Coasting in live-bearing fish: the drag penalty of being pregnant

Timing matters: female receptivity and mate choice in the zebrafish (Danio rerio)

Abstract

Female choice has been documented in many animal taxa, and how we test it has been refined through years of studies on the topic. However, when designing mate choice experiments some variables, surprisingly, often remain overlooked, including receptivity and reproductive stage. Here, we aimed to assess whether the female reproductive stage influences strength and direction of mate choice in the zebrafish, Danio rerio. Females were offered a choice between two males differing in body size. We found that female choice in our experimental setup was significantly repeatable and that females preferred larger males. Nonetheless, the level of choosiness of females was affected by the time since the last spawning. Females spent more time choosing when tested 7 and 10 days after spawning rather than 4 days, indicating a higher receptivity to males from one week after the last spawning. Moreover, females preferred larger males only when tested 7 and 10 days after spawning. Our results suggest that female mate choice should take female receptivity into account, by standardizing time since the last spawning across females. More broadly, this suggests that 7–10 days since the previous spawning is the ideal time interval for zebrafish female receptivity to peak, with implications for facilities and researchers to increase egg production in natural spawning events and manual egg collection.

Significance statement

The role of pre-copulatory female mate preference has long been recognized in sexual selection. Nonetheless, female receptivity often remains overlooked in mate choice experiments especially in external fertilizing species. In the present study, we investigated if the female reproductive stage affects the strength and direction of female mate choice in an external fertilizing fish, the zebrafish, Danio rerio. We found that, when tested 7 and 10 days after spawning rather than 4 days, females spent more time choosing, demonstrating an increased receptivity to males from 1 week following the last spawning. Furthermore, only at 7 and 10 days after spawning females exibith a clear preference for the bigger males. Our study highlights the importance of considering the female receptivity in future studies assessing mate choices in this and other externally fertilizing species, and also for zebrafish facilities to increase egg production in natural spawning events and manual egg collection.

Multiple paternity in superfetatious live-bearing fishes

Superfetation, the ability to carry several overlapping broods at different developmental stages, has evolved independently multiple times within the live-bearing fish family Poeciliidae. Even though superfetation is widespread among poeciliids, its evolutionary advantages remain unclear. Theory predicts that superfetation should increase polyandry by increasing the probability that temporally overlapping broods are fertilized by different fathers. Here, we test this key prediction in two poeciliid species that each carry two temporally overlapping broods: Poeciliopsis retropinna and P. turrubarensis. We collected 25 females per species from freshwater streams in South-Eastern Costa Rica and assessed multiple paternity by genotyping all their embryos (420 embryos for P. retropinna; 788 embryos for P. turrubarensis) using existing and newly developed microsatellite markers. We observed a high frequency of unique sires in the simultaneous, temporally overlapping broods in P. retropinna (in 56% of the pregnant females) and P. turrubarensis (79%). We found that the mean number of sires within females was higher than the number of sires within the separate broods (2.92 sires within mothers vs. 2.36 within separate broods in P. retropinna; and 3.40 vs 2.56 in P. turrubarensis). We further observed that there were significant differences in the proportion of offspring sired by each male in 42% of pregnant female P. retropinna and 65% of female P. turrubarensis; however, this significance applied to only 9% and 46% of the individual broods in P. retropinna and P. turrubarensis, respectively, suggesting that the unequal reproductive success of sires (i.e. reproductive skew) mostly originated from differences in paternal contribution between, rather than within broods. Together, these findings tentatively suggest that superfetation may promote polyandry and reproductive skew in live-bearing fishes.

The Fish Family Poeciliidae as a Model to Study the Evolution and Diversification of Regenerative Capacity in Vertebrates

The capacity of regenerating a new structure after losing an old one is a major challenge in the animal kingdom. Fish have emerged as an interesting model to study regeneration due to their high and diverse regenerative capacity. To date, most efforts have focused on revealing the mechanisms underlying fin regeneration, but information on why and how this capacity evolves remains incomplete. Here, we propose the livebearing fish family Poeciliidae as a promising new model system to study the evolution of fin regeneration. First, we review the current state of knowledge on the evolution of regeneration in the animal kingdom, with a special emphasis on fish fins. Second, we summarize recent advances in our understanding of the mechanisms behind fin regeneration in fish. Third, we discuss potential evolutionary pressures that may modulate the regenerative capacity of fish fins and propose three new theories for how natural and sexual selection can lead to the evolution of fin regeneration: (1) signaling-driven fin regeneration, (2) predation-driven fin regeneration, and (3) matrotrophy-suppressed fin regeneration. Finally, we argue that fish from the family Poeciliidae are an excellent model system to test these theories, because they comprise of a large variety of species in a well-defined phylogenetic framework that inhabit very different environments and display remarkable variation in reproductive traits, allowing for comparative studies of fin regeneration among closely related species, among populations within species or among individuals within populations. This new model system has the potential to shed new light on the underlying genetic and molecular mechanisms driving the evolution and diversification of regeneration in vertebrates.

Behavioral Variation in the Pygmy Halfbeak Dermogenys collettei: Comparing Shoals With Contrasting Ecologies

Variation in biotic and abiotic factors among populations affects individual behaviors by transforming the social landscape and shaping mating systems. Consequently, describing behaviors in natural populations requires consideration of the biological and physical factors that different individuals face. Here, we examined variation in socio-sexual and locomotor behaviors in a small, livebearing, freshwater fish, the pygmy halfbeak Dermogenys collettei, across natural populations in Singapore. The pygmy halfbeak is a surface feeding fish that spends most of the time near the water surface, making it ideal for non-invasive behavioral observations. We compared behaviors between sexes among 26 shoals while simultaneously accounting for environmental variation. We demonstrated that sexual interactions and locomotor behaviors differed among shoals with varying levels of canopy cover and water flow. Specifically, in areas with greater canopy cover, sexual interactions decreased, whereas time spent in a stationary position increased. Sexual interactions were more numerous in still water, where fish spent less time swimming. Variation in the expression of socio-sexual and locomotor behaviors were not associated with differences in the amount of aquatic vegetation, water depth or halfbeak shoal size. Agonistic interactions were robust to environmental effects, showing little variation among environments. However, there were strong sex effects, with males performing more agonistic behaviors and spending less time in a stationary position compared to females, regardless of the environment. Moreover, sexual interactions, measured as actively performed by males and passively received by females, were on average more frequent in males than in females. Our findings help us explore the proximal causes of intraspecific behavioral variation and suggest that fundamental information on socio-sexual behaviors from wild populations can lead to a better understanding of how sexual selection operates when the strength of natural selection varies across environments.

Predator Environment Does Not Predict Life History in the Morphologically Constrained Fish Alfaro cultratus (Cyprinodontiformes: Poeciliidae)

Predation is known to have a significant effect on life history diversification in a variety of species. However, physical constraints of body shape and size can sometimes limit life history divergence. We test this idea in the Costa Rican livebearing fishAlfaro cultratus. Individuals in this species have a narrow body and keeled ventral surface, and females do not develop a distended abdomen when pregnant like other livebearing fishes. Here, we describe the life history ofA. cultratusfrom 20 different populations across both high-predation and low-predation environments. We found significantly lower reproductive allotment in females from high-predation environments than in females from low-predation environments, but no significant difference in female or male size at maturity, number of offspring produced by females, or size of offspring. We found thatA. cultratusexhibit isometric patterns of allocation for clutch dry mass in relation to female dry mass in high-predation and low-predation environments. Our results suggest that body shape constraints in this species limit the life history divergence we typically see between populations from high-predation and low-predation environments in other species.

Thriving artificial underwater drag-reduction materials inspired from aquatic animals: progresses and challenges

In the past decades, drag-reduction surfaces have attracted more and more attention due to their potentiality and wide applications in various fields such as traffic, energy transportation, agriculture, textile industry, and military. However, there are still some drag-reduction materials that need to be deeply explored. Fortunately, natural creatures always have the best properties after long-term evolution; aquatic organisms have diversified surface microstructures and drag-reducing materials, which provide design templates for the development of thriving artificial underwater drag-reduction materials. Aquatic animals are tamed by the current while fighting against the water, and thus have excellent drag reduction that is unparalleled in water. Inspired by biological principles, using aquatic animals as a bionic object to develop and reduce frictional resistance in fluids has attracted more attention in the past few years. More and more aquatic animals bring new inspiration for drag-reduction surfaces and a tremendous amount of research effort has been put into the study of surface drag-reduction, with an aim to seek the surface structure with the best drag-reduction effect and explore the drag-reduction mechanism. This present paper reviews the research on drag-reduction surfaces inspired by aquatic animals, including sharks, dolphins, and other aquatic animals. Aquatic animals as bionic objects are described in detail, with a discussion on the drag-reduction mechanism and drag-reduction effect to understand the development of underwater drag-reduction fully. In bionic manufacturing, the effective combination of various preparation methods is summarized. Moreover, bionic surfaces are briefly explained in terms of traffic, energy sources, sports, and agriculture. In the end, both existing problems in bionic research and future research prospects are proposed. This paper may provide a better and more comprehensive understanding of the current research status of aquatic animals-inspired drag reduction.

Predation risk shapes the degree of placentation in natural populations of live-bearing fish

The placenta is a complex life‐history trait that is ubiquitous across the tree of life. Theory proposes that the placenta evolves in response to high performance‐demanding conditions by shifting maternal investment from pre‐ to post‐fertilisation, thereby reducing a female’s reproductive burden during pregnancy. We test this hypothesis by studying populations of the fish species Poeciliopsis retropinna in Costa Rica. We found substantial variation in the degree of placentation among natural populations associated with predation risk: females from high predation populations had significantly higher degrees of placentation compared to low predation females, while number, size and quality of offspring at birth remained unaffected. Moreover, a higher degree of placentation correlated with a lower reproductive burden and hence likely an improved swimming performance during pregnancy. Our study advances an adaptive explanation for why the placenta evolves by arguing that an increased degree of placentation offers a selective advantage in high predation environments.

Superfetation reduces the negative effects of pregnancy on the fast-start escape performance in live-bearing fish

Superfetation, the ability to simultaneously carry multiple litters of different developmental stages in utero, is a reproductive strategy that evolved repeatedly in viviparous animal lineages. The evolution of superfetation is hypothesized to reduce the reproductive burden and, consequently, improve the locomotor performance of the female during pregnancy. Here, we apply new computer-vision-based techniques to study changes in body shape and three-dimensional fast-start escape performance during pregnancy in three live-bearing fishes (family Poeciliidae) that exhibit different levels of superfetation. We found that superfetation correlates with a reduced abdominal distension and a more slender female body shape just before parturition. We further found that body slenderness positively correlates with maximal speeds, curvature amplitude and curvature rate, implying that superfetation improves the fast-start escape performance. Collectively, our study suggests that superfetation may have evolved in performance-demanding (e.g. high flow or high predation) environments to reduce the locomotor cost of pregnancy.