Constraints on prey size selection by the three-spined stickleback: energy requirements and the capacity and fullness of the gut

An experiment was designed to study how gut fullness and encounter with 5-mm Asellus aquaticus influenced acceptance or rejection of less profitable 8-mm Asellus . 45-mm sticklebacks were found to always accept 5-mm prey whereas 8-mm prey were accepted with an initial probability of about 0.9. This probability decreased as the gut filled. Fish of differing sizes and sex had similar daily energy intakes per unit body size, however the acceptance of 8-mm prey was related to fish size. Whenever a fish orientated to a prey it was followed by pursuit and manipulation independently of prey size. The decision to accept or reject prey occurred after one manipulation, a criterion that was more variable for the larger prey. For one feeding session per day the total energy intake was almost constant despite the changing combination of prey sizes eaten. The fish ate prey with long handling times if the energetic contents of the stomach had not reached 450 J. Calculations were made of how many of each millimetre prey size group would satisfy the 450 J demand and how long the estimated number would take to handle. This showed that the best option is to consume 5-mm prey if given the choice.     

The influence of hunger on shoal size choice by three-spined sticklebacks, Gasterosteus aculeatus

Single sticklebacks ( Gasterosteus aculeatus ) were presented with a choice between (a) a shoal and an empty compartment, and (b) two different sized shoals of conspecifics. The choice of hungry test fish (food-deprived for 24 h and 72 h, respectively) was compared to that of fish which were fed to satiation once a day. It was hypothesized that due to the increased level of competition for food in large shoals, hungry fish would spend less time with the larger of two shoals than their well-fed conspecifics. The results showed that there was indeed a significant trend for test fish to spend a decreasing amount of time near the compartment with the larger number of fish with increasing duration of food deprivation.     

Human-induced water turbidity alters selection on sexual displays in sticklebacks

Changes in the environment due to human activities are becomingincreasingly common. A serious problem in aquatic environmentsis increased water turbidity due to phytoplankton algal growth.This may affect the breeding system of fishes, especially thosewith a visually based mating system. Here we show that increasedturbidity affects sexual selection in the threespine stickleback(Gasterosteus aculeatus) through impaired possibility for visuallybased mate choice. In a laboratory mate preference and matechoice experiment on sticklebacks from the Baltic Sea, whichis an area suffering from increased turbidity due to human activities,we found that females spent more time with and visited moreoften males in clear water than males in turbid water. For malesin turbid water to receive the same amount of interest fromfemales as males in clear water, they needed to court significantlymore. Thus, turbid water induced selection for higher courtshipactivity. However, the final spawning decision of the femalesdid not depend on water turbidity, which suggests that nonvisualcues determined the final spawning decision. Because visualcues are important in mate attraction, increased turbidity affectsan important evolutionary force, sexual selection, which mayhave further consequences for the evolution of the sexual displaysand preferences. Differences in visual conditions could hencebe one factor that has lead to differences among sticklebackpopulation in the use of sexual signals.     

Female mate choice and male red coloration in a natural three-spined stickleback (Gasterosteus aculeatus) population

Under laboratory conditions, female three-spined sticklebacks(Gasterosteus aculeatus L.) show a mating preference for intenselyred-colored males. We verified this female choice in the fieldby observing a freshwater stickleback population in its naturalhabitat. During the egg collection phase, individual courtingmales were localized with the aid of a dummy of a ripe female,caught and photographed under standardized conditions, and released.After males had stopped collecting eggs, we counted the numberof eggs in the nests. The more intense a male's red breedingcoloration, the more eggs he received. Simultaneous female choiceexperiments in the laboratory suggested that ripe females ofthis population preferred redder males. Breeding activitiesof the males in the field were clustered and seem to be synchronizedwithin clusters. At one of the breeding sites, more intensered males were in better physical condition, but this was notthe case at another site. Although several synchronized breedingcycles were observed, the majority of males seem to completeonly one breeding cycle.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Breeding ecology and evidence of reproductive isolation of a widespread stickleback fish (Gasterosteidae) in Nova Scotia, Canada

A stickleback with brilliant white dorsal breeding colours is widely distributed in north-eastern Nova Scotia, Canada, where it often breeds sympatrically with the threespine stickleback, Gasterosteus aculeatus. Breeding males are highly conspicuous and visible at distances of 20 m or more whereas sympatric G. aculeatus are cryptic and difficult to detect even at 2 m. The white stickleback nests only above the substrate in filamentous algae, where G. aculeatus nests only on the substrate. The white stickleback is smaller in size and more terete than G. aculeatus , but it is morphologically similar in having a complete row of lateral plates and similar lateral plate and gill raker numbers. The white stickleback occurs only in environments where there are filamentous algae (which appears to be an obligatory nesting substrate) and where the water is clear, saline and relatively still. Female choice tests in the laboratory show that the white stickleback is reproductively isolated from G. aculeatus , and field observations on natural spawning support this conclusion. We suggest that the bright breeding colouration may have evolved through sexual selection and/or to advertise unprofitability to predators.     

Parasitism, oddity and the mechanism of shoal choice

When choosing between shoals differing in Schistocephalus solidus infection status, uninfected test sticklebacks Gasterosteus aculeatus showed a preference for joining uninfected conspecifics when shoal sizes were equal, but reversed this preference when the relative size of the infected shoal was increased by a factor of 3. When given a choice between a shoal composed of size-matched minnows Phoxinus phoxinus and a shoal composed of the same number of all uninfected or all S. solidus -infected sticklebacks, test fish always preferred the sticklebacks, regardless of their infection status, over the minnow shoal. These observations suggest that species, parasite status and shoal size are all of importance when fish decide which shoal to join.     

Schooling affects growth in the three-spined stickleback, Gasterosteus aculeatus

Three-spined stickleback fry (mean length 20 mm, mean weight 14 mg) were reared for 14 days alone and in groups of six in a constant per capita water volume. The fish originated from two habitats (rock-pools, sea) of different predation pressure. The fry were fed nauplii of Artemia and commercial aquarium fish food ad libitum. Specific growth rates of solitary and schooling fry differed and were also affected by their origin. The specific growth rates of solitary fry from the sea averaged 1.0% day⁻¹ (length) and 6.0% day⁻¹ (weight) and those of solitary rock-pool fry 1.1 and 6.7%, respectively. For group-reared fish the corresponding values were 1.2% (length) and 6.6% (weight), and 1.3% (length) and 7.6% (weight). The finding that schooling sticklebacks grow faster than solitary ones implies that grouping enhances fitness in stickleback fry under the conditions of our experiments.     

Low level infection by eye fluke, Diplostomum spp., affects the vision of three-spined sticklebacks, Gasterosteus aculeatus

Experiments have shown thai infection by low intensities of the eye fluke affects the vision of three-spined stickleback when selecting prey, and also shortens the reactive distance to the prey. The results have consequences for the fitness of the individual, and also for the study of prey selection.     

The effect of hunger and cestode parasitism on the shoaling decisions of small freshwater fish

Individual minnows Phoxinus phoxinus and three-spined sticklebacks Gasterosteus aculeatus were provided with a mutually exclusive choice between joining a shoal of conspecifics and foraging alone in a maze. The shoaling decisions and foraging behaviour of individual fish were studied when the fish were satiated and after 24- and 48-h periods of food deprivation. Hunger level was found to have a significant effect on shoaling behaviour. When satiated, fish of both species spent a greater proportion of time within one body length of the shoal and spent less time out of visual contact with the shoal than after periods of food deprivation. The effect of the cestode parasite Schistocephalus solidus on the shoaling behaviour of stickleback hosts was complex. When satiated, infected fish spent less time than uninfected fish within one body length of the shoal, preferring to remain outside the shoal, yet within visual contact, although when food deprived there was no difference in the proportion of time spent by infected and uninfected fish close to the shoal. The possible ecological significance of this change in behaviour is discussed with reference to the manipulation hypothesis of host-parasite interactions.     

Putative causes and consequences of MHC variation within and between locally adapted stickleback demes

Genes of the major histocompatibility complex (MHC) have been a source of considerable research interest, owing in large part to the growing body of evidence that they may be subject to both natural and sexual selection. However, much remains to be learned about the dynamics of MHC genes in subdivided populations, particularly those characterized by divergent ecological pressures. In this study, we attempt to disentangle the relative roles of both parasite-mediated selection and MHC-mediated mate choice in an open estuarine system inhabited by two parapatric, adaptively divergent threespine stickleback (Gasterosteus aculeatus) demes. We sequenced the putative peptide-binding region (PBR) of an estimated four Class IIβ loci from 127 individuals, identifying 329 sequence variants (276 translated amino acid sequences). Demes differed significantly both in the frequency of MHC alleles and in the communities of helminth parasites infecting resident sticklebacks. Strong signatures of natural selection were inferred from analyses of codon substitutions, particularly in the derived (freshwater) rather than the ancestral (marine) deme. Relationships between parasite load and MHC diversity were indicative of balancing selection, but only within the freshwater deme. Signals of MHC-mediated mate choice were weak and differed significantly between demes. Moreover, MHC-mediated mate choice was significantly influenced by environmental salinity and appeared of secondary importance to tendencies towards assortative mating. We discuss the implications of these findings in respect to ecological adaptation and the potential demographic consequences of possible outcomes of MHC-mediated mate choice.     

Genetic differentiation of freshwater and marine sticklebacks, (Gasterosteus aculeatus) of Eastern Europe

Electrophoretic variation of 13 presumptive enzymatic loci was studied in sticklebacks (Gasterosteus aculeatus) at 6 marine (Baltic) and 13 freshwater sites from Poland. Marine samples are significantly more variable (proportion of polymorphic loci, p # 0.38) than freshwater Samles (p # 0.12). Also the mean number of alleles per locus is significantly higher in the marine samples (n # 1.54) than in freshwater samples (n # 1.14). There is however no significant difference in values of mean heterozygosity between marine and freshwater sticklebacks. Some alleles are found in Baltic sticklebacks only, but all these occur in very low frequencies. Genetic distance between samples is low ranging from D = 0.000053 to 0.031410. However D is significant smaller between marine samples than between freshwater Samles. We suggest than this probaily is the result of larger genetic subdivision of freshwater populations. The low genetic divergence of marine and freshwater populations of the stickleback from poland might be expfained by the recent postglacial colonization of this area from a common refugium.     

Ornaments or offspring? Female sticklebacks (Gasterosteus aculeatus L.) trade off carotenoids between spines and eggs

Hypotheses and models to explain female ornaments often assume that the elaborated traits are condition dependent; nevertheless, few empirical studies have addressed this topic. We studied a population of three-spined sticklebacks (Gasterosteus aculeatus) in which the females have conspicuous, carotenoid-based red coloration to their pelvic spines. The red coloration seems not to be condition dependent, as coloration is negatively associated with age and body length and not associated with condition. Furthermore, redder females did not have a lower density of leucocytes. We found a negative association between the females' red carotenoid-based coloration in the spines and the amount of carotenoids in the female's gonads. Males choosing red-coloured females will fertilize eggs with small amounts of carotenoids and appear not to gain any benefit from their mates' phenotypic quality that could result in offspring of improved quality. These results do not support the 'direct selection hypothesis' to explain the existence of the female ornaments.     

Sticklebacks from Greenland

Greenland freshwater sticklebacks Gasterosteus aculeatus were low-plated. Length distribution of 1 + and 2+ fish peaked at 33 and 46 mm, in early July. Pre-reproductive fish including 1 + fish were infected with Schistocephalus solidus . The diet consisted of chironomid larvae and pupae. Infected individuals ate smaller prey than non-infected ones, on one occasion where competition for food was likely to occur. Infected fish had lower stomach fullness, and the parasite compressed the stomach in heavily infected fish. The other fish species was charr, and the stickleback seemed to occupy a wide range of habitats.