Morphometric variation among populations of Ambrysus mormon montandon (Heteroptera: Naucoridae)

9 Pages·2000·4.3 MB·

by  R W Sites

PROC. ENTOMOL. SOC. WASH.
102(3), 2000. pp. 533-541
MORPHOMETRIC VARIATION AMONG POPULATIONS OF
AMBRYSUS MORMON MONTANDON (HETEROPTERA: NAUCORIDAE)
Robert W. Sites and Michael R. Willig
(RWS) Enns Entomology Museum, Department ofEntomology, University ofMissouri,
Columbia, Missouri 65211 (e-mail: [email protected]); (MRW) Program in
Ecology and Conservation Biology, Department ofBiological Sciences and The Museum,
Texas Tech University, Lubbock, Texas 79409-3131 and National Center for Ecological
Analysis and Synthesis, University of California, 735 State Street, Suite 300, Santa Bar-
bara, California 93101-5504 (e-mail: [email protected])
—
Abstract. Morphometric variation with respect to 15 mensural characters was assessed
for adult specimens from 13 populations ofAmbrysus mormon Montandon in the United
States. This provides a context from which to assess the degree of divergence of an
isolated population (Ash Warm Springs, Nevada), which possesses discrete-state charac-
teristics that differ markedly from those ofother populations ofthe species. A multivariate
analysis of variance revealed that interpopulational differences were dependent on sex, a
finding corroborated by the univariate perspective provided by Bonferroni's sequential
adjustment that additionally identified lengths of body, protibia, and protarsus as contrib-
uting to the interaction between population and sex. Moreover, all pairwise comparisons
of the 13 populations were significant for males as well as for females (F-tests from
discriminant function analysis). Indeed, the pattern ofmorphometric variation among pop-
ulations was similar in males and females but was not a consequence of differentiation
by geographic distance (Mantel analyses). Interindividual variation primarily was due to
differences in size, as the first axis from principal components analysis accounted for
92.2% of the total variation. Three additional axes represented shape, and each accounted
for at least 10% for the remaining variation among individuals. The population from Ash
Warm Springs differed markedly (larger protarsus relative to meso- and metatarsi, and
smaller pro-, meso-, and metatarsi relative to body length and synthlipsis) from the other
populations, suggesting the need for systematic revision of its subspecific affiliation. In
contrast, populations of A. m. heidemanni Montandon and A. m. minor La Rivers were
similar to the other populations of the nominate subspecies, suggesting their subspecific
recognition is questionable. The distinctiveness ofthe population from Ash Warm Springs
as an isolated remnant of the historically connected pluvial White River intimates that it
may be a taxon in the process of specific differentiation.
Key Words: Naucoridae, Ambrysus mormon, morphometries, phenetics, shape
Ambrysus mormon Montandon is the Mexico. Throughout its range, A. mormon
most widespread species ofthe genus in the occurs in diverse habitats ranging from cold
United States, ranging from Oregon and montane streams to thermal spring efflu-
Idaho east to South Dakota, and south ents. Typically, it is found in slow water
through Arizona and New Mexico into near margins of gravel-bottomed streams
534 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
(Usinger 1946). Four subspecies currently domen; body width, head length, head
are recognized (La Rivers 1971): A. m. aus- width, and all leg segments were longest
tralis La Rivers in Mexico, southern Texas, distances; pronotal length was measured
and New Mexico; A. m. heidemanni Mon- along the midline. Meso- and metanotal
tandon in thermal runoffs in Yellowstone lengths were not included because it is dif-
National Park; A. m. minor La Rivers in a ficult to obtain accurate measurements
thermal spring in Idaho; and the nominate without dissection of specimens. Each of
subspecies throughout the remainder of the these nota subducts below the preceding
range in the United States from Idaho and notum, and the visible length is variable
South Dakota south to Arizona and New and dependent on the degree of thoracic
Mexico. flexion. Generally, 10 specimens of each
Because of their aquatic habitat require- sex from each population were measured;
ments, occurrence in otherwise arid land- however, for four populations, fewer than
scapes, and limited dispersal abilities, nau- 10 specimens were available (Eel River,
corids in the southwestern United States are Hot Creek Falls, Utah, and Yellowstone).
characterized by a high level of endemism. All data were transformed to natural loga-
For example, A. relictus Polhemus and Pol- rithms to evaluate more effectively the con-
hemus and A. amargosus La Rivers, which tribution of shape (see Sites and Willig
is listed as a federally endangered species, 1994a, b) to interpopulational differences.
occur only in several fragile streams in a Statistical analyses were executed using
western Nevada desert oasis; A.funebris La SPSS (1990). Voucher specimens are de-
Rivers occurs only in Death Valley; and posited in the Enns Entomology Museum,
Limnocoris moapensis (La Rivers) occurs University of Missouri-Columbia, and the
only in the vicinity of Moapa, Nevada. John T. Polhemus Collection.
An isolated population of A. mormon at Data from labels of 18 of the 19 speci-
Ash Warm Springs, Nevada, possesses dis- mens of A. m. minor provided insufficient
crete morphological characteristics that dif- detail concerning site of collection (Table
fer from those of other populations of the 1, HCF) to ascribe individuals to the type
species. As a parallel study to an analysis locality for the subspecies with certainty.
of discrete characters (J. T. Polhemus, in More specifically, a single specimen from
litt.), we present an analysis of shape-relat- the type locality of Hot Creek Falls is in-
ed variation in A. mormon to determine the cluded in the analysis, along with a series
degree of divergence of the Ash Warm of 18 specimens labeled "nr. Bruneau."
Springs population from other populations Therefore, the morphometric affinities of
ofthe nominate subspecies, as well as from these specimens with that of the known to-
A. m. minor and A. m. heidemanni. potypic specimen were determined using
discriminant function analysis with groups
Materials and Methods
defined by combinations of sex and popu-
A
suite of 15 external mensural charac- lation.
ters (body length and width; head length Two-way multivariate analysis of vari-
and width; synthlipsis; pronotal length; ance (MANOVA) evaluated differences
lengths ofpro-, meso-, and metathoracic fe- among populations and between sexes
mur, tibia, and tarsus), previously deter- based on mensural characters. Univariate,
mined to be effective in discriminating two-way analyses of variance (ANOVAs)
among naucorid taxa (Sites and Willig were performed to assess the contribution
1994a, b), was measured for adult speci- of each particular character to multivariate
mens of 13 populations ofA. mormon (Ta- group differences. To minimize the likeli-
ble 1, Fig. 1). Body length was measured hood of overestimating the significance of
from the tip of the labrum to tip of the ab- individual characters that compose a large
VOLUME 102, NUMBER 3 535
Table 1. Acronyms and collection data for populations ofAmhrysus mormon. Numbers of measured male
and female specimens, respectively, appear parenthetically below each acronym.
AWS NEVADA: Lincoln Co. MOA NEVADA: Clark Co.
(10,10) Ash Warm Springs (10,10) Warm Springs; 32°C
3750 ft. elev.; 36°C 27 August 1989
21 July 1992; CL 2711 coll: J. A. Back
J. T. and D. A. Polhemus
NMX NEW MEXICO: Lincoln Co.
COL COLORADO: Pueblo Co. (10,10) Rio Hondo 2 mi E Hondo
(10,10) Burnt Mill Creek 23 Sept. 1988; 12°C
CL2683; 19-V-1992 coll: R. W. Sites
J. T. Polhemus veg. in flow at margin
EEL CALIFORNIA: Mendocino Co. NYE NEVADA: Nye Co.
(8,10) Eel River at Bell glen nr. Leggett (10,10) Hot Creek Spring
62°F; 2-VII-1959 7 mi. W. Hwy 93
R. K. Allen 30°C; CL2898
coll: J. T. and D. A. Polhemus
HCF IDAHO: Owyhee Co. (1)
(0,1) Hot Cr. Falls SDA SOUTH DAKOTA: Fall River Co.
NW
9-IX-1965 (10,10) Hot Springs; 2 mi Hot Water
E. J. Allen 22 June 1940; H. C. Severin
HCF IDAHO: Owhyee Co. (18) SHA CALIFORNIA: Shasta Co.
(10,8) nr. Bruneau (10,10) Rt. 44; Cow Ck nr Palo Cedro
lO-X-1975 28 Aug 1991; R. S. Zack and
A. D. Allen M. A. Valenti, collrs.
IDA
536 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
Fig. 1. Range (shaded) oiAmbrysus mormon in the United States and localities (black circles) represented
in analyses.
among individuals in multidimensional males and females using Mantel correlation
space and to view differences among pop- analysis (Fortin and Gurevitch 1993, Manly
ulations with regard to shape and size for 1994, Sokal and Rohlf 1995). Via a sepa-
males and females separately. If the mag- rate Mantel analysis, we evaluated the de-
nitude of differences in shape among pop- gree to which interpopulational differences
ulations is a consequence of the degree of in shape between males and females were
isolation derived from geographic distance, similar, regardless of geographic correlates.
then a correlation should exist between a
Results
matrix ofpairwise linear distances between
sites and a matrix ofpairwisemorphometric The 18 specimens labeled "nr. Bruneau"
distances based on population centroids of uncertain subspecific association likely
(PC2 through 4). We tested this hypothesis represent A. m. minor because they formed
of differentiation by distance separately for a well-defined cluster in morphometric
VOLUME NUMBER
102, 3 537
Table 2. Significance levels (PY of the character
suite (MANOVA) and each character separately (AN-
OVA) in distinguishing between sexes and among 13
populations ofAmbi-ysus mormon.
Population Pop X Sex
MANOVA
538 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
Table 3. Principal component loadings (correlations) for axes representing size (PCI) and appreciable attri-
butes ofshape (PC2 through 4).
Character
VOLUME NUMBER
102, 3 539
Males
2"
V
UO
540 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
ronmental temperatures during develop- bert 1893, Carpenter 1915), which persists
ment, yet the positions of group centroids as a tributary of the Colorado River. Since
(Fig. 2) indicate that these populations that time, extensive desertification of the
share similar attributes of shape. Although Great Basin has occurred (King 1958), sig-
thermal environment does influence allo- nificantly reducing the extent of the White
metric relationships during ontogeny (Sites River. As a result, —200 miles of dry river
et al. 1996), the greatest degree of shape- bed now cross the parched desert, isolating
related variation in A. mormon has been the Recent White River (north) from the
achieved independently ofthermal environ- Moapa River (south). Despite drastic reduc-
ment during ontogeny. tions in surface water, sporadically distrib-
The population ofA. m. minor (HCF) ex- uted springs and effluent fragments of the
hibits morphometric attributes distinct from pluvial White River persist.
those of the nearby population of nominate The fish fauna of the White River is dis-
A. mormon in the Bruneau River (IDA), tinct and includes an endemic cyprinid ge-
based on a pairwise F-test from DFA. De- nus, an endemic species of Crenichthys,
spite the close proximity ofHot Creek Falls and several indigenous species and subspe-
to Bruneau River (~1 km), distinct mor- cies (Hubbs and Miller 1948). Subspecific
phologies characterize the populations of affinities of isolated fish populations in
the two sites. A series of unnamed hot these springs and effluents strongly suggest
springs occurs along the Bruneau River, and that these habitats were continuous in the
A. mormon may occur in them. Because of past. Ambrysus mormon also occurs in
local irrigation demands, the watertable has many ofthese aquatic fragments, a result of
declined recently, resulting in a concomi- its almost certain occurrence in the pluvial
tant reduction in the size of the springs. White River and subsequent isolation fol-
During the last known visit to these springs lowing landscape wide desertification.
(1991), the pool at the base of Hot Creek Three populations included in our morpho-
Falls, known as Indian Bathtub, was re- metric assessment represent White River
duced tremendously in size (—1.0 m diam isolates [AWS, MOA, and NYE (see Table
X 0.3 m depth) and persisted only because 1)]. Specimens of the Ash Warm Springs
a piece of plastic had been placed on the population exhibit structural features that
downstream side of the pool, probably to are divergent from "typical" A. mormon,
allow bathing; the actual falls no longer ex- including reduction in posterolateral con-
ists (R. S. Zack, personal communication). nexival spines, narrow embolium, and dif-
Continued reduction in the water table may ferences in genitalic features.
threatenA. m. minor to the point that it may Evaluation of discrete-state characters of
soon become extinct. the isolated Ash Warm Springs population,
The White River system in eastern Ne- combined with the attributes of shape of
vada extends from the White Pine Moun- other members of the species, suggests that
tains near Ely south to the Moapa River this population may warrant taxonomic rec-
near Lake Mead. The lower end of the ognition, at least at the subspecific level.
White River near Moapa is known as the Although the length oftime that it has been
Muddy River. The White River system is of separated from the parent population from
hydrographic and faunistic interest because White River is uncertain, clear differences
of its recently restricted watercourse and in phenotype, including shape, have accu-
isolated fish populations. In the Pliocene mulated in isolation. Even if speciation is
and at least the early Pleistocene epochs, not yet complete, the population likely will
the pluvial White River was continuous continue to diverge from the other popula-
throughout its length to an isolated lake, tions of A. mormon because of the effects
now represented by the Moapa River (Gil- of drift and natural selection in environ-
.
VOLUME NUMBER
102, 3 541
mental isolation. If divergence has not pro- and Analysis of Ecological Experiments. Chap-
gressed to the point at which reproductive man and Hall, New York. 445 pp.
incompatibility exists between the Ash Gilbert, C. H. 1893. Report on the fishes ofthe Death
Valley Expeditioncollected in southernCalifornia
Warm Springs population and the remain- andNevadain 1891, withdescriptionsofnewspe-
der of A. mormon, then we are witnessing cies. North American Fauna 7: 229-234, pis. 5-6.
speciation in action in this remnant popu- Holm, S. 1979. A simple sequentially rejective multi-
lation ofA. mormon. Although the Nye Co. ple testprocedure. Scandinavian JournalofStatis-
and Moapa populations also represent iso- Hubbtsi,csC6.:L6.5a-n70d.R. R. Miller. 1948. The Great Basin,
lates along the White River system, these with emphasis on glacial and postglacial times. II.
populations have not diverged from typical The zoological evidence: Correlationbetweenfish
morphometric form of A. mormon as radi- distribution and hydrographic history inthedesert
cally as has the population at Ash Warm basins of western United States. Bulletin of the
University ofUtah 38(20): 18-166 + 1 map.
Springs (Fig. 2). Further studies involving King, R B. 1958. Evolutionofmodemsurfacefeatures
discrete state characters are needed to re- of western North America, pp. 3-60. In Hubbs,
solve the intraspecific taxonomic status of C. L., ed. Zoogeography. American Association
these populations ofA. mormon. fortheAdvancementofSciencePublicationNum-
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Acknowledgments La Rivers, I. 1951. A revision ofthe genus Ambrysus
in the United States (Hemiptera: Naucoridae).
We thank John T. Polhemus (Englewood, University ofCalifornia Publications in Entomol-
CO) and Richard S. Zack (Washington ogy 8: 277-338.
State University) for the loan and gift of cori.da1e9)6.3.BiTolwoogicnaelwSoAcimebtryyosfiN(eHveamdiaptOecrcaa:siNoanua-l
selected specimens for this project, and Ri- Papers 1: 1-7.
chard S. Zack for details concerning local- . 1971. Studies of Naucoridae (Hemiptera). Bi-
ities of A. m. minor. We also thank J. E. ological Society of Nevada Memoirs 2, iii +120
McPherson (Southern Illinois University) pp.
Manly, B. F J. 1994. Multivariate statistical methods:
and John T. Polhemus for critical reviews A primer, 2nd ed. Chapman and Hall, New York.
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Museum provided optical and computing Rice, W. R. 1989. Analyzing tables ofstatistical tests.
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while MRW was a Sabbatical Fellow at the Sites,finRi.tiWes. ainndAMm.brRy.sWuislli(gH.em1i9p9t4ear.a:InteNrasupceocriifidcaea)f.-
National Center for Ecological Analysis Proceedings of the Entomological Society of
and Synthesis, a center funded by NSF Washington 96: 527-532.
(DEB-9421535), the University of Califor- . 1994b. Efficacy ofmensural charactersindis-
nia at Santa Barbara, and the state of Cal- criminating among species of Naucoridae (Insec-
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vided to by the Provost's Office, logical Society ofAmerica 87(6): 803-814.
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was provided in part by University of Mis- phology, ontogeny, and adaptation of Ambrysus
mormon (Hemiptera: Naucoridae): Quantitative
souri project #PSSL0232. This is Missouri comparisons among populations in different ther-
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