Rhynchodemus sylvaticus (Leidy, 1851)

Vila-Farré, Miquel, Sluys, Ronald, Mateos, Eduardo, Jones, Hugh D. & Romero, Rafael, 2011, Land planarians (Platyhelminthes: Tricladida: Geoplanidae) from the Iberian Peninsula: new records and description of two new species, with a discussion on ecology, Journal of Natural History 45 (15 - 16), pp. 869-891 : 887-889

Rhynchodemus sylvaticus (Leidy, 1851)

( Figure 1 , Table 1)

Discussion

As was predicted in a previous paper ( Vila-Farré et al. 2008) new sampling has yielded one new locality in southern Spain, la Axarquía ( Table 1, Figure 1 ), where this species has been found under a flowerpot in a mango plantation. The origin of this population, native or introduced, is uncertain because of the anthropogenic character of the habitat.

Exotic species

Material examined

CRBA-3781a-b, Benamargosa, la Axarquía. province of Málaga (southern Spain), 25 December 2007, sagittal sections on two slides. CRBA-3782a-b, ibid., sagittal sections on two slides .

Discussion

In the same mango plantation where some individuals of R. sylvaticus were found, under the next flowerpot, we collected a number of asexual land planarians. They were characterized by an external morphology very similar to that of the former species but with only one dorsal line instead of the two of R. sylvaticus . Molecular analysis performed by Marta Álvarez (personal communication) placed these individuals within the genus Platydemus . Because of the anthropogenic origin of the habitat and as Platydemus is mainly found in the Indo-Pacific region, with only one African species ( Jones 1998), we tentatively consider these animals to be an exotic component of the Iberian fauna.

Ecology of Iberian terrestrial planarians

Land planarians are more sensitive than most other organisms to the humidity of their environment because they lack any special mechanism of water conservation ( Kawaguti 1932). The animals are not good burrowers because of the absence of any fluid-filled body cavities so they have limited access to humid environments in deeper layers of the soil. However, the worms may use cracks and voids made by other organisms to retreat to a certain depth in the soil and so avoid desiccation.

The Iberian Peninsula is an area with an Atlantic climate in its northern sector and a Mediterranean one in the other parts, with populations of land planarians being widely distributed throughout the Peninsula (cf. Vila-Farré et al. 2008). Of the 58 records of land planarians properly georeferenced, at least 59% are located in the proximity of a river or stream. The constant humidity provided by the watercourses probably enables the establishment of populations in areas that are regularly affected by long dry periods, i.e. the Mediterranean zone of the Iberian Peninsula. In this region the animals usually occur in very low numbers (one to four animals collected per survey journey, with two collectors searching the under surface of stones for 6 hours), and the specimens are also rather small and frequently white. Generally, bigger specimens have been collected from the Atlantic region of the Iberian Peninsula, e.g. large specimens of M. terrestris and M. robusta from Galicia and unidentified specimens up to 8 cm long from Navarra.

We have only observed relatively large concentrations of animals (10 or more animals collected per survey journey) in the Sierra de Grazalema (southern Spain; M. aixandrei ) and in Sant del Bosc (northeastern Spain; M. hyalina ). Both species are very small, which is probably an advantage in Mediterranean areas that are subjected to periodic drought. In areas where watercourses are absent (for example in the Sant del Bosc, a slope in a holm oak forest with abundant vegetation and high humidity; type locality of M. hyalina ) and during long dry periods (from 2006 to April 2008) planarians can suddenly reduce their population size to such an extent that it is impossible to find any specimens, although this area was visited each year three or more times in spring, summer and autumn. Seasonal rarity may be the result of retreat into less accessible crevices rather than of population shrinkage. After a rainy period (May 2008), however, planarians were found again in large numbers (more than 10 individuals per survey journey in 2009, with two collectors searching the under surface of stones for 3 hours), suggesting that the populations of M. hyalina are well adapted to a Mediterranean climate. It is also possible that because of their small size the animals could be hidden during dry periods and that they re-appear rapidly when rain comes.

We have collected land planarians mostly in the vicinity of deciduous trees (oak, holm oak, beech and gallery forests) under stones and, occasionally, also under tree trunks, where they are more difficult to detect. We have never observed cocoons. Although some European species have been collected in coniferous or mixed forests (e.g. M. manherti Minelli, 1977 , collected in the Jura Mountains in a mixed woodland with Carpinus betulus L. and Picea glauca (Moench) (cf. Minelli 1977b), in Spain they have never been found in coniferous forests. We have only one record of a small white land planarian in a wheat field with some Pinus trees around. As the wheat field environment is extremely dry it may be that the animal was transported by recent rains from adjacent areas or even may have actively moved to the area under humid conditions.

Froehlich (1955) pointed out that land planarians cannot endure long periods of submersion in water. Nevertheless, we have often collected specimens from the banks of rivers in areas that are frequently flooded. One of the species, R. sylvaticus , on one occasion was even collected from a temporary pool (Clot d’Espolla, Girona, Northeastern Spain; Boix and Sala 2001). This species, although rare, is present in a wide variety of ecosystems in Spain: river banks (two records); dry areas in villages (Talarn, northeastern Spain; but only in very rainy periods and close to a very narrow temporary water canal); seasonal streams (two records); plant nurseries (one record); and mango plantations (one record).

General discussion

In this paper we have followed the classification for the triclad flatworms published by Sluys et al. (2009). In the comparative analyses of the new species presented above we have included all the known taxa of the genus Microplana . We considered it particularly important to analyse African species because the fauna of the Iberian Peninsula has a strong African component ( Ribera 2000; Pleguezuelos et al. 2008). In addition, the accidental ( Jones 1998) or voluntary ( Kawakatsu et al. 1992) introduction of land planarians is a well-known factor of dispersal in this group of animals. Therefore, an alien origin of the new Iberian species has been discounted after comparison with all currently known members of the genus.

A general overview of the distribution of Iberian land planarians has been provided by Vila-Farré et al. (2008) and Mateos et al. (2009). The present paper complements these analyses by reinforcing some of their conclusions and predictions. First, Iberian populations composed of individuals of similar external morphology and located in geographically distant areas, belong in fact to different species (e.g. M. aixandrei in the southern part and M. hyalina in the northeastern part of the Peninsula). Second, similar to the situation found in other animal groups, the Iberian fauna of terrestrial planarians exhibits a high degree of endemism. Furthermore, European species such as M. terrestris and M. monacensis are present in at least the northern sector of the Peninsula. We would also expect to find North African species during future samplings in the southern sectors of Spain and Portugal. Third, the study of the land planarians of the Iberian Peninsula is already yielding new and interesting ecological (innovation in reproductive strategies like the use of a true spermatophore) and biogeographic (high degree of endemism) information. In addition, populations of well known European species, such as M. terrestris , display on the Iberian Peninsula particular morphological characteristics suggesting the presence of interpopulation variability and / or cryptic species, a situation that may be analysed better with the help of molecular techniques. Fourth, although the presence of introduced and widely distributed species, such as Arthurdendyus triangulatus (Dendy, 1895) (cf. Boag and Yeates 2001), is still not established for the Iberian Peninsula, we have here recorded introduced land planarians in areas where continued human activity allows for their introduction.

Figures

Figure 1. Distributional records of terrestrial planarians from the Iberian Peninsula used for this study. Filled circles: localities with previously known species. Stars: localities with new species.