No assortative mating by diet, but male
alternative reproductive tactics in the invasive fish, round goby (Neogobius melanostomus).
Magnús Thorlacius
Degree project inbiology, Master ofscience (2years), 2011 Examensarbete ibiologi 45 hp tillmasterexamen, 2011
Biology Education Centre and Limnology department, Uppsala University Supervisors: Phillipp Hirsch and Richard Svanbäck
External opponent: Pia Bartels and Maria Cortazar Chinarro
1
Index
Abstract ...2
Introduction ...3
Genetic diversity and assortative mating ...3
Alternative reproductive tactics ...4
Materials and methods ...5
Sampling and study site ...5
Photography ...5
Stable isotope analysis ...5
Fish measuring ...6
Statistical analysis ...7
Results ...7
Stable isotope analysis ...7
Alternative Male Reproductive Tactics ...8
Discussion... 12
Acknowledgements ... 15
References ... 16
Appendix ... 19
Diet ... 19
2
Abstract
The colonization of invasive species in new habitats poses a serious threat to native biota and is commonly considered to be a key to the loss of biodiversity. The round gobies are an invasive littoral fish species in the Baltic Sea originated from the Ponto-Caspian area. They were first discovered in 1990 in the Gulf of Gdansk, but in 1995 they had spread throughout the Gulf and already in 1999 they were the dominant littoral fish species in the region.
Great genetic diversity has been found in the round goby populations of the Gulf of Gdansk, which is unusual for newly established populations. We investigated whether this could be caused by assortative mating by diet, i.e. genetic diversity is maintained by individuals selectively mating with individuals that share the same diet. Using stable isotope analysis, one can compare the diet composition of individuals and infere a females diet from the stable isotopic signature of her eggs. Males were sampled and eggs that they were found guarding. By comparing the isotopic signature of the males with that of their eggs we could establish whether the females shared the same diet as the males.
Male alternative reproductive tactics have been found in round goby populations in the Laurentian Great Lakes. In our study round gobies were sampled in their nests, but that makes it particularly convenient to study male alternative reproductive tactics. This we did by measuring their body size, head shape, removing and weighing their gonads and removing their opercular bones for measures of age and growth. Finally we photographed each individual and measured their pigmentation from the photos. Most studies of alternative reproductive tactics are conducted in laboratories under controlled conditions due to the difficulty of studying them in the field.
We did not find a correlation between the males isotopic signature and that of their eggs and therfore conclude that the genetic diversity can not be explained by assortative mating by diet. The likely explanation is stratified dispersal.
We found, however, substantial evidence for the presence of male alternative reproductive tactics in the form of two different male morphs. First there is the parental male, that grows more in his first year, has a darker pigmentation, a larger head and smaller gonads in proportion to body weight. Secondly, there is the sneaker male, that invests more in testes size than body size and hence, grows less in his first year and has larger gonads in proportion to body size. He also has a smaller head and a lighter, more cryptic, colouration much like the females.
We were able to correlate the male traits with the growth in their first year which indicates that their tactic is predetermined. Whether this indicates sexual selection is unclear but female round gobies have been found to selectively mate with males displaying sexual ornaments and parental care. It has also been suggested that the swollen heads and dark colouration has a purpose in male intraspecific competition, because parental males are known to defend nests and guard eggs layed by multiple females. The sneaker males however avoid the cost of courtship and nest guarding by sneak fertilizations.
We conclude that the round gobies in the Gulf of Gdansk mate randomly where they are and that there are predetermined male alternative reproductive tactics in these populations.
3
Introduction
Invasion of non-indigenous species to new habitats are commonly considered to be a key to the loss of biodiversity (Vander Zanden and Rassmussen, 1999).
The round gobies are an invasive species that has spread from its native Ponto-Caspian habitat to the Baltic region and to the Laurentian Great Lakes of North America, most likely by ships ballast water (Almqvist, 2008; Sapota and Skora, 2005). They are small aggressive fish that inhabit rocky substrates where they reproduce many times per year during the warmer half of the year (Wicket and Corkum, 1998). They were first caught in the Gulf of Gdansk, Poland in 1990, more specifically in the vicinity of Hel (Sapota, 2004; Sapota and Skora, 2005), where an expansion in population size was noticed in 1993. Already in 1994 they had colonized Puck lagoon (Sapota, 2004; Sapota and Skora, 2005). In 1995 they were caught outside of the Gulf of Gdansk, although near its borders (Kuczynski, 1995 in Sapota and Skora, 2005), indicating that they are spreading rapidly. Furthermore, in 1999 they were absolutely dominant in the port region of Hel, only nine years after first being discovered (Sapota, 2004; Sapota and Skora, 2005).
Round gobies pose a serious threat to the native biota of the invaded areas by eating the eggs of native fish species (Steinhart et al., 2004) and by competing with native species for food resources (Dubs and Corkum, 1996). Most species that colonize a new habitat successfully have short generation times, high fecundity and a high growth rate (Lodge 1993). Round gobies fit that description perfectly with a life span of three to four years (Sapota, 2004), nests within centimetres of one another in best conditions (Wickett and Corkum, 1998), each female spawns two to four times per year (Sapota, 2004) and they are also known to have great environmental tolerance (Charlebois et al., 2001). They tolerate high variation in salinity and temperature, which is also common for invasive species (Almqvist, 2008). As an example, even though their preferred habitat is a rocky substrate, they have been found in open, flat bottoms in the Gulf of Gdansk (Sapota and Skora, 2005). Furthermore, they are known to be opportunistic when it comes to diet composition, feeding on whatever prey is most abundant (Skora and Rzeznik, 2001). Their distribution in the Baltic Sea matches the route of freighters indicating that they are spread by ballast water (Almqvist, 2008). With their benthic morphology and normally small home range (Sapota, 2004), round gobies are not expected to have good abilities to disperse naturally (Hayden and Miner, 2009), especially upstream (Ray and Corkum, 2001), further supporting the ballast water theory.
Genetic diversity and assortative mating
Björklund and Almqvist (2009) found great genetic diversity in the round goby populations in the Gulf of Gdansk, even between sites very close to each other. They suggested that factors other than distance likely contributed to the genetic differences they found. One of the factors that can cause genetic differentiation, is assortative mating, which occurs when individuals selectively mate with individuals that are behaviourally or morphologically like themselves (Snowberg and Bolnick, 2008). In cases of polymorphism, this may in time lead to sympatric
4 speciation (Maynard Smith, 1966). It has been discovered that some fish species selectively mate with conspecifics that share the same diet (Snowberg and Bolnick, 2008).
Alternative reproductive tactics
Widespread in the animal kingdom, we find alternative reproductive tactics, which refer to behavioural and/or physiological traits that maximizes the individual fitness in two or more different ways when it comes to within species intrasexual reproductive competition (Taborsky et al., 2008). This is known to take place where there is much competition for mating opportunities, especially common in cases of external fertilization and male egg guarding, due to the possibility for cuckoldry. Since studying alternative reproductive tactics in the field is very difficult, many studies are carried out in a laboratory under controlled conditions (Stammler and Corkum, 2005; Marentette and Corkum, 2008; Wickett and Corkum, 1998). It is especially difficult to investigate courtship behaviour in the nests themselves, which we managed to do.
Hence, it was decided to look at alternative male reproductive tactics.
For many reasons, the Gulf of Gdansk is a particularly suitable study system when it comes to investigating male reproductive tactics. The newly established population was detected early in its colonization and has since then been the focus of numerous studies (Björklund and Almqvist, 2009; Corkum et al., 2004; Sapota, 2004; Sapota and Rzeznik, 2001; Sapota and Skóra, 2005). The population has a male biased sex ratio (70-80% males)(Skora and Rzeznik, 2001) and where conditions are most suitable, it has become extremely dense (Corkum et al., 2004), which combined with male‘s competing for nesting sites and defending their eggs, likely increases male intraspecific competition. Additionally, since nest density is limited by the availability of holes and every kind of hard substrate for attaching eggs, males make nests on every hard substrate there is (Wickett and Corkum, 1998), making them especially easy to locate and collect.
Previous studies have revealed that there are two different male morphs (Laframboise et al., 2011; Marentette et al., 2009). First, there is the parental male that is a larger, darker and more territorial morph that defends its nest and displays secondary sexual traits such as an enlarged head, the previously mentioned dark (or black) colouration (Marentette and Corkum, 2008) and the release of Pheromones (11-ketotestosterone) to attract females (Laframboise et.al., 2011; Marentette et.al., 2009). Secondly, there is the sneaker which is a smaller, lighter coloured morph that has a more female like appearance and invests more in testes size than in body size resulting in much larger ejaculate volumes in proportion to body size (Marentette et. al., 2009).
The females selectively choose to mate with the parental type due to his dark/black colouration (Yavno and Corkum, 2010; Marentette et al., 2009). The parental males construct nests in holes and cracks or on basically everything that is more solid than sand at depth of up to eleven meters.
In these nests many females will lay eggs that the males then guard against intruders (Sapota, 2004; Wicket and Corkum, 1998). Additionally, they have been observed guarding newly hatched larvae in Lake Michigan (Hensler and Jude, 2007).
5 The main focus of this study was to investigate whether assortative mating by diet is the cause of the genetic diversity found by Björklund and Almqvist (2009) among round goby populations in the Gulf of Gdansk. Secondly, we looked for evidence for the existence of alternative male reproductive tactics in these round goby populations.
Materials and methods
Sampling and study site
Sampling was carried out by Philipp Hirsch and associates in two different locations, all within the Bay of Gdansk in Poland in the spring of 2009 (Philipp Hirsch, personal communication).
More specifically, it was carried out in the vicinity of Kuznica and Puck. Particular effort was put into two of the sites for comparison of male and female diet, since Björklund and Almqvist (2009) found genetic differences between the same populations. In each site fish, mainly females were caught in nets that were left over night and the males located and collected where they were guarding their nests. For each nest, the males and females in or by the nest were labelled, the nest photographed with a coin next to it, for size measures, and samples of eggs eventually collected.
Additionally, suspected prey items were collected in both sites using an Ekman grabber (Philipp Hirsch, personal communication).
Photography
Each individual was pinned down and photographed next to a gray-scale for analysis of colouration. The luminosity of each male was measured using Photoshop Elements 9. A gray- scale in each photo was used for calibration.
Stable isotope analysis
As a measure of assortative mating by diet, we initially established whether the female’s diet could be inferred from their eggs. Having confirmed that, we compared the diet of the males that were caught guarding nests filled with eggs, with the diet of the females, inferred from the eggs, as Snowberg and Bolnick (2008) did in their study of assortative mating by diet in sticklebacks.
Should the male’s diet correlate significantly with the female’s diet, we will have positive assortative mating by diet.
To account for difference in diet we used stable isotope analysis, more specifically 13C and 15N that can be used as tracers for dietary analysis (Minagawa and Wada, 1984).
Additionally, it is possible to identify whether an animal’s diet is pelagic or littoral using carbon (13C) ratios, and trophic position using nitrogen (15N) ratios (Minagawa and Wada, 1984; France, 1995; Fry, 1988). France (1995) compared the δ13C distribution for pelagic and littoral consumers which revealed very little overlap. As previously mentioned the isotopic signature of the females correlated with that of their eggs, which means that we can compare the signature of the males with that of their eggs to establish whether the females that laid the eggs share the same diet as the males.
6 To prepare samples for stable isotope analysis, muscle tissue samples from the fishes and 5 eggs from each nest were taken and oven dried for 48 hours at 50°C. For the suspected prey items, amphipods, isopods, polychaetas and shrimps were dried whole, while the foot from snails and muscle from bivalves were dried. Later, all of the dried samples were ground using a mortar and pestle, and 1 ± 0,2 mg of the fine powder packed into tin capsules (6 x 4 mm), which were sent to UC Davis Stable Isotope Facility in California, USA for δ13C and δ15N analyses.
To determine the fish diet from the suspected prey items we used Isosource (Phillips and Gregg, 2003), which uses mixing models and yields output files that include all feasible combinations of the source with histograms on each sources (suspected prey items) distribution (Phillips and Gregg, 2003), see appendix. Since we used primary consumers instead of primary producers we added a fractionation to the mean nitrogen values of 3.4 as an increase of one trophic level (Minagawa and Wada, 1984; Vander Zanden et al., 1997).
Pelagic contribution to fish diet was calculated by using the equation:
where δ13CRG is the mean δ13C of the round gobies, δ13CPP is the δ13C of the pelagic prey and δ13CLP is the δ13C of the littoral prey, as a reversed edition of Vander Zanden and Vadeboncoeur (2002). In our case the value for the pelagic prey is the mean carbon ratio for all bivalves sampled in each site, and the littoral prey is the mean carbon ratio for the snails.
Organisms at the base of aquatic food webs have very variable δ15N values that tend to differ dramatically between locations/habitats and hence, so called baseline δ15N values are needed to account for fishes’ trophic position (Vander Zanden and Rasmussen, 1999). To account for this variation we used the mean δ15N of all primary consumers in each site as a baseline and estimated the trophic position following Anderson and Cabana (2007):
where δ15NRG is the δ15N value of round goby, δ15NBaseline is the previously mentioned baseline value, 2 is the estimated trophic position of the prey items used to estimate the baseline and 3.4 is the fractionation factor that represents an increase of one trophic level (Minagawa and Wada, 1984; Vander Zanden et al., 1997).
Fish measuring
Before the stable isotope samples were taken each fish was measured and weighed. Later, each individual’s body length, head shape (height, width and length from operculum to the tip of the mouth) and weight of gonads were measured. The operculum bone was removed, boiled in water and cleaned for age and growth measurements. Age of each individual was measured by
7 counting the winter bands on the opercula bone and the size of each individual then calculated, for the first and second year of their lives, using the equation:
where LOi is the length from the tip of the opercula to the i-th winter band, LOT is the total length of the opercula, Lf is the length of the fish and Y is the y-intercept of the equation for the linear regression between LOT and LF (which was calculated separately for each site). Size ate age one was then subtracted from the size at age two to acquire the growth in their first year.
Statistical analysis
Pearson’s correlations were used to compare the male’s and their eggs pelagic contribution and tropic position within each site separately and for all sites together. Before applying the data on fish size and shape, gonadosomatic index (weight of gonads/body weight*100) was calculated and a principle component derived from the length, width and height of the fishes head (independent of body size, explained 96,76 % of total variation in raw data with an eigenvalue of 2,9). Since only the isotope correlations for all of the sites together turned out to be significant, the data for all sites put together was used for comparisons with the other variables. In the interest of identifying whether two different male morphs (parental and sneakers) are found in this population, the male’s gonadosomatic index, pigmentation, PC of head shape and growth in their first year were correlated with each other. As they turned out to be distinguishable, the male’s pigmentation and the principal component of their head shape were compared with their pelagic contribution and trophic level, to identify a difference in feeding (benthic or pelagic) and tropic position, between the morphs. The statistical analyses were carried out in the program R and Statistica.
Results
Stable isotope analysis
Males guarding eggs were found in the sites near Kuznica and Puck, and in some cases females that were in the nest with them. Additionally some females that we caught outside the nests had fully developed eggs that could be analysed.
Correlating females pelagic contribution and trophic position with that of their eggs revealed a significant correlation for Kuznica separately and for both sites together (Table 1, Figure 1). In Puck however there was no significant correlation (Table 1, Figure 1). When comparing the males pelagic contribution and trophic position with that of their eggs we found a highly significant correlation for both sites together, but not for each site separately (Table 1, Figure 2).
8
Table 1 The results from a Pearson's correlation between the fish and their eggs pelagic contribution and trophic position, within separate sites and for all sites put together. The females and males were correlated separately.
Pelagic contribution Trophic level
r p r p
Females Kuznica 0,711 0,003 0,549 0,0339
Puck -0,841 0,0744 0,778 0,122
All sites 0,767 < 0,001 0,503 0,0236
Males Kuznica 0,065 0,656 0,233 0,107
Puck 0,237 0,113 0,12 0,427
All sites 0,629 < 0,001 0,388 < 0,001
Alternative Male Reproductive Tactics
We found that males that grew more in their first year have darker pigmentation (r=0,194, p=0,0389, Figure 3a), a larger head (r=-0,631, p<0,001, Figure 3b), and smaller gonads in proportion to body weight (r=-0,332, p<0,001, Figure 3c). It also turned out that males with larger heads have a darker pigmentation (r=-0,555, p<0,001, Figure 3d), males with darker pigmentation have proportionally smaller gonads (r=-0,417, p<0,001, Figure 3e), and males with larger heads have proportionally smaller gonads (r=0,589, p<0,001, Figure 3f).
When comparing the males sexual traits with their pelagic contribution and trophic position we found, marginally non-significantly, that males with larger heads have a more pelagic diet (r=-1884, p=0,0520, Figure 4a). Males with larger heads appear to have a higher trophic position but the correlation was not significant (r=-0,1672, p=0,06909, Figure 4b). We did not find a correlation between colouration and pelagic contribution (r=0,04758, p=0,6265, Figure 4c) but darker males turned out to have a higher trophic position (r=0,1980, p=0,0309, Figure 4d). We did not find a correlation between gonad size and pelagic contribution (r=- 0,06950, p=0,5057) or trophic position (r=0,06787, p=0,5067) and the same goes for growth in their first year (pelagic contribution: r=0,1351, p=0,1758 and trophic position: r=0,05404, p=0,5732).
9
Figure 1 Female plotted against egg pelagic contribution (left) and tropic position (right), all showing a solid linear regression line.
40 60 80 100 120
6080100120140160
(a) Kuznica
Female pelagic contribution (%)
Egg pelagic contribution (%)
2.8 2.9 3.0 3.1 3.2 3.3 3.4
2.62.83.03.2
(b) Kuznica
Female trophic position (delta15N)
Egg trophic position (delta15N)
0 10 20 30 40
859095100
(c) Puck
Female pelagic contribution (%)
Egg pelagic contribution (%)
2.0 2.5 3.0
2.702.802.903.00
(d) Puck
Female trophic position (delta15N)
Egg trophic position (delta15N)
0 20 40 60 80 100 120
6080100120140160
(e) Both sites
Female pelagic contribution (%)
Egg pelagic contribution (%)
2.0 2.5 3.0
2.62.83.03.2
(f) Both sites
Female trophic position (delta15N)
Egg trophic position (delta15N)
10
Figure 2 Male pelagic contribution (left) and trophic position (right) plotted against that of their eggs. All figures show a linear regression line that is solid for significant and dashed for non significant correlations.
40 60 80 100 120
0.91.01.11.21.31.41.5
(a) Kuznica
Male pelagic contribution (%)
Egg pelagic contribution (%)
2.5 3.0 3.5 4.0
2.62.83.03.23.4
(b) Kuznica
Male trophic position (delta15N)
Egg trophic position (delta15N)
10 20 30 40 50 60
0.70.80.91.01.1
(c) Puck
Male pelagic contribution (%)
Egg pelagic contribution (%)
2.6 2.8 3.0 3.2 3.4 3.6
2.52.62.72.82.93.03.13.2
(d) Puck
Male trophic position (delta15N)
Egg trophic position (delta15N)
20 40 60 80 100 120
0.60.81.01.21.4
(e) Both sites
Male pelagic contribution (%)
Egg pelagic contribution (%)
2.5 3.0 3.5 4.0
2.62.83.03.23.4
(f) Both sites
Male trophic position (delta15N)
Egg trophic position (delta15N)
11
Figure 3 Male traits plotted against each other and their growth in their first year. High pigmentation values indicate dark pigmentation and low head shape values indicate a large head. All figures show a linear regression line.
5 10 15 20 25 30 35
0.70.80.91.01.1
(a)
Growth in first year (mm)
Relative luminousity
5 10 15 20 25 30 35
-2-1012
(b)
Growth in first year (mm)
PC head shape
5 10 15 20 25 30 35
12345
(c)
Growth in first year (mm)
Gonadosomatic index
0.7 0.8 0.9 1.0 1.1
-2-1012
(d)
Relative luminousity
PC head shape
0.7 0.8 0.9 1.0 1.1
12345
(e)
Relative luminousity
Gonadosomaticindex
-2 -1 0 1 2
12345
(f)
PC head shape
Gonadosomatic index
12
Table 2 Alternative male reproductive tactics. Each row holds the results of one Pearson's correlation.
Parental male Sneaker male r p-value
Grow more in first year
Darker
pigmentation Grow less in first year
Lighter
pigmentation 0,194 0,0389 Grow more
in first year Larger head Grow less in
first year Smaller head -0,631 < 0,001 Grow more
in first year
Smaller gonads
Grow less in
first year Larger gonads -0,332 < 0,001 Larger head Darker
pigmentation Smaller head Lighter
pigmentation -0,555 < 0,001 Darker
pigmentation
Smaller gonads
Lighter
pigmentation Larger gonads -0,417 < 0,001 Larger head Smaller
gonads Smaller head Larger gonads 0,589 < 0,001 Darker
pigmentation
Higher trophic position
Lighter pigmentation
Lower trophic
position 0,198 0,0309
Discussion
We were not able to correlate the male diet composition with that of their eggs within each site but only when combining them. We were therefore forced to reject the null hypothesis that there is assortative mating by diet in the Gulf of Gdansk and that that is the cause for the genetic diversity found by Björklund and Almqvist (2009) in these populations. For the alternative male reproductive tactics however, we found substantial evidence for the presence of the two different male morphs in the round goby. The parental male grows more in his first year, has darker pigmentation, a larger head and smaller gonads in proportion to body size. Additionally we found some evidence for the parental male having a higher trophic position than the sneaker male.
The females’ pelagic contribution and trophic position correlated significantly with that of their eggs only for Kuznica and both sites together. However, for Puck, there were only five females caught with fully developed eggs and therefore those results are not reliable. Since the correlations for Kuznica were significant with only 15 females found with fully developed eggs, it was reasonable to assume that the egg isotope signatures can represent the female diet.
Although assortative mating by diet was not found in this study, stable isotope analysis has been used in a previous study where assortative mating by diet was found in a population of sticklebacks (Snowberg and Bolnick, 2008).
It appears that these round goby populations are isolated by distance and poor swimming capabilities (Ray and Corkum, 2001; Sapota, 2004) and that the males likely mate with as many females as they can regardless of diet. Furthermore, the females seem to select males from their
13
Figure 4 Male traits plotted against their pelagic contribution (left) and trophic position (right). High pigmentation values indicate dark pigmentation and low head shape values indicate a large head. All figures show a linear regression line that is solid for significant and dashed for marginally non significant correlations.
pigmentation (Yavno and Corkum, 2010) and/or pheromones (Laframboise et al., 2011). Our results suggest that the genetic diversity previously found is caused by human contribution. As previously mentioned the round goby was most likely brought to the Bay of Gdansk by ship ballast water (Almqvist, 2008). Judging from other studies and our results, multiple or constant colonisation is the most likely explanation for the apparent genetic diversity in the round goby population in the Gulf of Gdansk. Hayden and Miner (2009) found that juvenile round gobies swim to the surface at night where they feed on plankton, in many cases placing the larvae directly where the ships dock. Given the frequent trips of freighters between the Bay of Gdansk
-2 -1 0 1 2
20406080100
(a)
PC head shape
Pelagic contribution (%)
-2 -1 0 1 2
2.53.03.54.0
(b)
PC head shape
Trophic position (delta15N)
0.7 0.8 0.9 1.0 1.1
20406080100
(c)
Relative luminousity
Pelagic contribution (%)
0.7 0.8 0.9 1.0 1.1
2.53.03.54.0
(d)
Relative luminousity
Trophic position (delta15N)
14 and areas inhabited by round gobies (Almqvist, 2008), it is possible that they were brought there on multiple occasions from different populations. Björklund and Almqvist (2010) studied whether the number of colonisations could be estimated from genetic data, using the data from their previous study (Björklund and Almqvist, 2009) in the Gulf of Gdansk. In that study they found that the magnitude of alleles in this population can only be explained by constant colonisations from different source populations.
Newly established populations of round gobies are male biased (Gutowsky and Fox, 2011) with 75 to 80% of the population consisting of males in the Gulf of Gdansk (Skora and Rzeznik, 2001). As in all species with male bias, this may lead to sexual selection in which the males develop secondary sexual traits in order to attract females and compete with conspecific males (Tarabowsky et al., 2008). Though we cannot say from our results that we have found sexual selection in round gobies, females have been found to selectively mate with males of a larger body size (Stammler and Corkum, 2005), darker colouration (Yavno and Corkum, 2010), males with parental care (Kodric-Brown, 1990) or the combination of all of these traits (Kodric-Brown, 1990). Furthermore, male round gobies have been found to fight for shelter and nesting holes (Stammler and Corkum, 2005), guard the eggs (Charlebois et al., 2001) and to defend them aggressively against intruders with vocalizations, biting, chasing and by blocking the entrance (Corkum et al., 2004; Meunier et al., 2009; Wickett and Corkum, 1998). The swollen head, also found by Marentette and Corkum (2008) in the parental males has not been shown to effect female preference for males but may be useful when it comes to intraspecific competition. A larger head is likely useful when it comes to blocking the entrance of the nesting hole and to make the male appear larger to a rival male or an intruder. Finally, limited availability of nesting sites (Wicket and Corkum, 1998) facilitates the male intraspecific competition even further.
However, alternative male reproductive tactics are commonly found under those conditions (Tarabowsky et al., 2008). Where large males take up all the nests and possess secondary sexual traits preferred by the females (body size, head size, pigmentation and parental care), smaller males start to invest more in testes size than body size. With larger testis, a smaller body size, and a coloration similar to females they have a better chance of sneaking into a larger males nest while a female is spawning and fertilizing some of the eggs. This method allows the sneaker male to avoid the cost of courtship and nest guarding (Taborsky, 1998).
The fact that we found a significant difference in growth between parental males and sneaker males indicates that their trait or tactic is predetermined. Also interesting is that the sneaker males did not have the dark colouration but a more cryptic appearance much like the females. In some other fish species where male alternative reproductive tactics are found, the smaller males display a tactic of female mimicry, fooling the parental male into allowing him entrance to the nest along with an actual female (Dominey, 1981). Whether that is the case in round gobies has not yet been studied.
Our results help rule out the possibility that genetic diversity in newly established populations of invasive species is caused by assortative mating by diet. Should we have found
15 assortative mating, that would have been a possible first step towards speciation (Maynard Smith, 1966). But as the round gobies do not seem to mate selectively and new individuals are likely still carried regularly from different source populations, there is no sign of speciation.
Genetic diversity is normally rather scarce in newly established populations due to the limited number of colonizers (Ciosi et al., 2008), but genetic diversity, rather than just population size, is the fuelling factor for adapting to new environments (Lee, 2002). Our evidence of the apparent genetic diversity not being brought on by assortative mating by diet provides further support to Björklund and Almqvists (2009) theory of repeated colonisations.
We therefore conclude that the genetic diversity is rather caused by stratified dispersal, which is when local distribution, within the Gulf, is natural current-driven and long distance distribution is human facilitated, as found by Björklund and Almqvist (2009) in the Gulf of Gdansk and Bronnenhuber et al. (2011) in the Great Lakes of America. Since the larvae swim to the surface at night to feed (Hayden and Miner, 2009), at least a small proportion of them, will likely be carried with ocean currents to other areas or populations where they mate randomly. As previously mentioned, freighters likely transport round gobies by ballast water from the source populations during their frequent trips from the Ponto-Caspian region to the Gulf of Gdansk (Almqvist, 2008), and will presumably keep doing so. The round gobies in the Gulf of Gdansk seem to have reached permanent establishment and have started spreading to nearby regions (Almqvist, 2008).
Our study is the first to identify male alternative reproductive tactics in the Gulf of Gdansk and also to study these tactics in the nests themselves. Previous studies have shown different male morphs in round gobies but not that they are correlated with growth in their first year. This indicates that their tactic is predetermined and presumably inherited. As we focused on assortative mating by diet specifically, we cannot rule out completely the possibility of assortative mating facilitating the genetic diversity in these populations. It has been shown that female round gobies mate selectively with males displaying secondary sexual traits (Kodric- Brown, 1990; Laframboise et al., 2011; Marentette and Corkum, 2008; Marentette et.al., 2009;
Stammler and Corkum, 2005; Yavno and Corkum, 2010), but whether that is related to the genetic diversity is an opportunity for future studies.
Acknowledgements
I would like to thank Philipp Hirsch, one of my supervisors, for his excellent guidance and support throughout the project. I would also like to thank my other supervisor, Assistant Professor Richard Svanbäck, for his guidance and support. Additionally, I would like to thank Christoph Terlinden for his assistance in the Lab in the beginning of my project work, Pia Bartels for her assistance in the pigmentation analysis and finally, Pia Bartels and Maria Cortazar Chinarro for providing helpful comments on my thesis as my opponents.
16
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19
Appendix
Diet
Source partitioning revealed that bivalves were the most common food source in Kuznica, followed by crustaceans (Figure 5 and Figure 6). In Puck however, the difference between prey items was not as clear with four prey items for females (Figure 7a - d) and three for males that were all at similar proportions of the food source (Figure 8a - c).
Figure 5. Isosource source partitioning for females in Kuznica and Swarzewo. The values on the x-axis stand for likely proportion of diet.
Figure 6. Isosource source partitioning for males in Kuznica and Swarzewo. The values on the x-axis stand for likely proportion of diet.
Kuznica females
Bivalves
Frequency
0.1 0.3 0.5 0.7
0200400
Kuznica females
Crustaceans
Frequency
0.0 0.1 0.2 0.3 0.4 0.5
0100300
Kuznica males
Bivalves
Frequency
0.40 0.45 0.50 0.55 0.60
05152535
Kuznica males
Crustaceans
Frequency
0.30 0.35 0.40 0.45
051525
20
Figure 7. Isosource source partitioning for females in Puck. The values on the x-axis stand for likely proportion of diet.
Figure 8. Isosource source partitioning for males in Puck. The values on the x-axis stand for likely proportion of diet.
Resource composition in Kuznica is mostly bivalves followed by crustaceans, but round gobies in the Gulf of Gdansk are known to eat mainly bivalves except for the smaller (>4,5cm in length) and presumably younger individuals who feed mostly on crustaceans (Skora and
Puck females
Crustaceans
Frequency
0.0 0.1 0.2 0.3 0.4
02060
Puck females
Isopods
Frequency
0.0 0.1 0.2 0.3 0.4 0.5
0204060
Puck females
Gastropods
Frequency
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0204060
Puck females
Amphipods
Frequency
0.0 0.1 0.2 0.3 0.4 0.5
0204060
Puck males
Crustaceans
Frequency
0.0 0.1 0.2 0.3 0.4 0.5
01020304050
Puck males
Gastropods
Frequency
0.0 0.1 0.2 0.3 0.4 0.5 0.6
010203040
Puck males
Bivalves
Frequency
0.10 0.15 0.20 0.25 0.30
01020304050
21 Rzeznik, 2001). That was not the case in Puck where the diet composition was rather equally split between crustaceans, gastropods and bivalves for males and crustaceans, isopods, gastropods and amphipods for females. There are a number of possible explanations for this. One might speculate that the population has become too dense in Puck resulting in depletion or too much competition for bivalves, but as mentioned, only the males in Puck had bivalves in their diet, with the possible explanation that the males monopolize bivalves and the females are forced to utilize other resources. Another possible explanation is that there is, for some reason, more interspecies resource competition in Puck than in the other site, which would have the same effect. Sapota and Skora (2005) found round gobies on open flat bottoms in Puck which supports both of these explanations. Whether this resource variation in Puck had a negative effect on our study is impossible to say.
