Journal of Fish Biology (2005) 66, 454–467
doi:10.1111/j.1095-8649.2004.00611.x, available online at http://www.blackwell-synergy.com
Variability in diet and foraging behaviour between sexes
and ploidy forms of the hybridogenetic Squalius
alburnoides complex (Cyprinidae) in the
Guadiana River basin, Portugal
A. G O M E S -F E R R E I R A *, F. R I B E I R O *, L. M O R E I R A D A C O S T A *,
I. G. C O W X † A N D M. J. C O L L A R E S -P E R E I R A *‡
*Centro de Biologia Ambiental, Departamento de Biologia Animal, Faculdade de
Cieˆncias da Universidade de Lisboa, C2 – Piso 3, Campo Grande, 1749-016
Lisboa, Portugal and †International Fisheries Institute, University of Hull,
Hull, HU6 7RX, U.K.
(Received 7 May 2003, Accepted 19 October 2004)
Samples of the Squalius alburnoides complex, a hybridogenetic Iberian minnow composed of
diploid, triploid and, less commonly, tetraploid forms, were collected from a tributary of the
Ardila River (Guadiana River basin) between February 1999 and January 2001. Seasonal
variation in diet was evident, and was probably linked to prey availability. Distinct foraging
behaviours between ploidy forms were found towards several prey items, suggesting that
diploid adult males fed mostly near the surface, whereas diploid adult females tended to feed
near the bottom and submerged vegetation. Triploid females exhibited an intermediate foraging
behaviour, although there was greater affinity towards diploid male feeding behaviour. Diploid
males which, in contrast to diploid and triploid females, have non-hybrid genomes in the
Guadiana drainage, exhibited a higher specialization for food. Despite considerable dietary
overlap, there appeared to be spatial segregation of feeding niches between the three forms,
especially during dry periods when prey availability was lower, which may be a strategy for
diminishing competition for food. Therefore, considering asexual generalist and specialist
hypotheses, it appears that the different ploidy levels are generalist, opportunistic feeders that
# 2005 The Fisheries Society of the British Isles
partition the resources when limited.
Key words: diet overlap; hybrids; Iberian cyprinid; non-sexual vertebrates; polyploidy.
INTRODUCTION
The Squalius alburnoides (Steindachner) complex is an endemic Iberian cyprinid,
with a hybrid origin and an altered mode of reproduction called meiotic
hybridogenesis. Although diploid (2n ¼ 50), triploid (3n ¼ 75) and rare tetraploid (4n ¼ 100) males and females can be found, the previously described
reproductive modes imply cyclic polyploidy elevation and reduction, allowing
a bidirectional movement of genes between the various forms (Alves et al.,
‡Author to whom correspondence should be addressed. Tel.: þ351 21 7500000 ext. 24309; fax:
þ351 21 7500028; email: [email protected]
454
#
2005 The Fisheries Society of the British Isles
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
455
2001). This fish complex is one of the few examples of an asexual (i.e. nonrecombinant; Beukeboom & Vrijenhoek, 1998) vertebrate that include both
hybrid females and males that are fertile. It also has a non-hybrid all-male
diploid lineage, which has allowed the preservation of the ancestral paternal
genome and plays a significant role in the dynamics of the hybrid southern
populations (Alves et al., 2002).
Due to their total lack of or reduced meiotic recombination compared with
bisexuals, asexual vertebrates have been considered interesting for ecological
studies (Vrijenhoek, 1994; Schlosser et al., 1998). Two general models were
proposed to explain the coexistence of asexuals, either clones or hemiclones,
and their sexual relatives since sperm is always required to trigger embryonic
development: the General-Purpose-Genotype model (GPG) and the FrozenNiche-Variation model (FNV) (Jokela et al., 2003). The GPG model proposes
that those asexuals that best tolerate broad environmental conditions are the
ones that show higher persistence. Accordingly, in highly fluctuating habitats,
selection would favour a more generalized resource utilization. Conversely, the
FNV model predicts that in a heterogeneous environment selection would
favour asexuals with distinct ecological strategies to avoid both inter-clonal
and sexual relatives competition; in such cases in heterogeneous environments,
selection would eliminate ecologically similar asexuals and favour narrower
niche breadth.
As regards hybridogenetic complexes, the FNV model has been suggested to
promote a stable coexistence once resource partitioning, with concomitant
reduction of competition, has been accounted for (Vrijenhoek, 1994). Differences in resource partitioning between hemiclones have been described for
Poeciliopsis monacha-lucida (Schenck & Vrijenhoek, 1989; Weeks et al., 1992;
Gray & Weeks, 2001) and Rana esculenta L. (Rist et al., 1997; Negovetic et al.,
2001).
Although extensive genetic studies on the S. alburnoides complex have been
conducted (Alves et al., 2001), testing for resource partitioning in this fish
complex is lacking because it is not possible to recognize the ploidy forms in
the field. Martins et al. (1998), however, found some evidence of spatial segregation mainly outside the reproductive period between the diploid males and the
two female forms in the Guadiana River basin, which suggested differences in
resource use. In the only other published study examining differences between
ploidy forms (Ribeiro et al., 2003), no difference in growth rate was found
between diploid and triploid females, but diploid males tended to grow more
slowly than females. The same authors also found no significant differences in
fecundity for age between the female forms, but diploids tended to produce
bigger (diploid) oocytes than triploids. This lack of information on resource
partitioning is considered a major omission because of the evolutionary significance of maintaining a stable coexistence of the ploidy forms. It could also help
to explain inter-basin variations and intra-population fluctuations in the ploidy
forms and genotype constitution (Alves et al., 2001), suggesting variability in
population dynamics and biotic interactions within systems.
The present study compared the diet composition and foraging behaviour of
the coexisting forms of S. alburnoides complex, to identify if there is any niche
separation between the forms which could account for maintenance of the
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
456
A. GOMES-FERREIRA ET AL.
population structure. It was conducted using the same samples studied by
Ribeiro et al. (2003), collected over a 2 year period in a tributary of the Ardila
River, a major tributary of the Guadiana River in Portugal.
MATERIALS AND METHODS
STUDY AREA
The Guadiana River basin in southern Iberia has an irregular hydrological regime,
with severe floods and droughts, and >80% of rainfall between October and March.
During the summer (June to September) many tributaries either dry up or are reduced to
a series of isolated pools. The study was conducted at a single site (38 090 N; 06 580 W) on
the Múrtega River, a third order stream that forms a series of isolated pools during the
dry period. The sampling site was located upstream of the Ardila River, the major left
bank tributary of the Guadiana River, at 625 km from its confluence and at an altitude
of 180 m (Ribeiro et al., 2003).
At the study site, the S. alburnoides complex was the most abundant and common fish.
This population comprised mainly triploid females (566%), with diploid males (a nonhybrid important lineage in this drainage; Alves et al., 2002) representing 202% and
diploid females 228% (Ribeiro et al., 2003). Triploid males were very rare (04%) as in all
populations of this complex that have been studied (Alves et al., 2001).
SA M P LI NG P ROC ED UR E S
As described in Ribeiro et al. (2003), sampling was carried out by electrofishing
(300–500 V, 2–3 A, DC) between February 1999 and January 2001. During the first 5
months, samples were obtained at 15 day intervals but monthly thereafter, at the same
period of the day. On each occasion the sampling procedure tried to cover all available
habitats of the river stretch and a sub-sample of 40 fish, when possible, was collected for
analysis. Immediately after capture a blood sample was taken from all fish caught, mixed
with freezing solution (855% w/v sucrose, 118% w/v citric acid and 5% v/v dimethyl
sulphoxide), transported in liquid nitrogen to the laboratory, and stored at 80 C. This
blood sample was used to determine the ploidy level by flow cytometry (Epics
Profile II, Coulter), based on DNA content measurements following the protocol of
Collares-Pereira & Moreira da Costa (1999). All fish samples were immediately placed on
ice until they were stored at 20 C in the laboratory.
DI E T A NA L Y S IS
In the laboratory, all fish were measured (standard body length, LS, to the nearest mm)
and weighed (total mass, Mt, to the nearest 0001 g). The gut was extracted from each fish
and the fish reweighed (eviscerated mass, Me, to the nearest 0001 g). Gut mass was
estimated from Mt Me. Since cyprinids lack a differentiated stomach, all gut contents
were examined at 50 magnification. Prey items were identified to the lowest taxon
according to Tachet et al. (2000), and numerically quantified by identifying pieces that
most often survive cyprinid digestion and mastication. Gut contents were grouped in the
following categories: Diptera adults, Chironomidae larvae, Simulidae larvae, Diptera
nymphs, Ephemeroptera nymphs, Coleoptera adults, Corixidae, Gastropoda, Ostracoda,
Ectoprocta statoblasts, Spongillidae gemmules, seeds, plant material, inorganic material,
unidentified material and others. The ‘others’ category included items with a frequency of
occurrence <5% in all three forms, such as Coleoptera larvae, Formicidae, Trichoptera
larvae and fish eggs. Unfortunately, the gut contents could not be weighed or their
volume determined since the pharyngeal teeth had severely macerated all the food.
Consequently, a percentage contribution was attributed to each food item that could
not be counted (plant, inorganic and unidentified materials) and also to the gastric
mucus, based on the Hynes points method (Hynes, 1950).
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
457
Feeding activity was evaluated from the fullness (estimated fullness of each gut),
feeding activity (100 number of guts with food divided by total number of guts) and
‘repletion’ [100(Mt Me) Mt1] indices.
DATA ANALYSES
Numerical abundance (AN, %), based on total number of prey in the sample, and the
frequency of occurrence (FO, %), omitting empty guts, were calculated according to
Hyslop (1980). A modification of the Costello (1990) graphical representation was
applied to assess foraging behaviour and determine dominant prey items (Amundsen
et al., 1996). This method classifies the species prey selection behaviour by plotting the FO
and the prey-specific abundance (AP), defined as the percentage a prey taxon contributes
to all prey
in only those fish in which the actual prey occurs:
P items
P
APi ¼ 100( Si) ( St)1, where APi is the prey-specific abundance of prey i, Si the gut
content (in this case, number) comprised of prey i, and St the total gut content in only
those fish that consumed prey i. This graphical representation determines prey importance (rare prey will be located near the lower left corner of the graph and dominant prey
near the upper right corner) and also the feeding strategy (most points at the bottom of
the graph reflect generalization and most points at the top reflect specialization). The
relationship between feeding strategy and the between- or within-phenotype contributions to the niche width are also represented. The lower right represents a high withinphenotype component, and the upper left represents a high between-phenotype
component.
To examine seasonal variation in diet using w2, equivalent months in consecutive years
were grouped. This was deemed acceptable because environmental and meteorological
variations between years were not significant during the study period (Ribeiro et al.,
2003).
Ontogenetic diet variation was studied using seven, 10 mm interval, LS classes. Since
capture of the different classes was not constant, results may have been somewhat
distorted by seasonal variation.
Percentage overlap between diets of the three forms, for each month and between size
classes, were calculated using the Renkonen overlap index where values >60% are
regarded as high (Krebs, 1989; Marshall & Elliott, 1997).
To determine differences regarding overall, seasonal and ontogenetic variations for the
three dominant ploidy forms of the S. alburnoides complex, a Kruskal-Wallis test was
applied, firstly, to the repletion index, and then to the AN of each item, with the exception
of items which could not be quantified in discrete units, in which case the FO were used.
When significant differences were found, Dunn’s post hoc test was applied to locate
groups among which these differences were found (Sokal & Rohlf, 1981; Mosteller &
Rourke, 1993; Lehner, 1996).
The data and statistical analyses were performed using the software EXCEL 2000 and
SPSS version 10.0.1.
RESULTS
SA M P L E C H A R A C T E R I ZA T I O N
A total of 290 triploid females (27–90 mm LS), 147 diploid females (31–85 mm
LS), 114 diploid males (27–57 mm LS) and four triploid males (40–54 mm LS)
were analysed. The sub-sample of triploid males was not considered further due
to the small sample size. Only triploid females attained age 5þ years, whilst
diploid females reached age 4þ years and diploid males age 3þ years. The best
represented age classes were 2þ and 3þ years for triploid females (306 and
255%, respectively) and diploid females (261 and 312%, respectively), and 1þ
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
458
A. GOMES-FERREIRA ET AL.
and 2þ years for diploid males (578 and 202%, respectively). As described by
Ribeiro et al. (2003), both these ploidy forms had a protracted spawning period
between late March and early June.
F O O D C O N SU M P T IO N
The average repletion and feeding activity indices and the average fullness
were all highest for triploid females (Table I). There were significant differences
for the repletion index between diploid males and diploid females (Dunn’s post
hoc test, Q ¼ 458, k ¼ 3, P < 005), and between diploid males and triploid
females (Dunn’s post hoc test, Q ¼ 686, k ¼ 3, P < 005). The average fullness
coefficient was lowest for diploid males and the feeding activity index was
lowest for diploid females.
The diet of S. alburnoides complex consisted mainly of macroinvertebrates,
chiefly insects (Table II). Significant differences in food intake were found
between ploidy groups for only Diptera adults, Coleoptera adults and plant
material (Dunn’s post hoc test). Diploid females fed less frequently on Diptera
adults than triploid females (Q ¼ 308, k ¼ 3, P < 005) and diploid males
(Q ¼ 387, k ¼ 3, P < 005). Plant material was more commonly found in triploid
(FO ¼ 3125%; Q ¼ 372, k ¼ 3, P < 005) and diploid (FO ¼ 2615%) females
than in diploid males (FO ¼ 667%; Q ¼ 240, k ¼ 3, P < 005) (Table II). Overall
the different ploidy forms were all considered to be generalists with some
specialization for the four most abundant items (Diptera adults, Chironomidae
larvae, Simulidae larvae and Ephemeroptera nymphs) (Fig. 1).
SE A SO NA L DIE T V A R IA T IO N
The repletion index indicated seasonal variation in food intake (w2, d.f. ¼ 11,
P < 005), since mean gut mass decreased during the dry season (late spring and
early summer). Amongst the three forms, however, no relevant seasonal variation was found and February had the highest number of prey items per gut.
Some seasonal variation in the AN of several items occurred (Fig. 2). In the
summer months, when prey availability (diversity) was lower, the ingestion of
plant, inorganic and unidentified material increased whereas Simulidae larvae,
Diptera adults and Ephemeroptera nymphs had a lower AN. Chironomidae
larvae were the item consumed more regularly throughout the year, although
their occurrence decreased in late summer and early autumn (Fig. 2).
TABLE I. Average feeding indices for the three ploidy forms of the Squalius alburnoides
complex in the Múrtega River (Guadiana River basin)
Triploid females
Sample size (n)
Repletion index (%)
Feeding activity index (%)
Average fullness (%)
#
290
421
9379
5760
Diploid females
147
389
8 844
5 524
Diploid males
114
377
9211
4553
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
459
TABLE II. Numerical abundance (AN) and frequency of occurrence (FO) for the items
found in the guts of the three ploidy forms of the Squalius alburnoides complex in the
Múrtega River (Guadiana River basin)
Triploid females
Food item
Macroinvertebrates
Diptera adults*
Chironomidae larvae
Simulidae larvae
Diptera nymphs
Ephemeroptera nymphs
Coleoptera adults*
Corixidae*
Gastropoda*
Ostracoda*
Seeds*
Plant material*
Inorganic material
Unidentified material*
Ectoprocta statoblasts
Spongillidae gemmules*
Others*
Diploid females
Diploid males
AN (%)
FO (%)
AN (%)
FO (%)
AN (%)
FO (%)
8668
2127
2999
1506
206
1255
381
073
037
084
427
–
–
–
168
517
034
8529
4743
5662
3566
1029
3603
1544
331
221
221
1875
3125
3235
4301
478
919
184
9005
1180
2971
1414
290
1406
630
732
186
196
402
–
–
–
163
129
213
8231
3077
5231
3231
1077
3692
1692
1462
923
692
1846
2615
3923
4462
615
231
769
9669
3055
3686
1176
034
1361
201
114
000
042
090
–
–
–
000
145
004
9143
5238
5524
2857
476
3714
571
667
000
095
476
667
1810
2571
000
667
095
*Statistical differences (P < 005) between the ploidy forms (Kruskal–Wallis test).
O N T O G E N E T I C DI E T V A R I A T I O N
The diet variation among the different LS classes for the three ploidy forms of
the complex is shown in Fig. 3. Juvenile (<30 mm) and adult diploid males
>50 mm had a narrower diet niche than 30–50 mm fish (Fig. 3). Diploid females
showed no evidence of variation in diet between size classes, but this form
exhibited a great variability in niche width. Triploid females also ate a greater
variety of food types than the other forms, but the consumption of Diptera
adults and Simulidae larvae increased with size. Conversely, the occurrence of
unidentified and inorganic material decreased with size.
DIET OVERLAP
Considerable overlap in diet (>60%) was found between the three forms. Diet
overlap was highest between diploid and triploid females (8742%), followed by
diploid males and triploid females (7947%) and diploid females and males
(7105%).
Diet overlap between the different ploidy forms was lower during the summer
months, when river flow stopped and the fish were restricted to isolated pools,
than in winter and spring months and after the re-establishment of flow
(September to October) (Fig. 4). Testing of diet overlap between ploidy forms
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
460
A. GOMES-FERREIRA ET AL.
80
(a)
70
60
50
40
30
20
10
0
0
80
10
20
30
40
50
60
10
20
30
40
50
60
10
20
30
40
50
60
(b)
70
AP (%)
60
50
40
30
20
10
0
0
80
(c)
70
60
50
40
30
20
10
0
0
FO (%)
&
FIG. 1. A modified graphical representation (plots of prey-specific abundance and frequency of occurrence; Costello, 1990) for the diet (^, Diptera adults; &, Chironomidae larvae; ~, Simulidae larvae;
, Diptera nymphs; , Ephemeroptera; þ, Coleoptera adults; -, Corixidae larvae; , Gastropoda; ,
Ostracoda; &, seeds; n, Ectoprocta statoblasts; *, Spongillidae gemmules) of the three ploidy
forms: (a) triploid females, (b) diploid females and (c) diploid males of the Squalius alburnoides
complex in the Múrtega River (Guadiana River basin).
of different size groups was constrained by lack of samples in some size groups,
especially diploid males. Diet overlap between both female forms was lowest for
the 40–50 mm LS class.
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
461
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
(a)
100
31
27
37
55
25
9
15
3
5
18
30
12
8
10
9
10
31
6
5
10
19
11
42
80
60
40
20
0
(b)
100
5
3
5
11
9
14
80
AN (%)
60
40
20
0
(c)
100
1
1
5
11
13
80
60
40
20
0
Jan
Mar
May
Jul
Sep
Nov
Month
FIG. 2. Seasonal variation in the numerical abundance of the diet ( , Others; , Spongillidae gemmules;
, Ectoprocta statoblasts; , seeds; , Ostracoda; &, Gastropoda; , Corixidae larvae; , Coleoptera adults; , Ephemeroptera nymphs; , Diptera nymphs; , Simulidae larvae;
, Chironomidae
larvae; , Diptera adults) of the three ploidy forms: (a) triploid females, (b) diploid females and (c)
diploid males of the Squalius alburnoides complex in the Múrtega River (Guadiana River basin). The
numbers above each column represent sample size.
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
462
A. GOMES-FERREIRA ET AL.
(a)
100
1
36
62
52
17
28
32
7
35
57
5
≤ 30
30–40
40–50
50–60
59
45
17
80
60
40
20
0
(b)
100
31
16
6
80
AN (%)
60
40
20
0
(c)
100
80
60
40
20
0
60–70
70–80
80–90
LS (mm)
FIG. 3. Variation in the numerical abundance of the diet ( , others; , Spongillidae gemmules; , Ectoprocta
statoblasts; , seeds; , Ostracoda; &, gastropoda; , Corixidae larvae; , Coleoptera adults; ,
Ephemeroptera nymphs; , Diptera nymphs; , Simulidae larvae; , Chironomidae larvae; , Diptera
adults) with standard length of the three ploidy forms: (a) triploid females, (b) diploid females and (c)
diploid males of the Squalius alburnoides complex in the Múrtega River (Guadiana River basin). The
numbers above each column represent sample size.
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
463
90
Diet overlap (%)
80
70
60
50
40
30
20
10
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
FIG. 4. Monthly dietary overlap between diploid females and triploid females (F2n-F3n; ), diploid
females and diploid males (F2n-M2n; &) and diploid males and triploid females (M2n-F3n; ~) of
the Squalius alburnoides complex, in the Múrtega River (Guadiana River basin). Points above the
horizontal line represent significant diet overlap.
DISCUSSION
The overall diet of the different ploidy forms of S. alburnoides complex
suggests that it is a generalist feeder, confirming the general conclusions of
Magalhães (1993) and Coelho et al. (1997) based on aggregated data from all
ploidy forms in studies on the Tejo River basin.
All ploidy forms of the complex fed preferentially on insect adults and larvae,
probably because these prey types were locally abundant (unpubl. data) and
have a high calorific value. Diploid males had significantly lower amounts of
plant material in their diet and had a narrower dietary niche than, for example,
triploid females. Diploid females exhibited a more generalist diet, consuming
substantially less Diptera adults than the other two forms and more of the less
frequently occurring prey items (‘others’). This suggests differences in resource
use possibly associated with different space use by the distinct forms, as
suggested by Martins et al. (1998) who found that diploid females prefer deeper
water and a coarse substratum in the non-reproductive period, while triploid
females prefer higher current velocity and a high proportion of instream cover.
The feeding activity index, average fullness and repletion index were highest
for triploid females. Although the reasons for this are unclear it may be related
to the larger size of triploids in general, giving them greater access to a wider
range of food items and potentially being more aggressive in their feeding
behaviour. This is probably linked to resource partitioning, which may help
explain the continued coexistence of the different ploidy forms, and verify
whether the GPG model or the FNV model applies to this complex. Further
work on the dominance of the different forms is required.
In intermittent rivers, peak emergence in most insects tends to occur after the
first autumn rainfalls (Puig et al., 1986), and the relative abundance of the prey
(Tachet et al., 2000) was generally consistent with the seasonal variability found
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
464
A. GOMES-FERREIRA ET AL.
in the guts of the S. alburnoides complex. Thus the various ploidy forms seem to
exhibit an opportunistic behaviour that is well adapted to environmental variability, focusing on alternative food items, when preferential (or energetically
more advantageous) prey types are not available. In a heterogeneous diet,
however, several prey can be important due to their high abundance in only
some guts or to their high frequency of occurrence despite a low numerical
abundance (Costello, 1990). The modification to the Costello (1990) method
suggests a mixed feeding strategy, with varying degrees of specialization and
generalization on different prey types. The occurrence of adult stages of Diptera
and Ephemeroptera nymphs in the guts suggests that this complex consumes the
most available prey, e.g. invertebrates within aquatic vegetation as well as
drifting organisms on the surface and on the water column, as previously
suggested by Coelho et al. (1997).
Plant material was also an important item in the diet. It was frequently found
in small quantities and mostly during the dry months; the same was found for
inorganic and unidentified (mainly detritus) material. The ingestion of these
food items may be accidental, when the fish tries to ‘snap’ small benthic animals
from plants or bottom substrata, or this may be due to a lower defecation rate
when the volume of animal food is lower, which causes the accumulation of this
least digestible material in the guts, leading to an overestimate of these items.
One other alternative is that the ingestion of plant and inorganic material may
be an alternative resource when animal food resources are scarce. Several other
authors (Brabrand, 1985; Cowx, 1989; Lobón-Cerviá & Rincón, 1994) found
that cyprinids often increase their consumption of plant material when availability of animal prey is reduced. Indeed such seasonal consumption of plant
material is believed to be a dietary refuge buffering seasonal variation in the
availability of animal food resources (Persson, 1983).
Smaller fish (<30 mm) fed on a small range of items, which can be explained
by a limited capacity to capture other prey, but it may also suggest a difference
in the spatial niches occupied. The diet of juveniles resembles the characteristic
feeding habits of benthic fishes, so it is possible that smaller fish inhabit river
beds, whereas adults tend to feed from the surface and in the water column.
This may be a strategy to reduce interaction with adults, increasing survival, but
it may simply be a behavioural effect with smaller individuals staying closer to
shelter associated with the bed. The reduced choice of food in such microhabitats is counterbalanced against tradeoffs in energy to maintain station in
the water column and reduced predation risk.
Considerable overlap in diet was observed between the ploidy forms of the
complex. The overlap values, however, were consistently higher between diploid
and triploid females, than between both diploid forms. The time of least overlap
in diet was during the summer months when the fish are restricted to reduced
habitat because of the drought conditions. It appears that the triploid and
diploid forms are selecting for different food types, i.e. chironomid and simulid
larvae, and coleopteran adults and corixid larvae respectively, which suggests
they may be feeding in different parts of the water column, perhaps to reduce
competition. From the data it is not known which ploidy form is locally
dominant, and whether there is partitioning of the resources to reduce competition during periods when resources are likely to be limited, i.e. during the dry
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
465
season. The high dietary overlap during the wet season coincides with greater
abundance of food resources, which by inference will reduce competitive pressures. Both situations will contribute to the continued coexistence of the various
ploidy forms within the community, and when linked to the spatial segregation
identified by Martins et al. (1998) points towards a strategy of partitioning of
resources.
According to Schenck & Vrijenhoek (1989), seasonally stable spatial segregation (over both large and small sampling scales) is widespread among coexisting
sexual and clonal forms of Poeciliopsis, another hybridogenetic complex. These
authors also showed that differences in the natural diets of these fish were
strongly affected by microhabitat heterogeneity. In the case of S. alburnoides
complex, Coelho et al. (1997) compared its overall diet with a close sexual
relative, Squalius pyrenaicus (Gunther), and found reduced overlap between
them, which they attributed to seasonal differences in their specific foraging
activities. Although such a hypothesis could not be empirically demonstrated
within the ploidy forms of the S. alburnoides complex, the reduced similarity in
diets during the dry season (after the spawning period) when prey availability
was also lower, and the differential feeding habitats, suggest such mechanisms
are probably occurring within this hybridogenetic complex. Therefore,
considering the asexual generalist and specialist hypotheses, it appears that the
different ploidy levels are generalist, opportunistic feeders that partition the
resources when limited. Thus, this species complex is closer to the GPG model.
The comparatively narrower resource use of diploid non-hybrid males, which
are known to have normal meiotic recombination and act as a continuous
source of genetic variation of the complex in the southern populations (Alves
et al., 2002), might provide an opportunity to test for competition-avoidance
strategies, once there are well-established populations where they coexist with
hybrid (non-recombinant) diploid males. Such microhabitat scale analyses need
to be performed to confirm the hypothesized niche diversification and achieve a
progressively better understanding of the intra- and interpopulation dynamics
of this hybridogenetic fish.
We are grateful to T. Marques, A.F. Filipe and M. Gromicho, who contributed with
very helpful discussions, and also to C. Mieiro and P. Tiago for their help in fieldwork
and prey identification. This work was supported by Empresa de Desenvolvimento e
Infra-Estruturas de Alqueva (EDIA, S.A. and PEDIZA/FEDER), in the frame of the
project PM2.1 (‘Programa de Minimização para o Património Natural – Áreas de
Regolfo de Alqueva e Pedrógão’). We acknowledge the Direcção Geral das Florestas
for permission to collect specimens. We also thank two anonymous referees for valuable
comments.
References
Alves, M. J., Coelho, M. M. & Collares-Pereira, M. J. (2001). Evolution in action through
hybridisation and polyploidy in an Iberian freshwater fish: a genetic review.
Genetica 111, 375–385.
Alves, M. J., Collares-Pereira, M. J., Dowling, T. E. & Coelho, M. M. (2002). The
genetics of maintenance of an all-male lineage in the Squalius alburnoides complex.
Journal of Fish Biology 60, 649–662. doi: 10.1006/jfbi.2002.1883
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
466
A. GOMES-FERREIRA ET AL.
Amundsen, P. A., Gabler, H.-M. & Staldvik, F. J. (1996). A new approach to graphical
analysis of feeding strategy from stomach contents data – modification of the
Costello (1990) method. Journal of Fish Biology 48, 607–614.
Beukeboom, L. W. & Vrijenhoek, R. C. (1998). Evolutionary genetics and ecology of
sperm-dependent parthenogenesis. Journal of Evolutionary Biology 11, 755–782.
Brabrand, A. (1985). Food of roach (Rutilus rutilus) and ide (Leuciscus idus): significance
of diet shift for interspecific competition in omnivorous fish. Oecologia 66,
461–467.
Coelho, M. M., Martins, M. J., Collares-Pereira, M. J., Pires, A. M. & Cowx, I. G. (1997).
Diet and feeding relationships of two Iberian cyprinids. Fisheries Management and
Ecology 4, 83–91.
Collares-Pereira, M. J. & Moreira da Costa, L. (1999). Intraspecific and interspecific
genome size variation in Iberian Cyprinidae and the problem of diploidy and
polyploidy, with review of genome sizes within the family. Folia Zoologica 48,
61–76.
Costello, M. J. (1990). Predator feeding strategy and prey importance: a new graphical
analysis. Journal of Fish Biology 36, 261–263.
Cowx, I. G. (1989). Interactions between roach, Rutilus rutilus, and dace, Leuciscus
leuciscus, populations in a river catchment in the south-west of England. Journal
of Fish Biology 35, 279–284.
Gray, M. M. & Weeks, S. C. (2001). Niche breadth in clonal and sexual fish (Poeciliopsis):
a test of the frozen niche variation model. Canadian Journal of Fisheries and
Aquatic Sciences 58, 1313–1318.
Hynes, H. B. N. (1950). The food of freshwater sticklebacks (Gasterosteus aculeatus and
Pygosteus pungitius) with a review of methods used in studies of the food of fishes.
Journal of Animal Ecology 19, 36–58.
Hyslop, E. J. (1980). Stomach contents analysis – a review of methods and their
application. Journal of Fish Biology 17, 411–429.
Jokela, J., Lively, C. M., Dybdahl, M. F. & Fox, J. A. (2003). Genetic variation in sexual
and clonal lineages of a freshwater snail. Biological Journal of the Linnean Society
79, 165–181.
Krebs, C. J. (1989). Ecological Methodology. New York: Harper Collins.
Lehner, P. N. (1996). Ethological Methods, 2nd edn. Cambridge: Cambridge University
Press.
Lobón-Cerviá, J. & Rincón, P. A. (1994). Trophic ecology of red roach (Rutilus arcasii) in
a seasonal stream; an example of detritivory as a feeding tactic. Freshwater Biology
32, 123–132.
Magalhães, M. F. (1993). Feeding of an Iberian stream cyprinid assemblage: seasonality
of resource use in a highly variable environment. Oecologia 96, 253–260.
Marshall, S. & Elliott, M. (1997). A comparison of univariate and multivariate numerical
and graphical techniques for determining inter- and intraspecific feeding
relationships in estuarine fish. Journal of Fish Biology 51, 526–545.
Martins, M. J., Collares-Pereira, M. J., Cowx, I. G. & Coelho, M. M. (1998). Diploids v.
triploids of Rutilus alburnoides: spatial segregation and morphological differences.
Journal of Fish Biology 52, 817–828.
Mosteller, F. & Rourke, R. E. K. (1993). Estatı´sticas Firmes. Lisboa: Edições Salamandra.
Negovetic, S., Anholt, B. R., Semlitsch, R. D. & Reyer, H.-U. (2001). Specific responses
of sexual and hybridogenetic European waterfrog tadpoles to temperature.
Evolution 82, 766–774.
Persson, L. (1983). Food consumption and the significance of detritus and algae to
intraspecific competition in roach Rutilus rutilus in a shallow eutrophic lake.
Oecologia 41, 118–125.
Puig, M. A., Ferreras-Romero, M. & Rojas, A. M. G. (1986). Ecosistemas de rios
temporales: ecologia de las poblaciones de Efemeropteros de la cuenca del rio
Bembezar (Sierra Morena). Anales de Biologia 8, 65–69.
Ribeiro, F., Cowx, I. G., Tiago, P., Filipe, A. F., Moreira da Costa, L. & Collares-Pereira,
M. J. (2003). Growth and reproductive traits of diploid and triploid forms of the
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467
FORAGING IN A HYBRIDOGENETIC CYPRINID COMPLEX
467
Squalius alburnoides cyprinid complex in a tributary of the Guadiana River,
Portugal. Archiv für Hydrobiologia 156, 471–484.
Rist, L., Semlitsch, R. D., Hotz, H. & Reyer, H. U. (1997). Feeding behaviour, food
consumption, and growth efficiency of hemiclonal and parental tadpoles of the
Rana esculenta complex. Functional Ecology 11, 735–742.
Schenck, R. A. & Vrijenhoek, R. C. (1989). Coexistence among sexual and asexual
Poeciliopsis: foraging behaviour and microhabitat selection. In Evolution and
Ecology of Unisexual Vertebrates (Dawley, R. M. & Bogart, J. P., eds), pp. 39–48.
Albany, NY: New York State Museum.
Schlosser, I. J., Doeringsfeld, M. R., Elder, J. F & Arzayus, L. F. (1998). Niche
relationships of clonal and sexual fish in a heterogeneous landscape. Ecology 79,
953–968.
Sokal, R. R. & Rohlf, F. J. (1981). Biometry. San Francisco, CA: W. H. Freeman &
Company.
Tachet, H., Richaux, P., Bournaud, M. & Usseglio-Polatera, P. (2000). Inverte´bre´s d’eau
douce – Syste´matique, biologie, e´cologie. Paris: CNRS Éditions.
Vrijenhoek, R. C. (1994). Unisexual fish: model systems for studying ecology and
evolution. Annual Review of Ecology and Systematics 25, 71–96.
Weeks, S. C., Gaggiotti, O. E., Schenck, R. A., Spindler, K. P. & Vrijenhoek, R. C.
(1992). Feeding behaviour in sexual and clonal strains of Poeciliopsis. Behavioural
Ecology and Sociobiology 30, 1–6.
#
2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 454–467