Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 8

Biology

Most aspects of the ecology and life history of P. coeruleum are poorly known. Information on British Columbiapopulations in particular is virtually non-existent and based on unpublished observations by the authors.Burke et al. (2000) provided a summary of the species’ biology in the United States as a part of management recommendations. McGraw et al. (2002) studied the diet of the species in Oregon.


Life Cycle and Reproduction

This species appears to have an annual life cycle, maturing and reproducing within one year (Burke et al. 2000). An absence of observations of adults in the early spring, both in British Columbia and the United States, suggest that few or no individuals survive to their second year. The slugs probably over-winter as eggs, which then hatch in the spring. All records from British Columbia are from late autumn, from October to December; all individuals found were adults.

Like other pulmonate gastropods, P. coeruleum is oviparous and simultaneously hermaphroditic, each individual possessing both male and female reproductive organs. There is no evidence of self-fertilization in this species, however, and cross-fertilization may be the norm as in most other pulmonate species. Clutch size and details of the reproduction are unknown.


Diet and Predation

In Oregon, fungi form a large part of the diet of P. coeruleum, based on the presence of fungal spores and hyphae in the feces (McGraw et al. 2002). Fungal hyphae and spores were the most common items found and were present in 90% of the samples examined (spring and autumn samples combined). Although fungi were a major item in the samples during both seasons, the frequency of fungal spores was much greater in the autumn (62%) than in the spring (24%). The spores recovered from the samples represented 10 different families of fungi. Most of the fungi were mycorrhizal associates of vascular plants, including beneficial fungi associated with roots of conifers. Other food items found in fecal samples consisted of plant tissues (frequency of occurrence = 59%; spring and autumn data combined) and lichens (25%). In British Columbia slugs kept in captivity consumed fresh vegetable matter (lettuce and carrots) and survived for several months (K. Ovaska, unpublished data).

A variety of predators, both vertebrate and invertebrate, feed on slugs and probably also on P. coeruleum (Burke et al. 2000). Lancetooth snails (Haplotrema and Ancotrema species) and carabid beetles (Coleoptera: Carabidae) are common in habitats where the species is found and feed extensively on gastropods. Like other species in the genus Prophysaon, this species is capable of self-amputation of the tail, an adaptation that is an effective anti-predation mechanism (Hand and Ingram 1950).


Dispersal/Migration

These slugs are thought to have very limited dispersal capabilities, in the order of tens to hundreds of metres per generation, but few data are available (Wilke and Duncan 2004; N. Duncan, pers. comm.). Slugs from Oregon kept in enclosures moved very little (N. Duncan, pers. comm.). Whether there is a dispersal period at some stage of the life cycle is unknown, but it is unlikely that the slugs would be able to move far within a generation without assistance from other animals or physical forces. There are no known or suspected animal vectors or other means of dispersal (such as wind or water) for this species.

Transport by humans is possible but unlikely to be important. Unlike exotic slugs that are common in disturbed areas throughout most of North America, this species is not known to be invasive, is an inhabitant of forests rather than open, disturbed areas, and has an extremely patchy distribution throughout the northern portion of its range in Canada and the United States. Therefore, inadvertent introduction of slugs or eggs by humans, such as with nursery stock or in soil adhering to footwear, is unlikely for this species.

Although discovered only recently, P. coeruleum has most likely been present on Vancouver Island for long periods. The following lines of evidence support this conclusion (R. Forsyth, pers. comm.). First of all, very few of B.C.’s native terrestrial molluscs show any propensity towards being introduced. Introduced species are almost always synanthropic. Non-native species that occur in “natural” forests, for example, are just a subset of the non-native species that occur in city gardens. There are no introduced species that only inhabit natural forests. Secondly, the known range of P. coeruleum on Vancouver Island does not present an anomaly; rather, it fits well with the non-Canadian range for this species and there are parallels with other terrestrial mollusc species as well.


Interspecific Interactions

Many species of exotic gastropods occur within the range of P. coeruleum in British Columbia (Forsyth 2004). Exotic species found at sites with records of this species include the Chocolate Arion (Arion rufus), Giant Garden Slug (Limax maximus), and Worm Slug (Boettgerilla pallens) (K. Ovaska and L. Sopuck, unpublished data). It is unknown whether competition for food or shelters with exotic species or predation is a problem for P. coeruleum populations, but adverse effects are possible.


Adaptability

This species can tolerate some degree of disturbance to the habitat as evidenced by its occurrence in second-growth forests and in forest edge habitats in British Columbia. The availability of shelter (provided by decaying logs and other cover) and shady, moist conditions might be more important than forest age. However, it remains unknown whether the slugs prefer second-growth habitats or persist in available marginal habitats in the absence of older forest stands.

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