First report on mass aggregation of opiliones in
China
Aeshita Mukherjee 1, Burkhard Wilske 2& Chen Jin 2
1,2 Xishuangbanna Tropical Botanical Garden, Chinese Academy of
Sciences, Mengla, Yunnan Province, Menglun 666303, China
Email: 1 aesh2003@yahoo.com
Date of publication (online): 26 May 2010
Date of publication (print): 26 May 2010
ISSN 0974-7907 (online) | 0974-7893 (print)
Editor:Glauco Machado
Manuscript
details:
Ms # o2296
Received 25
August 2009
Final
received 06 March 2010
Finally
accepted 30 March 2010
Citation: Mukherjee, A., B. Wilske & C.
Jin (2010). First report on mass aggregation of opiliones in
China. Journal of Threatened Taxa 2(5): 892-893.
Copyright: © Aeshita Mukherjee, Burkhard Wilske
& Chen Jin 2010. Creative Commons Attribution 3.0 Unported License. JoTT
allows unrestricted use of this article in any medium for non-profit purposes,
reproduction and distribution by providing adequate credit to the authors and
the source of publication.
Acknowledgement: We are thankful to Prof. Yael Lubin
(BIDR, Israel) and Prof. Les Underhill (ADU, South Africa) for their kind help
and suggestions, and to Prof. Nobuo Tsurusaki for the identification of the
studied species.
Most harvestman species live in
moist habitats and are frequently found under fallen trunks, among the leaf
litter and inside caves, where some species can form large aggregations
(Holmberg et al. 1984; Machado 2002). In
fact, gregariousness is a widespread behavior among harvestmen, with reports
for species from South and North America, Europe, and India (review in Machado
& Macías-Ordóñez 2007). There are
several hypotheses to explain why harvestmen aggregate, including: (1)
collective selection of places with low risk of dehydration and with low light
exposure, (2) reduction of air movement around each individual and consequent
decreasing in evaporation rate, (3) strengthening of the chemical defenses by
the collective action of the repulsive fluids secreted by the individuals, and
(4) decrease in individual predation by a dilution effect (Holmberg et al.
1984; Machado 2002).
During a field trip to a seasonal
rainforest on limestone in Xishuangbanna (21.910N & 101.280E),
in the southern prefecture of Yunnan Province, southwestern China, a
mono-species aggregation of the harvestman Pseudogagrella sp. (Sclerosomatidae: Gagrellinae) was found on the ground
vegetation. The ground vegetation in the area of the aggregation was 30-40 cm
tall and was dominated by the Malabar nut Adhatoda vasica Nees (Image 1). The
aggregation consisted of five adjacent, clearly distinguishable
sub-aggregations (hereafter referred to as patches), alongside a concrete-paved
forest walk. Distances between the
patches were between 2 and 10 m. The
aggregation was first observed on 01 January 2005, and was monitored every two
weeks during morning hours until 29 April 2005, which are winter months of the
tropical-subtropical transition zone. A
square plot (1 x 1 m) was used to estimate the number of individuals per
patch. Sample individuals (n = 5) from
each patch were brought to the laboratory for species and sex determination.
The majority of the individuals in
the three-dimensional aggregation were made up of females (n ≈ 500). Only few males were found within each patch
(n ≈ 10). Due to large aggregation size, since estimation of actual sex
ratio was not feasible, we crudely estimated that less than 10% of the
individuals in each patch were males. Individual patches covered an area
between 1 and 6 m2, and the total area occupied by the aggregation
was about 15m2. Harvestman
densities within individual patches ranged from 10 to 30 individuals (mean ± SD
= 20.0 ± 2.4 individuals, n = 45). Extrapolation of these densities resulted in an estimated aggregation
size of ca. 300,000 individuals. To the
best of our knowledge, this is the largest harvestman aggregation recorded so
far – surpassing the previous record of 70,000 individuals of Leiobunum “cactorum” (Sclerosomatidae: Leiobuninae) aggregated on a candelabrum
cactus in a Mexican desert (Wagner 1954).
During our observations, we found a few individuals (n ≈ 20) moving between patches. It was not evident whether there was a
pattern of exchange of individuals between the patches. Upon disturbance, whenever we approached less
than 1m close to a patch, all individuals moved away about 50cm over the
vegetation but were nevertheless found in the original patch location during
subsequent visits (n = 2 times). These
observations suggest some kind of habitat selection and roosting fidelity, like
that described for another sclerosomatid, Prionostemma sp., in Panama (Donaldson & Grether 2007; Grether &
Donaldson 2007).
Although we do not know the precise
date when the aggregation was formed, the aggregation persisted during the dry
winter months i.e., between November and March. By 23 February, the number of individuals in the entire aggregation had
decreased to about 3000. Although all the patches still existed, the majority
of the individuals were now accumulated in a single patch. On 8 March, the aggregation still existed in
the same place, and a few individuals were seen running at the sites previously
occupied by the earlier patches, but they no longer formed an aggregation in
these places. The total number of
individuals was estimated to be around 1500-2000. At this time, we saw many of them carrying
flowers of Malabar nut, but we did not see whether they were actually feeding
on them. On 12 April, only
about 200 individuals remained in the patch. They were inactive and reluctant to move on disturbances compared to the
previous month. On our last visit, on 29
April, we did not find any individual. We checked the entire area in case they had shifted their roosting site,
but the entire aggregation had disappeared.
Our observations suggest that
individuals of Pseudogagrella sp. aggregate for a few months and
then they probably disperse, die or hibernate until the next season. Winter temperatures in the
subtropical-tropical transition climate usually range far above the freezing
point (4-100C), even during the coldest month. This suggests a similarity between the
aggregation we found in China and the over-wintering aggregations described for
harvestmen from temperate zones, which were reported to occur inside caves or
mines (Holmberg et al. 1984). The
moderate ambient temperature within the caves from the temperate zone during
chilling winter may be comparable to the temperatures under which the
aggregation occurs in the ground-vegetation of a seasonal rainforest.
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