Dance of the flame-throated fish
In rivers and streams across the country, male sticklebacks are preparing for an intricate courtship and the challenges of fatherhood
as river banks start to come alive with fresh greenery, one fish has donned a mantel of intense colour under the sparkling waters. With a throat as red as flame, the male stickleback performs extravagant displays, to attract the interest of roving females. He starts constructing a nest more advanced than those of many birds. The coming weeks are important, as this may be his only chance to reproduce and pass his genes on to a new generation. He has invested heavily for this moment, turning aggressively on former allies in his quest to mate. Britain’s common three-spined stickleback, Gasterosteus aculeatus, is found across the northern hemisphere and throughout the UK. Some inhabit coastal zones, with clear reaches to the sea. However, inland lakes, ponds and streams with muddy or sandy bases and ample vegetation all provide suitable habitats. The only essential is that they do not dry out at any point. These fish can either adopt an exclusively freshwater existence, or they are anadromous, spending the majority of their life in the sea. These specimens only move to freshwater to spawn.
Mating colours
Outside of the breeding season the male stickleback appears nondescript and featureless. Once the reproductive phase starts however, there are big changes. Its eyes become sapphire bright, its breast flame red. The intensity of the red advertises his worth as a suitor. The pigments responsible for the colour are carotenoids, bright antioxidants ingested as a result of good diet. This includes benthic midge larvae, small aquatic insects and tiny fish for the freshwater populations, and drifting, planktonic organisms for marine fish. Biologically, the fish with stronger reds have higher fertility rates. Eating such colourful diets, so rich in antioxidants, has beneficial outcomes for a fish’s sperm. For most of their lives, these tiny fish are socially gregarious. During the endless hunt for their favourite foods, they are at risk of predation. For protection, they forge lifelong relationships with other trusted members of a shoal. Two fish cooperate to inspect threats, working in pairs to establish risk levels on a tit-for-tat basis. One fish moves, then the other, until the threat has been deemed harmless, or both fish have been forced to flee.
This is forgotten, however, in the hunt for a mate. Triggered by a break in the winter cold, and the following elevated temperatures, male sticklebacks abandon their allies to become solitary. The saltwater populations start spawning behaviour early, swarming inland from the seas in a patchy stickleback armada from February onwards. They spawn with the females that have joined them on the journey. The freshwater fish do not follow suit for another month or more. Both types move to shallower waters at the edges of ponds, rivers and lakes, looking for territories at least 20in (50cm) across to claim as their own. The males police their territories with tyrannical fervour. Weaker rivals and premature females who amble into their domains before they are ready are ruthlessly driven away.
Nest building
When his territory is secured, the male stickleback builds his nest, which takes the form of a soft tunnel. He amasses scraps of vegetation, moulding it into a tubular shape before securing it with a gluey substance called spiggin. This is secreted from his kidneys and expelled through his vent, an opening underneath the body. Once the bower is constructed, the male starts to advertise his presence. He displays to the watchful plump drab olive or silvery females, patrolling the many different nests. If a female lingers on the outskirts of his territory, he starts to dance for her. His three spines are erected like flagpoles and his proud red chest prominently displayed. Making little movements to the left and right, he closes the distance between them. Intermittently he breaks off to rush at her with his wide-open mouth. Finally, after repeatedly dancing in a line between the nest and the female, he succeeds in enticing her to follow him down to the entrance. He clearly indicates where it is, by pointing his head directly at the opening aperture. Once she comes closer, the male quivers beside her, encouraging her to enter. She worms her way through the close shroud, laying from 40 to 500 eggs before leaving through the other side. The male then follows through to fertilise them. Animosity now returns with a vengeance, as he chases the female away. Now his attention is focused on caring for his new brood. The females will move on to visit any males that they consider worthy, spreading their genes around several potential fathers.
Caring for the young
Three-spined sticklebacks have fewer young than many fish species, but offer them a high degree of paternal care. While the females have nothing further to do with their progeny, the male tends to them from egg stage onwards. He chases off intruders who come near, while remaining vigilant for the opportunity to eat his neighbours’ eggs. Unattended eggs in static waterways low in oxygen would run the risk of suffocation, especially in the closed tent of the nest. To avoid this the father positions himself at the entrance to the nest. Here he begins to swim on the spot, fanning through fresh oxygen-rich water. This is kept up day and night for seven to eight days, until the young start to hatch out. As hatching gets closer, the male makes another opening in the roof of the nest, allowing for even better breathing. Upon hatching, the parent stickleback acts as guard dog. The
fry are free swimming and prone to putting themselves in danger. If they swim off, he catches the young fish in his mouth, spitting them back into the cluster.
Leaving the nest
The young fish become bolder as they grow, and their father lets them drift further from view. Once they have consumed their yolk sacs, they will drift away altogether. Growth rates depend on the amount of food available, but it is the formation of social groups that takes the juvenile fish to their next stage in life. Such shoals become increasingly obvious, with smaller ‘wolf packs’ of fish forming larger, obvious congregations as late as July and August. Depending on whether the sticklebacks are freshwater or marine in origin, these shoals either drift towards deeper parts of their ponds or lakes, or move back downstream towards the sea. There they swim in clusters around floating seaweed. The juveniles spend the coming years fattening up, preparing to make their own spawning journey. For some freshwater fish, especially those found at extreme latitudes, this may happen in just one year, though most tend to wait two or three. Meanwhile, the male will have rebuilt his nest, and done his best to attract a second or even a third female. This, together with the exertion of guarding his young, takes its toll. A freshwater stickleback returns to the lower levels of water where he tries to either reforge old alliances or strike up new ones. The marine fish expended considerable energy swimming inland to begin with. They also had to adapt to different food sources. For them, the trip is usually one-way. Lacking the strength to follow their young families back to the sea, the sticklebacks – male and female – die of their exertions and strained physiologies. All the young need to do now is survive for as long as they can. Then, when the time comes, the males flare red breasts of their own, taking up their roles as the dancers that follow the tradition of a million generations. The circle of life continues.