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First published online March 2, 2006
Journal of Experimental Biology 209, 1112-1121 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02119
Differential parental nutrient allocation in two congeneric pipefish species (Syngnathidae: Syngnathus spp.)
Department of Biology, West Virginia University, PO Box 6057, Morgantown, WV 26506, USA
* Author for correspondence (e-mail: jripley{at}mix.wvu.edu)
Accepted 23 January 2006
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Key words: brood pouch, nutrient allocation, nurse eggs, sex ratios, Chesapeake bay
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;
Alexander and Borgia, 1979).
Anisogamy, gametes of two different sizes from the male and female, underlies
the evolution of sex differences in morphology and behavior including parental
care (Maynard-Smith, 1978).
Evolutionary theory predicts that in species with female parental care, males
will compete for access to females
(Trivers, 1972). When males
exhibit paternal care, they incur a loss of fitness due to increased
predation, decreased foraging time and territory and limited mating
opportunities relative to males of other species
(Williams, 1966;
Williams, 1975;
Gross and Shine, 1981;
Svennsson, 1998; Vincent et al.,
1992). Caring males are accordingly expected to be more
discriminating in their choice of mates, and females will compete for access
to receptive males. Sex roles are therefore historically defined by mating
competition (Emlen and Oring,
1977). Species exhibiting greater competition by females than
males for mates are said to be sex-role reversed
(Gwynne, 1991). The occurrence
of sex-role reversal offers an opportunity to explore the evolution of sex
differences. For example, an extreme case of sex-role reversal has been
suggested to result in complete sex reversal where anisogamy is inverted as
indicated by a loss of nutritional investment from female gametes
(Pagel, 2003).
Pipefishes and seahorses of the family Syngnathidae are characterized by a
unique mode of ovoviviparous reproduction in which the male carries the
developing embryos in a special organ referred to as the brood pouch
(Lockwood, 1867). The degree
of brood pouch closure varies considerably and has classically been grouped
into three types: ventral gluing, two pouch flaps that meet midline and a
completely sealed sac (Duncker,
1915; Herald,
1959). A female oviposits her eggs directly onto the brooding skin
where they are fertilized. `Internal' fertilization in males assures paternity
and is probably one of the selective pressures behind the evolution of this
structure (Helfman et al.,
1997; Jones and Avise,
1997; Jones et al.,
1998; Jones et al.,
1999). The developing embryos remain in the brood pouch for a
lengthy incubation period before being released as independent young without a
yolk sac.
The brood pouch is believed to protect, aerate, osmotically buffer and
nourish the embryos (for a review, see
Azzarello, 1991;
Carcupino et al., 1997;
Drozdov et al., 1997;
Carcupino et al., 2002). An
investigation on the pipefish Syngnathus acusimilis demonstrated that
the dorsal epithelial lining of the brood pouch is well vascularized and
composed of thick cuboidal cells similar to those known to have a secretory
function (Drozdov et al.,
1997). The syngnathid brood pouch is expected to be an
epithelochordal placenta, because eggs are deposited with a large amount of
yolk and a yolk sac is still visible once the fry hatch in the pouch.
Therefore, the syngnathid brood pouch resembles that of squamate lizards,
conferring inorganic ion exchange
(Stewart, 1992). Several
authors have suggested an ionic exchange function that regulates the
osmolality of the brood pouch fluid to that of paternal blood, facilitating
embryonic development (Linton and Soloff,
1964; Quast and Howe,
1980; Watanabe et al.,
1999). A nourishment function for the brood pouch has been
suggested in Syngnathus scovelli, because smaller eggs are absorbed
by the pouch epithelium and thought to serve as `nurse eggs'
(Ahnesjo, 1996). The pouch has
also been suggested to transfer steroid or growth hormones to the embryos, but
this function has yet to be fully investigated (Haresign and Shumway, 1981;
Azzarello, 1991;
Mayer et al., 1993).
The extent to which the various pouch types perform these physiological
roles is unknown. For instance, in two related pipefish species in which fry
measure 11–13 mm total length (TL) at release, removal of
S. scovelli embryos at least 4 mm TL resulted in normal
development (Azzarello, 1991).
Conversely, S. acusimilis young could not survive outside the brood
pouch until 11–12 mm TL
(Drozdov et al., 1997).
Molecular phylogenic analyses indicate the syngnathid brood pouch underwent
rapid diversification evidenced by repeated evolution of a number of pouch
types (Wilson et al., 2001).
Rapid diversification of placental function in females has been documented in
another teleost system (Morrell,
2002; Resnick et al.,
2002). These studies suggest the function of the paternal brood
pouch is a fairly plastic trait and that syngnathid groups may differ in brood
pouch physiology.
Regardless of the degree of closure, all syngnathid brood pouches were once
assumed to provide developing embryos with protection, osmoregulation and
nutrients (Vincent et al.,
1992; Jones and Avise,
1997). However, sex-role reversal as predicted by extensive male
investment was not consistently reported within this taxa
(Vincent et al., 1992). Recent
ultrastructural comparisons of three syngnathid species representing each of
the brood pouch types refuted the hypothesis of uniform functionality by
suggesting the epithelium has different functions in each type of enclosure.
Specifically, more enclosed pouches were observed to contain greater
anatomical complexity and secretory function
(Carcupino et al., 2002).
Based on this positive correlation between the degree of pouch closure and
male parental care (Berglund et al.,
1986; Masonjones,
2001; Carcupino et al.,
2002), the frequency of sex-role reversal across Syngnathidae was
examined as a measure of female competition. The predicted relationship of
frequent sex-role reversal in taxa with more enclosed brood pouches was not
observed (Carcupino et al.,
2002; Wilson et al.,
2003).
Our comparison of two closely related, sympatric species, the northern
pipefish Syngnathus fuscus and the dusky pipefish Syngnathus
floridae, supports the rejection of brood pouch functional uniformity in
syngnathids, as well as providing a hypothesis for the lack of relationship
between sex-role reversal and pouch structure. We propose that pouch closure
is not indicative of the degree of physiological allocation of nutrients by
brooding males to embryos, but rather, brood pouch physiology varies between
related species with similar brood pouch appearance. Brood enclosure for
S. fuscus and S. floridae is intermediate within syngnathids
with two pouch folds sealing along the midline but not permanently fusing. Our
species comparison of nutrient concentrations in mature, unfertilized eggs and
newly released fry, brood pouch morphology and nutrient levels of fluid from
inside the brood pouch and blood plasma were used to evaluate this hypothesis.
Further, characterization of parental nutrient allocation to offspring
examines the proposition of the progression of some species of syngnathids
toward complete sex reversal (Pagel,
2003).
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).
Initially, all adult males and females were retained for laboratory
dissections, and juveniles were held briefly in aerated tanks onboard the boat
and returned before leaving the site. Once the data set neared completion, the
developmental state of the brood was estimated by examining the embryos
through the translucent pouch flap, and only animals filling gaps in the data
were collected from catches. Pipefish were held for a maximum of 3 days at the
field station in aerated tanks with daily water changes. Adults were
transported in Kordon® breathing bags (Novalek Inc., Hayward, CA, USA)
chilled with ice to the laboratory for tissue collection.
Tissue collection and nutrient analysis
In the laboratory, pipefishes were held in same sex groups of 10–12
fish in filtered 37.8 l tanks maintained at 24±1°C on a 14 h:10 h
light:dark cycle (on 6:00 h: off 20:00 h). Pipefish were given a recovery
period of at least 16 h in the laboratory with food withheld for a minimum of
36 h during travel and acclimation. Fish were not fed prior to tissue
collection to avoid influences on nutrient levels from food intake, or the
lack thereof. All tissues were collected within 2 days oflaboratory arrival.
We anesthetized pipefish with 3-aminobenzoic acid ethyl ester (MS222; Sigma
Aldrich, St Louis, MO, USA) in saltwater until opercle movement ceased and the
fish failed to respond to pinching the caudal peduncle. Pouch fluid from
brooding and non-brooding males was collected with microcapillary tubes
(Drummond Scientific Co., Broomall, PA, USA) by inserting the tube along the
junction of the pouch flaps at the anterior of the brood pouch by the anus.
The capillary tube could then be used to separate the pouch flaps and collect
fluid from all areas of the brood pouch. In non-brooding males the pouch flaps
are not fused and saltwater could mix with any secretions produced by the
male. Pouch fluid was collected from these animals to determine how different
this fluid is from the surrounding saltwater. Pouch fluid was immediately
stored at –80°C until analysis. Blood was collected from the heart
of brooding and non-brooding males and females with heparinized microcapillary
tubes. Blood samples were transferred to microcentrifuge tubes containing 2
µl of a 6.5 mg ml–1 solution of sodium heparin salt in
water, and centrifuged at 21 000 g for 10 min at 4°C. The
plasma fraction was removed and stored in a microcentrifuge tube containing
phenylmethylsulfonyl fluoride (PMSF, 99% purity; Sigma Aldrich) at
–80°C until analysis. Brooding embryos and eggs were collected
directly from the brood pouch or ovary, respectively, with fine point forceps.
Developing embryos were anesthetized with MS222 in saltwater. Before fry
release, brooding males were isolated so that newly released fry could be
collected with nets. They were then anesthetized in MS222 in saltwater. For
each brooding male in the study, 20 embryos or newly released fry (within 12
h) were pooled and homogenized with 40 µl of homogenization buffer (0.1 mol
l–1 Tris base, 1 mmol l–1 EDTA) for 60 s in
the case of yolk sac embryos or 120 s for late stage fry. Females with
immature follicles that could not be removed intact from the ovary were termed
immature females to distinguish them from gravid females with mature eggs that
could be separated. From gravid females, 20 eggs were removed from the
posterior section of the ovary. The pipefish ovary has been classified as an
asynchronous type with follicles in all stages of development
(Wallace and Selman, 1981;
Begovac and Wallace, 1987). By
collecting those closest to the ovipositor, the sample contained mature eggs
ready for fertilization. Eggs were homogenized in buffer for 60 s and the
homogenate centrifuged at 21 000 g for 30 min at 4°C. The
supernatant was removed and stored at –80°C until analysis.
An additional sample of five eggs, embryos or fry from each fish were
viewed with an Olympus SZ40 dissecting microscope (Melville, NY, USA) and
photographed with a Spot Insight color camera (Diagnostic Instruments Inc.,
Sterling Heights, MI, USA; model 3.2.0). These samples were used to determine
the stage of development of fertilized embryos. Mature, unfertilized eggs were
considered stage 0. Embryonic development was divided into seven stages based
on the presence of the embryo stripe (state 1), development of eye spots and
cups (state 2), detachment of the tail from the yolk sac (i.e. hatched; state
3), heart development (state 4), presence of fins (state 5), development of
striped pigmentation (state 6) and complete absorption of the yolk sac (state
7). Newly released fry were classified as state 8. Both S. fuscus and
S. floridae embryos fitted these developmental stages with the
distinction of S. floridae fry exhibiting longer snout lengths in
states 6–8. In this study, these embryonic development classifications
were considered distinct stages in place of specific time periods from the
date of fertilization because observations indicated factors such as the
number of embryos in the pouch, water temperature and male size influenced
brooding periods (Berglund et al.,
1989; Ahnesjo,
1992). Individuals of both species at every stage were included in
our analysis. Egg diameter and standard length of newly released fry were
measured with the software Image Pro Plus (Media Cybernetics, Silver Spring,
MD, USA). We averaged the five measures for a single data point for each
individual. Any excess eggs, embryos or fry collected were frozen or fixed in
formalin for analysis in concurrent studies.
Nutrients, specifically proteins, lipids and carbohydrates, were measured
in tissue samples as follows. Protein content was measured from dilutions of
blood plasma (1:99), pouch fluid (1:99) and the supernatants from homogenized
eggs (1:199) and fry (1:99). Total protein concentrations were determined
using a Bio-Rad protein dye reagent and a standard curve of bovine serum
albumin. The samples were transferred to a 96-well plate and read at 490 nm
using a Tecan GENios (Durham, NC, USA). Lipid content was determined by
acidifying methanol/chloroform extracts, adding a vanillin reagent and
comparing the sample to known amounts of soybean oil
(Wheeler and Buck, 1992;
Lotufo et al., 2000). Samples
were analyzed at 595 nm.Carbohydrates, diluted 1:3 for all samples, were
quantified at 490 nm using an anthrone assay and sucrose standards
(Van Handel, 1985;
Wheeler and Buck, 1992). For
each nutrient analysis, the sample was measured in triplicate and averaged for
a single data point. Normality and homogeneity of data sets were assessed
prior to the use of parametric statistics. All statistical analyses were
performed with JMP 5.1. Variability measures calculated from these data can be
used in power analyses to determine required sample sizes for future
experiments.
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2=147.717, P<0.001;
Fig. 1) but not for S.
floridae (2=0.949, P=0.917;
Fig. 1).
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Morphological observations of developing embryos held within the brood
pouch of S. fuscus and S. floridae show a previously
undescribed close association between fry and the pouch lining. Unlike most
fish species in which the outer membrane hardens following fertilization to
protect the embryo from water loss and environmental adversities, these
pipefish embryos lack a rigid chorion
(Jobling, 1995). In S.
fuscus, the two flaps of the male brood pouch seal independently to the
ventral body surface forming two chambers
(Fig. 3A). Embryos are embedded
under a clear membrane on the vascularized flaps of the brood pouch until
release (Fig. 3A,B). Although
most embryos develop at the same rate, one row on either side of the midline
seal frequently remains underdeveloped relative to the rest of the brood.
Limited vascularization and epithelial tissue in this region are likely to
decrease the connectivity between embryos and the brood pouch. For S.
floridae, the two flaps forming the pouch seal at the midline resulting
in a single large brood chamber (Fig.
3C). One side of the fertilized egg becomes strongly attached to
the ventral flap of the pouch (Fig.
3C,D). As the yolk sac is absorbed, this connection dissolves and
embryos are contained within the pouch. Undeveloped eggs and lipid droplets
are found interspersed with embryos all at the same stage of development
(Fig. 3D). When fry are
released, a clear matrix similar in appearance to a honeycomb and
approximately the size of the pouch is also released. Overall our observations
reveal differences between S. floridae and S. fuscus in the
arrangement of undeveloped eggs and the connectivity of embryos to paternal
tissue.
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Nutrients potentially available for uptake by developing embryos were measured by determining protein, lipid and carbohydrate content of fluid collected from the pouch of brooding (N=21 per species) and non-brooding males (N=15 per species). When the two species are grouped together, brooding males with embryos in various developmental stages harbour pouch fluid rich in proteins (Table 2, Fig. 4A), lipids (Table 2, Fig. 4B) and carbohydrates (Table 2, Fig. 4C) relative to non-brooding males. Nutrient concentrations in the pouch fluid do not differ between males of the two species. However, in S. floridae, protein concentrations of the pouch fluid begin high for newly fertilized broods and decline rapidly over development (Table 3, Fig. 4A). A significantly more gradual decline characterizes the depletion of lipid from S. floridae pouch fluid (Table 3, Fig. 4B). Whereas carbohydrate content of pouch fluid for S. floridae and S. fuscus significantly decreased over embryonic development, species differences in the rate of decline were not observed (Table 3, Fig. 4C). To examine control of pouch fluid content in non-brooding males, nutrient levels were compared to saltwater containing MS222. Pouch fluid from non-brooding males of both species contained significantly higher protein, lipid and carbohydrate concentrations than saltwater with MS222 (Table 2).
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Blood plasma concentrations of total protein, lipids and carbohydrates are similar between brooding males and gravid females in both species. Considering species and reproductive state, significant differences in protein content were not detected (2-way ANOVA, F=1.9132, P=0.0983; Table 4). Furthermore, when plasma protein concentration was examined over embryonic development in brooding males, trends did not emerge in either species (S. fuscus, R2=0.1017, P=0.1480, slope=–1.5663; S. floridae, R2=0.0868, P=0.1529, slope=–1.7106). There was a significant difference in lipid content of blood plasma, with gravid females circulating lower lipid levels than non-brooding males (2-way ANOVA, F=3.2819, P=0.0087; post-hoc Tukey HSD, Q=2.3791, P<0.05; Table 4). In addition, S. floridae overall contained higher concentrations of plasma lipids than S. fuscus (post-hoc Student's t-test, t=1.9840, P<0.05). Significant changes in plasma lipid content over the brooding period were not evident (S. fuscus, R2=0.0493, P=0.2970, slope=–0.0701; S. floridae, R2=0.0475, P=0.3300, slope=–0.0710). Examination of plasma carbohydrate concentrations revealed brooding S. floridae males had higher levels compared with brooding S. fuscus (two-way ANOVA, F=3.1526, P=0.0110; post-hoc Tukey HSD, Q=2.9057, P<0.05; Fig. 5, Table 4). Again, examining changes over embryonic development yielded no trends in plasma carbohydrate levels (S. fuscus, R2=0.0188, P=0.5324, slope=–1.2927; S. floridae, R2=0.0108, P=0.6533, slope=0.7158).
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). Indeed, lower concentrations of proteins, lipids and
carbohydrates were measured in fry than in eggs of both species. Still,
significant species differences in the nutrient reserves of eggs indicate that
S. floridae embryos are more adequately supplied by the egg. S.
fuscus females produce eggs with considerably less nutrients than S.
floridae, which must be balanced by elevated paternal provisioning. These
observations of S. fuscus follow predictions of evolutionary theory
and the progression to complete sex reversal
(Pagel, 2003).
The paternal brood pouch serves as a source of nutritional supplementation
during development for both species, by bathing embryos in a fluid rich in
macronutrients. Since the chorion is absent, nutrients and hormones may be
readily absorbed. Differences between seawater and pouch fluid indicate
concentrations of proteins, lipids and carbohydrates in the pouch can be
regulated physiologically. Pouch fluid nutrients are nearly depleted over the
brooding period. These decreases may reflect changes in the male's
contribution, providing less to fry as they approach release, or a change in
utilization of the pouch fluid by the embryos, increasing intake in relation
to reduced yolk reserves. In placental sharks, embryos are initially reliant
on yolk reserves sequestered in the egg until maternal supplementation is
activated (Hamlett et al.,
1987; Hamlett et al.,
1993). A comparable series of events probably transpires in
pipefish. Our study shows that species differ in the rate of decline of pouch
fluid nutrients. The presence of yolk droplets floating in the pouch of S.
floridae suggests utilization of `nurse eggs', or release from fertilized
embryos, provides a nutrient source in pouch fluid. Conversely, paternal
contribution is more pronounced in S. fuscus with males encompassing
relatively nutrient-poor eggs in epithelia supplied by brood pouch
vasculature. The steep nutrient drop in the pouch fluid over embryonic
development corresponds with differential use of protein from nurse eggs in
S. floridae and lipids provided by the brooding male in S.
fuscus. A level supply of protein across S. fuscus development
is consistent with males continually secreting nutrients into the brood pouch.
However the steeper drop in lipids may reflect dependence of S.
fuscus embryos on brood pouch secretions for metabolic fuel. For both
species, fry are released once paternal resources are exhausted.
Undeveloped nurse eggs in the S. floridae brood pouch are believed
to have originated from overripe oocytes
(Teixeira and Vieira, 1995).
Other syngnathid species frequently possess undeveloped eggs as well. In
Syngnathus typhle, these eggs and less developed embryos attach to
the placenta-like structure in the pouch
(Ahnesjo, 1992). The presence
of lipid cells in the pouch epithelium of the seahorse Hippocampus
brevirostris suggests egg yolk can be re-absorbed
(Rauther, 1925). From these
observations, the syngnathid pouch fluid is believed to be at least partially
derived from nurse eggs. Specifically, the protein hormone, prolactin, is
released into the pouch causing enzymatic breakdown of the egg to form a
placental fluid (Boisseau,
1967; Ahnesjo,
1996). In pipefish, the presence of undeveloped eggs in the brood
pouch is more likely to indicate that a nurse egg system is being employed
rather than eggs simply not being fertilized. Internal fertilization in these
species involves sperm released into the pouch, and therefore, the rate of
fertilization is high (Fielder, 1954). Employing a nurse egg system in S.
floridae and thus decreasing paternal nutrient allocation may translate
into a more equal sex ratio, as well as higher circulating nutrient
concentrations for brooding S. floridae males over S.
fuscus.
The matrix construct observed with fry release in S. floridae may
be a structure produced by the male during brooding to support developing
embryos. In two Stigmatopora pipefish, males incur an additional
energetic cost in making membranous egg-holding compartments within the brood
pouch. These specialized structures are not present in the non-brooding S.
floridae males or Stigmatopora, implying this structure must be
rebuilt every time a male broods (Berglund
et al., 1986; Steffe et al.,
1989). The functional significance of this matrix has yet to be
explored.
A different embryonic supplementation mechanism revealed by our study
involves a paternally derived, nutrient-rich pouch fluid. S. fuscus
brood pouch flaps have epithelial coverings and extensive vascularization that
are only visible when males are brooding. Histological examinations of several
syngnathids show similar morphological changes in the pouch wall concurrent
with brooding (Lockwood, 1867;
Huot, 1902;
Steffe et al., 1989;
Carcupino et al., 1997;
Drozdov et al., 1997;
Carcupino et al., 2002). Most
commonly noted is the development and growth of capillaries in the epithelium
of the pouch walls (Gill,
1905; Thevenin,
1936; Carcupino et al.,
1997; Drozdov et al.,
1997). Carcupino et al.
(Carcupino et al., 2002)
hypothesized that nutrients may be transferred from the paternal blood to the
pouch by transcytosis or may be synthesized or modified in the epithelial
cells. In S. abaster, large intercellular spaces form at the bases of
epithelial cells functioning as a freeway to facilitate the passage of
molecules from capillaries to the lumen of the pouch
(Carcupino et al., 1997).
Accordingly, we believe profusion of blood vessels in the pouch flaps and
implantation of embryos adjacent to these vessels is evidence for paternal
provisioning in S. fuscus.
Blood plasma nutrient levels provide further evidence of greater paternal
investment in S. fuscus. We document equivalent plasma nutrient
levels in brooding males and conspecific gravid females, suggesting brooding
males incur a cost for parental care in both species. A study on threespine
sticklebacks, measuring lipid, glycogen and protein, found the concentrations
of all these substances peaked at the beginning of the breeding season. At the
end of the season, males that brooded had lower energy reserves and higher
mortality rates than males that did not breed
(Chellappa et al., 1989).
Thus, low nutrient levels in spawning fish indicate allocation to gamete
production and/or parental care. Our data reveals circulating carbohydrate
concentrations are lower in brooding males of S. fuscus than S.
floridae. Because the pipefish diet is nearly exclusively composed of
proteins and lipids, carbohydrate levels are a measure of the availability of
metabolic fuel (Ryer and Boehlert,
1983; Huh, 1986;
Jobling, 1995). Lower
concentrations in S. fuscus are indicative of the higher metabolic
cost of brooding.
Our data concerning egg and fry size support the hypothesized paternal
provisioning mechanisms suggested by the adult nutrient analyses. If changes
in weight from egg to fry state are considered for species with various modes
of development, viviparous organisms commonly loose up to 50% in weight,
oviparous 20–30% and matrotrophic ovipararous gain 11–369%
(Needham, 1942;
Amoroso, 1960;
Hamlett et al., 1993). Rather
than measure weight changes, we examined drops in nutrient levels. The
difference in nutrient stores between mature, unfertilized eggs and released
fry was always greater for S. floridae. When compared to S.
fuscus, the nutrient declines in S. floridae were greater in
magnitude by 2.4 for protein, 9.6 for lipids and 0.9 for carbohydrates.
Considering the brood period for both species overlap (personal observation)
(Bigelow and Schroeder, 1953),
S. fuscus would necessarily acquire more nutrients from the pouch
fluid.
Brooding in syngnathids has been described as carrying a cost of parental
care which exceeds that of most vertebrates
(Breder and Rosen, 1966;
Clutton-Brock and Vincent,
1991). Within this family, it has been stated that species with
less complex brood pouches spend less energy brooding young than do males with
more enclosed pouches (Berglund et al.,
1986; Masonjones,
2001; Carcupino et al.,
2002). Although the physiological cost of parental care is not
equivalent to parental investment in offspring, energy expenditures may be
positively correlated with parental care in many cases
(Wilson et al., 2003). Higher
costs of parental care are likely to be reflected in a lower frequency of
potential mates, and therefore a skewed operational sex ratio in breeding
populations (Trivers, 1975;
Clutton-Brock and Vincent,
1991; Gwynne,
1991). Defined as the ratio of fertilizable females to sexually
active males, the operational sex ratio is dependent upon several factors in
addition to differences in parental care, including spatial and temporal
clumping of the limited sex and life history differences between the sexes
(Emlen, 1976;
Emlen and Oring, 1977;
Berglund and Rosenqvist, 1993;
Andersson, 1994). Hence, if
other factors are constant or very similar, paternal nutrient provisioning and
the cost of reproduction for male syngnathids should be reflected in sex
ratios. Breeding populations of the two species in this study overlap
spatially and temporally. Although we cannot completely rule out other
factors, a significant difference in the proportion of wild-caught adult males
between S. fuscus and S. floridae breeding populations
suggests higher relative paternal energy expenditure in S. fuscus.
The costs of parental care and parental investment to brooding offspring in
these species need to be tested through an examination of lifetime
reproductive success.
If, as our data suggest, S. fuscus males contribute a greater
proportion of parental care than S. floridae males, we predict a
corresponding divergence in sex roles, mating competition and the evolution of
secondary sexual characteristics between these species. Some of these
predictions are supported by accounts in the literature. S. fuscus
females are reported to roam a larger area and to develop dimorphic banding
coloration to find and attract mates
(Roelke and Sogard, 1993;
Berglund et al., 1997;
Bernet et al., 1998). More
balanced investment in progeny between the sexes in S. floridae
predicts comparable reproductive rates for males and females. The sexes would
not be expected to evolve strongly dimorphic behaviors and traits in this
species, and descriptions of S. floridae report no obvious secondary
sexual characters (Jones and Avise,
2001). Regardless of these accounts, the behavioural ecology of
these species needs to be studied to support the above hypothesis.
Our results highlight the importance of basic reproductive physiology in
understanding the functional significance of the brood pouch. The location and
enclosure of the male brood pouch defines primary taxonomic groupings (Dunker,
1915; Herald, 1959). In
general, syngnathid phylogeny is largely based on the three paternal brood
pouch types (Herald, 1959;
Wilson et al., 2001). Our
comparison reveals physiology differs considerably within a pouch type.
Perhaps both phylogeny and taxonomy should be reinvestigated with more
detailed reproductive physiological data. Syngnathids offer a plethora of
opportunities to explore the evolution of placental-like structures and their
role in development (Milius,
2000). Connectivity of embryos to the brood pouch of S.
floridae and S. fuscus differs considerably, and the
physiological mechanism for this distinction may provide a greater
understanding of selective pressures for parental investment. The comparison
of the eggs from these two species provides evidence of female restriction in
gametic provisioning. Our results imply that female allocation to offspring in
oviviparous animals can be altered with relative selective pressure. Since
females are defined through the production of large, nutrient-laden gametes, a
drop in egg nutrient content represents the logical progression from a
sex-role reversed to a fully sex-reversed species
(Pagel, 2003). The comparison
of S. fuscus and S. floridae offers unique insight into the
evolution of mechanisms of parental care, and provides an interesting
mechanism to explore the distinction between the sexes based on gamete
production.
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