), where the specimens will fall, and the bait container holds the bait (e.g., blue cheese, liver or fresh fish) avoiding its contact with the propylene glycol. The bait in the bait container is covered by a piece of mesh, and the bait container is fastened with a perforated cap allowing the bait odour to exit while preventing animals from reaching the bait. The third component of the trap is a silicone cap closing the top of the pipe, like those used by plumbers as temporary caps to protect pipe threads (part A in Fig. 1).
Suitable MSS is difficult to locate from the surface terrain, but easier to detect when the terrain is cut for building roads. The traps can be set a few meters upslope from such a location (Fig. 3). Once the site is selected, a narrow vertical hole is made by hand from the surface down to reach the MSS, using a 1 m long, sharp pointed iron bar. The PVC pipe is installed inside the hole such that the perforations of the pipe stay at the MSS level, and the top remains a few centimetres below the ground surface. The small stones previously removed when digging the hole are used to fill up the empty space left around the pipe, just above the highest perforations of the latter (Fig. 4). The rest of the cavity is filled with compacted soil to isolate the system and limit the entrance of surface fauna. The tray, loaded with propylene glycol and baited bait container, is lowered to the bottom of the pipe, leaving the nylon cord handle protruding from the tube. The top of the pipe is then closed with the silicone cap trapping the nylon handle, and the whole system is covered with soil and litter to hide and protect the trap. A stone or a conspicuous marker is used to indicate the trapʼs location. At the end of a sampling interval, the top of the trap is opened, taking care to avoid soil falling inside, and the tray is lifted to the surface with the nylon handle. The content of the trap is filtered and saved for sorting in the lab, and the tray is deployed again with fresh propylene glycol and bait.
Propylene glycol is ideal for genetic studies because it preserves the DNA of specimens in good condition12. In locations with high subterranean humidity, water condenses on the inner walls of the pipe, producing water droplets that slip down into propylene glycol, diluting its DNA preservative properties. To avoid this problem the number of holes in the pipe should be increased.
The time necessary to obtain troglobionts with these traps depends on the MSS conditions (humidity, soil covering of surface, degree of porosity) and on the surface conditions where traps are installed (vegetation cover, humidity, exposure to the sun). Sometimes we have obtained good results in a few weeks, while in other cases the best results have been reached after one or two years.
These traps have been working on five islands with good results. For instance, on Gran Canaria, studies made in the 1980ʼs 4 at five MSS locations (35 traps) produced one subterranean adapted species, whereas the new traps at six different locations (15 traps) have produced 19 troglobiont species, 18 of which were previously unknown.
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Figure 1. A: silicone cap sealing the pipe, with lateral cuts for easy removal. B: PVC pipe with abundant holes.
Figure 2. A: tray with propylene glycol (pg), the bait container (bc) and the nylon cord tied with small plastic tubes (pt). B: detail of the union between the bait container (yellow) and the tray (grey) with a screw, washers and silicone (blue).
Figure 3. The trap installed in the MSS close to a road cut.
B
A
Figure 1
Figure 2
pt
pg
bc
A
B
Figure 3
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Figure 4. Filling stones into the hollow around a trap installed in the MSS (La Gomera island).
Acknowledgements We are grateful to Rafael García and Agustín Aguiar for the explanations about their traps for subterranean sampling, to David Culver for encouraging us to publish the design of this trap and for a first revision of the manuscript, and to an anonymous referee and particularly to Daniel Fong who have much improved it with their suggestions. Literature Cited: 1. Gers C. 1992. Écologie et biologie des populations dʼarthropodes terrestres
du milieu souterrain superficiel: fonctionnement et écologie évolutive. Thèse dʼÉtat, UPS Toulouse III: 392 pp.
2. Juberthie C., Bouillon M. & Delay B. 1981. Sur l´existence d´un milieu souterrain superficiel en zone calcaire. Mémoires Biospéologie 8, 77-94.
3. Juberthie C., Delay B. & Bouillon M. 1980. Extension du milieu souterrain en zone non-calcaire: description d´un nouveau milieu et de son peuplement par les Coléoptéres troglobies. Mémoires Biospéologie 7, 19-52.
4. Medina A.L. 1991. El medio subterráneo superficial en las Islas Canarias: caracterización y consideraciones sobre su fauna. Doctoral thesis (unpublished), Universidad de La Laguna, Tenerife: 205 pp.
5. Medina A.L. & Oromí P. 1990. First data on the superficial underground compartment on La Gomera (Canary islands). Mémoires Biospéologie 17, 87-91.
6. Medina A.L. & Oromí P. 1991. Wolltinerfia anagae n.sp., nuevo coleóptero hipogeo de la isla de Tenerife (Coleoptera, Carabidae). Mémoires Biospéologie 18, 215-218.
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7. Oromí P. 2004. Canary Islands: Biospeleology. In Gunn J. (Ed.) Encyclopedia of caves and karst science. Fitzroy Dearborn, New York, pp. 179-181. URL: http://www.informaworld.com/smpp/content~db=all~content=a740990225
8. Oromí P. 2008. Biospeleology in Macaronesia. Association for Mexican Cave Studies Bulletin 19, 98-104.
9. Oromí P., Martín J.L., Medina A.L. & Izquierdo I. 1991. The evolution of the hypogean fauna in the Canary Islands. In Dudley E.C. (Ed.) The unity of evolutionary biology, vol 2. Dioscorides Press, Portland, pp 380-395.
10. Oromí P., Medina A.L. & Tejedor M.L. 1986. On the existence of a superficial underground compartment in the Canary islands. Actas del IX Congreso Internacional de Espeleología, Barcelona 2, 147-151.
11. Owen J.A. 1995. A pitfall trap for the repetitive sampling of hypogeal arthropod faunas. Entomologistʼs Record 107, 225-228.
12. Rubink W.L., Murray K.D., Baum K.A. & Pinto M.A. 2003. Long term preservation of DNA from honey bees (Apis mellifera) collected in aerial pitfalll traps. The Texas Journal of Science 55, 159-168.
