Multi-Species Aggregations in Neotropical Harvestmen (Opiliones, Gonyleptidae)

3 Pages·1998·2.7 MB·English

by  Glauco Machado

1998. The Journal of Arachnology 26:389-391
RESEARCH NOTE
MULTI-SPECIES AGGREGATIONS IN NEOTROPICAL
HARVESTMEN (OPILIONES, GONYLEPTIDAE)
Harvestmen are generally vagile and soli- nae) and Holoversia nigra Mello-Leitao 1940
tary but some species may be found in sta- (subfamily Gonyleptinae). Nine individuals
tionary aggregations. Gregariouseess in har- (9.7%) were found isolated, not associated
vestmen has been recorded in some Palpatores with any aggregation: four D. montanus
(Cockerill 1988 and inch ref.; Coddington et (Id'S?), three Eugyndes sp. (1(32$) and two
al. 1990; Holmberg et al. 1984) and Lania- H. nigra (16'1$). The remaining 84 individ-
tores species (Capocasale & Brano-Trezza uals (90.7%) were found in five multi-species
1964; Acosta et al. 1993; Pinto-da-Rocha aggregations. The average number of individ-
1993). Multi-species aggregations have been uals per cluster was 16.8 (SD = 11.1; range
reported once for three Leiobuninae species 5~34; n = 5). Despirus montanus and Eugyn-
(Palpatores) in the southern USA (Cockerill des sp. were the most commonly encountered
1988). species in the aggregations since 95.2% ofthe
On 3 and 16 November 1996, multi-species individuals in all aggregates belonged to these
aggregations ofharvestmen were found in the species (Table 1). In both species, the sexratio
low vegetation in a swampy area of Serra do was not si—gnificantly different from 1:1—(D.
Cipo (19°17'S, 43°35'W, 1200 m elevation), montanus ^ 2.38; Eugyndes sp.
Minas Gerais State, Brazil. The local vegeta- = 1.68; df ^ 1; P > 0.05). Holoversia nigra
tion consists of Montane Fields (“campo ru- was present in three aggregations and repre-
pestre”): mainly shrubs and low herbs grow- sented by only a few individuals—-generally
ing in thin and rocky soil. Water infiltration one or two per aggregation. The two isolated
into the ground was retarded by the layers be- individuals of this species were found in the
m
low the thin soil. As a result, with any minor swamp area at distances of 1 and 2 from
rainfall, the whole area became a swamp (Joly the nearest aggregation where this species was
1970). We sampled a 460 m^ plot in a swamp not found. Table 1 shows the occurrence of
in the early afternoon. All harvestmen found species within aggregates.
were collected and immediately preserved in When the aggregated individuals were dis-
70% ethanol. During the survey we also ob- turbed, it appeared that only H. nigra released
served the behavioral responses of the indi- repugnatory substances. After the discharge,
viduals to disturbance (such as attempts to es- the individuals of this species abandoned the
cape, immobility, discharge of odorous aggregations slowly. Disturbed individuals of
secretions). Voucher specimens were deposit- D. montanus and Eugyndes sp. fled the place
ed in the arachnological collection ofthe Mu- of disturbance or hid themselves in the base
seu de Zoologia daUniversidade de Sao Paulo of the root clumps. Even when these species
(MZUSP). were manipulated they did not release detect-
Harvestmen were found either in clumps of able defensive secretions. Some laniatorid
roots of gramineous species, or partially bur- species use their repugnatory substances very
ied in the mud below these clumps. The soil sparingly, if at all (see Cokendolpher 1987;
in the basal portion ofclumps was still muddy Roach et al. 1980).
but not saturated to the point that pools of Even though the water had receded at least
water were still present. We found 93 individ- one night before the harvestmen were found,
uals belonging to three species of the family they were still aggregated. Thus we reject the
Gonyleptidae: Despirus montanus Mello-Lei- hypothesis that the harvestmen had aggregat-
tao 1941 (subfamily Mitobatinae), Eugyndes ed in the root clumps to avoid drowning and
sp. (a new species of the subfamily Pachyli- had not had time to disperse. Gregarious be-
389
390 THE JOURNAL OF ARACHNOLOGY
—
Table 1. Species occurrence within five different multi-species aggregations of 84 harvestmen in a
swamp in Serra do Cipo, Minas Gerais State, Brazil.
Holoversia nigra Eugyndes sp. Despirus montanus Total # of
Male Female Male Female Male Female individuals
0 1 1 1 1 1 5
1 0 1 1 3 4 10
0 0 6 3 1 5 15
1 1 8 6 2 2 20
0 0 7 4 9 14 34
4.8% 45.2% 50.0% 84
havior in harvestmen has been interpreted in 1981). By living in groups, the chemically-
several ways according to Holmberg et al. protected species may diminish the risk ofbe-
1984. The first interpretation is that the ag- ing preyed upon because there are more
gregations are formed in optimal places in or- chances that another individual be the victim.
der to avoid dehydration and exposure to Although H. nigra was not found in all clus-
light. The risk of dehydration must be low in ters, it is possible that it was present in the
the basal portion of the gramineous clump, formation of the aggregation. Both D. mon-
since this microhabitat is constantly moist. tanus and Eugyndes sp. showed a tendency to
Besides, all swamp area was under similar aggregate, and individuals found isolated may
light conditions and the aggregation places temporarily have moved away from aggrega-
were not patches of darkened areas. Thus the tions or have been expelled by other individ-
&
microhabitat explanation is not adequate to uals ofthe group. Capocasale Bruno-Trezza
explain why the aggregations are formed in (1964) suggested that the expulsion of con-
very different locations in the swamp. We can specifics from clusters was the reason why
not reject the hypothesis, however, that multi- isolated individuals were found in Acantho-
species aggregations are formed in more pro- pachylus aculeatus (ICirby 1818), but the rea-
tected areas (due to greater interpenetration of sons for this were not stated in that work.
the grass roots, for example) and thereby Even if mating groups could be one of the
serve as hiding places from predators. reasons to the formation of mono-species ag-
Another hypothesis is that the gregarious gregations (Holmberg et al. 1984), other ef-
behavior increases the defensive ability fects like dilution and chemical protection
against predators by the collective action of could result in the incorporation of additional
the repulsive substances secreted by these an- species to this groups. In the future, experi-
imals. This hypothesis may be supported by mental studies should be performed in order
the fact that disturbance ofan aggregate is im- to understand the evolutionary significance of
mediately followed by a discharge of a sub- gregarious behavior in harvestman.
stance with a strong, sour smell, produced by ACKNOWLEDGMENTS
at least one species of harvestmen. Thus the
multi-species aggregations ofharvestmenmay We thank R. Pinto-da-Rocha andA.B. Kury
rely on the fact that the non-chemically pro- for identification of the harvestmen; A.A.
tected species (i.e., those species which rarely Giaretta, A.V.L. Freitas, A.S. Mello, RS. Oliv-
released these chemicals: D. montanus and eira, J.C. Cokendolpher, K. Brown and two
Eugyndes sp.) may get protection from aggre- anonymous reviewers for their comments on
gating with chemically-protected species (H. an earlier draft; and J.R. Lima for help with
nigra). On the otherhand, harvestmen that se- English.
crete noxious chemicals may gain benefit LITERATURE CITED
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