Whitefish in northern Sweden
A study of head morphology between different morphs of whitefish
Magnus Enbom
Degree Thesis in Biology 15 ECTS Bachelor’s Level
Report passed: 2013-06-07 Supervisor: Göran Englund
Whitefish in northern Sweden
A study of head morphology between different morphs of whitefish
Magnus Enbom Abstract
Fishes in post-glacial lakes of the northern hemisphere are known for their ecological variation, with divergence along different ecological axes. In that conxt the European whitefish (Coregonus lavaretus L.), is interesting to study because it is the most divergent fish of all coregonides. The objective of this study was to investigate how different factors are causing the whitefish head morphology to diverge, by comparing 9 introduced populations and one natural population, of different age. The study should answer if there is a significant difference in head morphology between different habitats and between different whitefish populations, depending on the age of introduction and the presents of pike. The head of each fish was photographed and analyzed by using landmarks on the head, a geometric
morphometric method. One result was that the pelagic morph had relatively bigger eyes and a more oblong head then the littoral morph. Another result was that the distance (i.e. the difference in morphology between littoral and pelagic populations of whitefish) was increasing with increasing population age. This correlation between distance and age of introduction was significant for pike lakes but not for non-pike lakes, suggesting that pike is important for whitefish divergence. In conclusion this study showed that the head
morphology differs between littoral and pelagic whitefish and that the age of introduction determines the degree of morphological divergence in whitefish.
Key words: whitefish, head, morphology, diverge, pike
Register
1. Introduction
...12. Methods
...23. Results
...43.1. Pike lakes, sorted by age of introduction………5
3.2. Non-pike lakes, sorted by age of introduction……….7
4. Discussion
...95. Acknowledgement
...106. References
...111
1. Introduction
Adaptive radiation describes the process in evolutionary biology when a common ancestor diverges into several species that occupy different ecological niches (Schluter 2000). One of the most famous examples of adaptive radiation is probably Darwin´s ground-finches, where the beak size and shape are adapted to feeding on specific seeds (Grant 1999). In nature there are many degrees of ecological and morphological divergence. This could be seen as a
continuum from high inter-individual variation within a population, to polymorphism and lastly totally reproductively isolated species (Smith and Skúlason 1996). The degree of divergence is thought to be determined by ecological opportunity, which means the strength of competition, the niche availability and the amount of available resources (Schluter 2000).
For instance, lakes that were colonized early after the latest ice age should have the highest diversity (Siwertsson et al. 2010). Furthermore lakes that are deep, large and productive (i.e.
high ecological opportunity) should have the most pronounced divergence in terms of differences between morphs and the number of morphs (Siwertsson et al. 2010).
Fish species inhabiting postglacial lakes are known for their ecological variation (Robinson and Parson 2002). Adaptive radiation among these fish has usually occurred, which is mainfested as different morphs living in pelagic and littoral habitats (Schluter and McPhail 1993). In that context the European whitefish (Coregonus lavaretus L.) is interesting to study because it is the most divergent species of all coregonids (Svärdson 1979). The different morphs of whitefish usually occupy all the available habitats (Siwertsson et al. 2010). In the littoral there is a large morph with few gill rakers and in the pelagic there is a smaller morph with many gill rakers (Siwertsson et al. 2010). In some lakes there is a third morph, co- existing with the others, that is also small but has few gill rakers (Siwertsson et al 2010). This third morph has its niche in the profundal zone, foraging on benthic macroinvertebrates (Siwertsson et al. 2010). The littoral morph is also adapted to forage on benthic
macroinvertebrates whilst the pelagic morph forages on zooplankton (Amundsen et al.2004).
It is likely that all the whitefish morphs in Fennoscandia derived from a single ancestor and then the adaptive divergence has happened within each lake separately (Bernatchez and Dodson 1994). The ancestor is probably the littoral morph that is a generalist using a variety of habitats and diets when living in allopatry (Ostbye et al. 2006).
Traditionally, whitefish have been categorized into different morphs by counting gill rakers (Kahilainen and Ostbye 2006). That is because gill rakers are considered as environmentally stable (Svärdson 1979), and highly heritable traits (Svärdson 1950). On the other hand, because differences in gill rakers are highly heritable, their divergence should be observed relatively late in the divergence process (Öhlund 2012). Differences in morphology, diet, size etc. might be more plastic and should therefore be observed earlier (Öhlund 2012). For that reason head morphology will be used in this study.
The use of different habitats and food by whitefish could lead to adaptations of their head morphology (Kahilainen and Ostbye 2006). Previous studies have showed that the head morphology of Arctic charr (Salvelinus alpinus) gets more robust when feeding on
zoobenthos, compared to feeding on zooplankton (Snorrason et al. 1989). Amongst cichlids, divergences in mouth morphology is connected to suction and biting feeding (Cooper et al.
2010) and in three-spined stickelbacks (Gasterosteus aculeatus L.) the head gets deeper and the mouth gap bigger, when exposed to larger prey (Day and McPhail 1996).
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I also relate the head morphology to the presence of pike (Esox lucius L.) as pike could play an important role in promoting phenotypic diversity, by size-structuring the fishes into different habitats (Öhlund 2012). Pike is the most important predator on coregonids with high densities in littoral and profundal habitats (Bohn et al. 2002). What separate pike from other piscivorous species are the large gape size and therefore the capacity to predate on large prey (Öhlund 2012). Öhlund (2012) showed that pike induces rapid divergence in
Scandinavian populations of whitefish. This was mainly caused by the whitefish
predisposition to avoid predation risk, either by staying in the almost predator-free pelagic or by growing large in the littoral (Öhlund 2012).
The objective of this study was to investigate how different factors are causing whitefish head morphology to diverge, by comparing 9 introduced populations of different age (53-188 years) and one natural population (approx. 10 000 years).
Specifically I will examine if there are significant differences in head morphology between different habitats and between different whitefish populations, depending on the time since whitefish were introduced to the lakes and the presence of pike.
2. Methods
The study was carried out during the spring of 2013, using whitefish that were caught during the summers of 2011 and 2012. The fish were taken from 10 lakes in northern Sweden (Table 1) with various years of introduction, ranging between 53 and 188 years ago, and one natural population. These lakes are in different stages of divergence (Öhlund 2012). The reason why we have this gradient of different introductions is because whitefish have been an important food resource for people living in northern Sweden for a long time and have therefore been introduced in many lakes. That also gives us the unique opportunity to study divergence in whitefish at difference stages. Northern pike (Esox lucius L.) was present in 6 out of 10 lakes.
Whitefish were caught in both the littoral and pelagic zones using gillnets (multi-mesh, 35mm, 45mm). The fish have been stored in a freezer until the analysis.
Table 1. Overview of the studied lakes, including name of the lake, approximate year of whitefish introduction and the presence/absence of pike.
Lake Introduction y ear Pike
Bölessjön 1 82 5 present
Görv ikssjön 1 845 present
Gunnarv attnet 1 900 absent
Hetögeln 1 960 present
Idsjön 1 0 000 y ears ago present
Mev attnet 1 92 5 absent
Rissjön 1 92 6 present
Ström sv attudal 1 860 present
Torringen 1 844 absent
Tuv attnet 1 865 absent
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Fish were taken from the freezer daily and left to thaw to the next day. For the analysis, the head of each whitefish was photographed (Quevedo et al. 2009) using a digital camera installed on a tripod. The camera was set on 3 times the zoom and using the macro function.
In order to make the analysis easier a pin was attached to the beginning of the head. Each picture was scaled using the ruler in the picture to avoid size-bias due to differences in photography. In total approximately 330 fishes were analyzed, 33 fishes per lake with a distribution of about 50/50 between pelagic and littoral habitats. The head morphology was analyzed using a geometric morphometric software called ”tpsDig”. The analysis is based on landmarks, often called “homologous points” (Zelditch 2012). The landmarks are points on the head that always exists, but their exact position could vary. For example you always have an eye, but the position of the eye can differ among individuals (Zelditch 2012). For this study 9 landmarks were positioned on the head (Fig. 1) and these were: tip of the nose (1), tip of the upper lip (2), front of the eye (3), back of the eye (4), beginning of the head (5), insertion of operculum (6), the “triangle” of operculum (7), upper insertion of fin (8) and lower insertion of fin (9).
Figure 1. Landmarks (1-9) used in this study.
Furthermore the software “tpsRelw” (Rohlf 2005 A) was used to analyze the relative position of each landmark and variation in body shape by calculating partial warps and uniform scores for each individual. The partial warps and uniform scores were analyzed with a discriminiant function analysis (DFA). The DFA combines all partial warps and uniform scores for each fish into morphological scores. To compare whitefish morphology between habitats the partial warps and uniform scores were analyzed separately for each lake, with the classification in the DFA based on habitat (i.e. littoral and pelagic). In order to visualize this variation in head morphology between pelagic and littoral individuals, the software “tpsRegr”
(Rohlf 2005 B) was used.
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A regression analysis was made between the morphological “distance” (i.e. the difference in morphological score amongst the mean littoral and mean pelagic morphs within each lake) and the age of introduction, in order to see if the distance was affected by increased age of population. Age of introduction was log-transformed to fulfill model assumptions.
Furthermore correlation analysis between the distance and age of introduction was made for all the lakes in total and also for pike lakes and non-pike lakes separately. Finally an
ANCOVA (i.e. analysis of covariance) was performed to test whether pike presence affected morphological distance between lake pairs of similar introduction age (Table 2). Since introduction dates differ among lake pairs, introduction date was included as a covariate in the model.
Table 2. Overview of the lakes used in the ANCOVA analysis. Distance stands for the difference in morphological score amongst the mean littoral and mean pelagic morphs within each lake.
Lake Distance Pike Age of introduction (years)
Rissjön 2 ,502 82 present 87
Görv ikssjön 3 ,2 92 3 present 1 68
Ström sv attudal 3 ,7 63 93 present 1 53
Mev attnet 2 ,3 7 53 8 absent 88
Torringen 2 ,67 2 88 absent 1 69
Tuv attnet 2 ,82 3 97 absent 1 48
3. Results
Morphological distance between littoral and pelagic morphs increased (Fig. 2) with increasing age of whitefish populations (R-squared=0,61).
Figure 2. Regression analysis between age of introduction and the distance.
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The correlation analysis showed a significant correlation (P=0,044) between distance and age of introduction in pike lakes. But the correlation was not significant (P=0,256) in non-pike lakes (Table 3).
Table 3. Summary of correlation analysis between distance and age of introduction.
Summary of Correlation analysis P-value Pike lakes (distance; log(age of intro) 0,044 Non-pike lakes (distance; log(age of intro) 0,2 56 All lakes (distance; log(age of intro) 0,007
A general morphological difference between the morphs within each lake was observed (Fig.
3-7) where the littoral individuals had negative morphological scores whilst the pelagic morphs had positive scores. The degree of overlap differed between lakes. Idsjön (Fig.3 ) was the lake with the least visual overlap followed by Strömsvattudal (Fig. 4). A general pattern was that the pelagic individuals had relatively bigger eyes and a flatter forehead than the littoral morphs. This pattern was clearly seen in Bölessjön (Fig. 3), Idsjön, Görvikssjön (Fig.
4), Strömsvattudal and Rissjön (Fig. 5).
The results for the mouth morphology were not very clear. In Idsjön and Gunnarvattnet (Fig.
7) the littoral morphs had a mouth pointing downwards and the pelagic morphs had a mouth pointing upwards. But in some lakes the mouth morphology was inverse, like in Torringen (Fig.6 ) and Strömsvattudal. Furthermore in some lakes, for example Hetögeln (Fig. 5) and Mevattnet (Fig. 7), the mouth looked similar between pelagic and littoral individuals.
3.1. Pike lakes, sorted by age of introduction
Figure 3. Idsjön and Bölessjön. Upper graph: frequency distribution of whitefish’s morphological scores. Lower graph: visualizes the mean head morphology within the littoral group (left) and the pelagic group (right) in each lake.
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Figure 4. Görvikssjön and Strömsvattudal. Upper graph: frequency distribution of whitefish’s morphological scores. Lower graph: visualizes the mean head morphology within the littoral group (left) and the pelagic group (right) in each lake.
Figure 5. Rissjön and Hetögeln. Upper graph: frequency distribution of whitefish’s morphological scores. Lower graph: visualizes the mean head morphology within the littoral group (left) and the pelagic group (right) in each lake.
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3.2. Non-pike lakes, sorted by age of introduction
Figure 6. Torringen and Tuvattnet. Upper graph: frequency distribution of whitefish’s morphological scores.
Lower graph: visualizes the mean head morphology within the littoral group (left) and the pelagic group (right) in each lake.
Figure 7. Gunnarvattnet and Mevattnet. Upper graph: frequency distribution of whitefish’s morphological scores.
Lower graph: visualizes the mean head morphology within the littoral group (left) and the pelagic group (right) in each lake.
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There was no significant difference in divergence in head morphology between pike and non- pike lakes (table 4).
Table 4. Results of the ANCOVA. With the morphological distance responding to the presents/absents of pike and the age of introduction.
Anova Table (Type II tests)
Response: distance
Df F value P-value
pike 1 4.2988 0.1739
log(age) 1 5.2385 0.1493
pike:log(age) 1 1.1133 0.4020
The no significant result in divergence in head morphology is also illustrated by a barplot (Fig. 8) with overlapping standard error.
Figure 8 . Barplot of the mean values (+- standard error) from lakes with pike present (blue bar, Rissjön, Görvikssjön, Strömsvattudal) and with pike absent (red bar, Mevattnet, Torringen, Tuvattnet).
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5
Morphological distance
Pike present Pike absent
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4. Discussion
The results from this study showed that morphological distance increased with age of population in whitefish. This supports previous studies, indicating that the time available since colonization plays a vital role in the process of speciation (Coyne and Orr 2004).
Furthermore, there is no correlation between age of introduction and divergence in no-pike lakes, indicating that pike plays an important role in divergence (Öhlund 2012). A
hypothesized mechanism is that pike forces whitefish to either grow fast out of the predation window in the profitable littoral or to stay as dwarfs in the almost predator-free pelagic (Öhlund 2012).
The graphs over the morphological scores showed that lakes with the oldest introduction date and pike present, had the clearest distinction between the two morphs. It was assumed that the head morphology would differ between the morphs, as a result of adaptations to foraging in the different habitats (Kahilainen and Ostbye 2006). Previous studies have shown that head morphology in fish could be connected to feeding techniques such as suction and biting (Cooper et al. 2010). Exposure to larger prey could result in a larger gape and a deeper head (Day and McPhail 1996), whilst planktivorous feeding would generate a longer and more oblong head, providing better suction force (Wainwright et al. 2004). Furthermore, pelagic fishes should have relatively bigger eyes and a mouth pointing upwards compared to littoral fish with a mouth pointing downwards and relatively smaller eyes (Garduno-Paz and Adams 2010). My results agree with these expectations; the pelagic morphs had a more oblong head with relatively bigger eyes, while the littoral morphs had a deeper head and relatively smaller eyes. However, the result for the mouth morphology was very unclear. In some lakes it agreed with the expectations, in other lakes there was no difference at all or the results were totally the opposite.
The unclear results for the mouth morphology are probably caused by “human error” during the landmarking process, rather than the assumption in itself being incorrect. The analyses with geometric morphometrics using landmarks are exposed to a certain degree of
subjectivity (Zelditch 2012). Some traits interpret as more stable than others (Zelditch 2012).
The landmark, tip of the upper lip, was probably the hardest landmark to position correctly every time and that could affect the result for the morphology of the mouth.
Northern pike is probably the most important predator on coregonids in many lakes of Fennoscandia (Bohn et al. 2002) and are assumed to play an important role in promoting phenotypic diversity (Öhlund 2012) because of its high abundance in the littoral habitat (Bohn et al. 2002). With pike present, the small fish will be restricted to the low risk pelagic habitat, whilst the bigger morphs can survive in the more profitable littoral habitat (Saksgård et al. 2002). Öhlund (2012) has also showed that, in Scandinavian populations of whitefish, the presence of pike leads to fast divergence into dwarf- and giant morphs. To examine if the morphological distance was responding to the precence of pike I used an ANCOVA, with age of introduction as covariate because introduction date differs among lake pairs. There were three replicates each for pike presence (Rissjön, Görvikssjön, Strömsvattudal) and pike absence (Mevattnet, Torringen, Tuvattnet).
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Unexpectedly there was no significant difference in the divergence in head morphology between pike and no-pike lakes. Based on the results from this study it seems that pike does not affect head morphology in whitefish, although there were very few replicates resulting in low statistical power. It is Rissjön, a pike lake with a low morphological distance, who “fell out” and made the whole test insignificant. Thus, with more replicates I suspect that the results would be significant.
In conclusion this study shows that head morphology differs between littoral and pelagic whitefish and that the age of introduction determines the degree of morphological divergence in whitefish, supporting previous studies that time since colonization is an important factor for divergence (Schluter 2000). This study also shows that pike could play an important role in whitefish divergence because there was no significant correlation between distance and age of introduction in non-pike lakes. The results for the morphological score reinforces previous studies of other fish species (Day and McPhail 1996, Wainwright et al. 2004, Cooper et al. 2010), that head morphology differs between littoral and pelagic ecotypes as an
adaptation to foraging.
5. Acknowledgement
I want to thank Göran Englund for the opportunity to write my bachelor thesis as a part of his whitefish research programme. In particular because I have experienced it is not easy to find a suitable subject during the spring semester, when most of the fieldwork standstill.
Especially I want to thank Pia Bartels for the practical supervision with the study. Without her dedication this would have been much more problematic. Lastly I want to thank Daniel Gonzalez for the labwork, above all the “chilling” in the freezer!
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5. References
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Bernatchez, L. and Dodson, J.J. 1994. Phylogenetic relationships among Palearctic and Naearctic whitefish (Coregonus sp.) populations as revealed by mitochondrial DNA variation. Canadian Journal of Fisheries and Aquatic Sciences, 51:240-251
Bohn, T., Amundsen, P-A., Popova, O., Reshetnikov, Y.S. and Staldvik, F.J. 2002. Predator avoidance by coregonids: Can habitat choise be explained by size-related prey vulnerability?
Advances in Limnology 57: Biology and Management of Coregonid Fishes – 1999, 57:183- 197
Cooper, W.J., Parson, K.J., McIntyre, A., Kern, B., McGee-Moore, A. and Albertson, R.C.
2010. Bentho-Pelagic divergence of cichlid feeding architecture was prodigious and
consistent during multiple adaptive radiations within African riftlakes. PLoS ONE, 5:e9551 Coyne, J.A. and Orr, H.A. 2004. Speciation. Sinauer Associates, Sunderland.
Day, T. and McPhail, J.D. 1996. The effect of behavioural and morphological plasticity on foraging efficiency in the three spined stickleback. Oecologica, 108:380-388
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gradients. Dissertation thesis. Department of Ecology and Environmental Science. Umeå University.
Dept. of Ecology and Environmental Science (EMG) S-901 87 Umeå, Sweden
Telephone +46 90 786 50 00 Text telephone +46 90 786 59 00 www.umu.se
