Table Of ContentTHE NAUTILUS 125(2):83-85, 2011 Page 83
Do Philomycus ccirolinianus (Gastropoda: Philomycidae)
prefer to congregate?
TimothyA. Pearce
Katherine A. Porter
Section of Mollusks
Carnegie Museum ofNatural History
Pittsburgh, PA 15213 USA
[email protected]
ABSTRACT Huddling and congregation are not unique to
Philomycus species. Orstan (2007) noted huddling by
Field observations of land slugs Philomycus carolinianus individuals ofthe same and ofdifferent species ofslugs,
(Bose, 1802) together at shelters led us to ask whether they but we do not know whether the same mechanisms
were attracted to conspecifics versus attracted to a limited
trherseoeurscheelt(ee.rgs.,toshtehlrteeer)s.luOgsurinleaabcohrarteoprlyicaetxep,ewriitmhenttheoefxfpeerce-d atiroen,acatlitnhgouignhiwnteer-noawndhaivnetrab-estpteecrifuincdehurdsdtlaensd.inIgnoaldtdhie-
tation that slugs would occurtogether under asingle shelter ii systematic^ of Philomycidae (Fairbanks, 1986; 1989;
they preferred to be with conspecifics, while slugs would 1990; 1993; 1998; Tsai et ah, 2005; Tsai and Wu, 2008),
occur singly under each shelter if they avoided conspecifics. we still know relatively little about their biology. In this
We found that slugs chose shelters regardless ofthe presence note, we examine congregation and huddling behavior
of conspecifics, suggesting that their occurrence together in among individuals ofa single species, the eastern North
nature is due to the sharing of limited shelter resources. American slug Philomycus carolinianus (Bose, 1802).
Additionalkey words: landslugs, shelter, limited resource
MATERIALS AND METHODS
Slugs were wild-caught in spring 2009 from the Patux-
INTRODUCTION ent National Wildlife Refuge, Prince George’s County,
Maryland (39o03' N, 76°49' W). The slugs were used in
Animals congregate for many reasons. In some cases, another experiment over the summer, then provided
theybenefit from the presence ol others. In other cases, to us for this study, which was conducted from 27
they occur together in an area because a resource is October to 21 November 2009. Voucher specimens,
presentthere. Explanations forcongregation behavior in preserved in 80% EtOH within a day post mortem,
terrestrial mollusks include prevention of water loss are deposited at Carnegie Museum of Natural History
(Dundee et ah, 1975; Cook, 1981; Waite, 1988) and (CM104587).
escape from ground-level heat (Cook, 2001). Wepreparedthree arenas. Eachwas 44 x 28 x 22.5cm
In nature, gastropods are commonly associated with high. The arenas were corrugated cardboard boxes, each
mm
woody debris, and can be found congregated in areas lined with a plastic bag and covered with a 1.4 mesh
with large amounts ofsuch debris (Kappes et ah, 2009). screen. To maintain humidity in the arenas, each screen
Individuals of species ol slugs in the genus Philomycus was covered with a stack of 12 moistened paper towels.
Rafinesque, 1820 are sometimes seen in close proximity Each paper towel had a surface area ol 276 cirri.
to each other, especially near coarse woody debris. In Each arenafloorwas coveredwith soil approximately 1 cm
some cases, these slugs are found in physical contactwith deep. In each arena,weplacedthreebarkshelters measur-
eachother(aphenomenon known as huddling). However, ing approximately 12 cm long, 10 cm wide, and 1.5 cm
Philomycus individuals do not always exhibit huddling in thick. Each arena also contained a water dish and a food
thewild, and thepresence ofmultiple slugs inclose prox- dish that contained fish food flakes and mushroom slices.
imityisyetto be explained. Twopossible explanations are The amounts offood and water provided were ample for
thatindividualsofPhilomycus species aregregarious (pre- the number of slugs in each arena, ensuring that neither
fer to associate with each other) and that they tend to food nor moisture was limiting. Shelter was the limiting
congregate at the locations oflimiting resources, regard- factor; other than the bark, food dish, and water dish, the
less ofthepresenceofotherslugs. arenawas devoidofobjects thatcouldfunction as shelters.
.
Page 84 THE NAUTILUS, Vol. 125, No. 2
We placed three mature slugs in each arena. For each Because these slugs appear to assort randomly,
replicate, we observed slug positions once daily for six congregations of slugs found in nature may indicate
days. Observations were made during the daytime when the presence of a scarce resource (e.g., shelter) in that
slugs would more likely be resting under shelters. Time particular location. Therefore, observations ofslug hud-
ofday when observations were made ranged from 7:42- dling could be used as evidence for resource scarcity.
21:44 with a mean of 12:55 (95%, Cl 9:00-16:50). For example, some land slugs appear to cluster to con-
Because we had a limited number of slugs, we re-used serve moisture (Dundee et al., 1975; Cook, 1981;
slugs in subsequent trials to maximize the number of Waite, 1988; although see Welsford et ah, 1990). In
replicates. The assigning of individuals to arenas was this experiment, food and moisture were non-limiting.
not strictly random. Rather, with the exception of 2 A repetition of the experiment in which moisture,
individuals in the last trial, the slugs were grouped in a rather than shelter, was the limiting factor might
manner ensuring that the individuals together in one allow us to study whether individuals of Philomycus
trial would not be together in a subsequent trial. carolinianiis do huddle to conserve moisture. Investiga-
We compared the observed slug positions with a null tions in which other resources were limiting could also
model, which assumed a random distribution of slugs address whether huddling occurs due to the scarcity of
under the shelters. We generated the null model by other resources.
determining the 18 possible arrangements of three All ofthe slugs used in this investigation were mature.
slugs under three shelters. This null model predicts An investigation of the effects of age on congregation
nine instances of singletons (slugs alone under shel- behavior could provide additional data on the be-
ters), six instances of pairs (two slugs under the same havior of Philomycus species. Studies involving species
shelter), and three instances of triplets (three slugs other than P. carolinianiis could determine whether
under the same shelter). Triplets would be expected to other species show similarbehaviors.
be more common if slugs are gregarious and less com-
mon if they prefer to be solitary. We used a chi-square ACKNOWLEDGMENTS
test to determine the statistical significance of the
observations. We are grateful to Megan Paustian for lending us the
slugs to use inthese trials. Cagin Unalhelpeddiscuss the
design ofthe experimentand helped set up the trials. An
RESULTS AND DISCUSSION anonymous reviewer provided helpful comments that
improved the manuscript.
Table shows the observed and expected numbers of
1
shelters with different numbers of slugs. With eight
replicates (a total of 83 observations), we observed 46 LITERATURE CITED
instances of slugs alone under shelters, 31 instances of
Cook, A. 1981. Huddling and the control ofwater loss by the
tthwroeeslsulgusgsunudnedrerthethesasmaemesheslhteelrt,era.ndWesixdiinssctaarndceeds 1o8f s29l:ug289U-m29a8x. pseudoflavus Evans. Animal Behaviour
observations in which at least one sing was not under a Cook, A. 2001. Behavioralecology: on doingthe right thing, in
shelter or in which a slug was using the food or water therightplaceattherighttime. In: G.M. Barker(ed.)The
dish as a shelter. We also discarded two replicates in Biology of Terrestrial Molluscs. CABI Publishing, New
which one ofthe slugs died. The observed and expected York, NewYork, pp. 447—487.
values are not significantly different from random [yj = Dundee, D.S., M. Tizzard, and M. Traub. 1975. Aggregative
5.33, 2 degrees offreedom, 0.1>P>0.05). behaviorinveronieellid slugs. The Nautilus 89: 69-71
This result suggests that slugs choose shelters ran- Fairbanks, H.L. 1986. The taxonomic status of Philomycus
domly,withoutregardtowhetheranotherslugispresent. togatus (Pulmonata: Philomycidae): a morphological and
Of particular interest is the observed trend for fewer eMlaelcatcroolpohgoiraet2i7c:c2o7m1p-a2r8i0s.onwithPhilomycuscarolinianiis.
triplets than predictedbythe random model (6vs. 13.8). Fairbanks, H.L. 1989. The reproductive anatomy and taxo-
Although this trend is not statistically significant, it does nomic status of Philomycus venustus Hubricht, 1953
suggestthatthese slugs are not gregarious. and Philomycus bisdosus Branson, 1968 (Pulmonata:
Philomycidae). The Nautilus 103: 20-23.
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Table 1. Number of shelters having 1, 2, or 3 slugs cies of Megapallifera (Gastropoda: Philomycidae). The
underneath. y“ analysis indicates the slugs were behaving Nautilus 104: 71-75.
randomly (0.1>P>0.05). Fairbanks H.L. 1993. The reproductive anatomyofPhilomycus
sellatus Hubricht, 1972 and Philomycus virginicus
Numberofslugsinshelter Observed Expected XOp Hubricht, 1953 (Gastropoda: Philomycidae). The Nautilus
107: 9-13.
1 46 41.5 0.49 Fairbanks, H.L. 1998. Clarificationofthetaxonomicstatusand
2 31 27.7 0.40
reproductiveanatomyofPhilomycusbatchi Branson, 1968
3 6 13.8 4.44
(Gastropoda: Pulmonata: Philomycidae). The Nautilus
Totals 83 83 5.33
1 12: 1-5.
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