Killifish are highly adaptable to their surroundings
The killifish on the Caribbean island of Trinidad are ideal subjects for studying evolution because they can quickly adapt to environmental change. Some have even become amphibious, spending time out of water, probably to avoid predation, while others lay their eggs out of water on damp moss as an evolved adaptation to predation. However, eggs out of water bear the additional risk of desiccation which needs to be balanced against the risk of predation in water.
This paper by Professor Matthew R. Walsh, a biology professor at the University of Texas at Arlington, Arlington, TX, USA and his laboratory technician, Christopher Roden shows that the eggs of fish that lay them out of water hatch earlier than those in water, reducing the risk of desiccation, showing how predation has driven a change in the rate of embryo development to reduce the risk of desiccation, while avoiding predation.
Their findings are published open access in Proceedings of the Royal Society B and explained in the University of Texas press release:
Tell me all about the killifish on the island of Trinidad, please. Killifish on the island of Trinidad, particularly the Trinidadian guppy (Poecilia reticulata) , are fascinating subjects of study due to their unique ecological and evolutionary dynamics. Here’s an in-depth look at these fish:
Habitat and Distribution
Killifish, including guppies, are found in various freshwater habitats across Trinidad, such as streams, rivers, and ponds. They thrive in environments ranging from slow-moving to fast-flowing waters. Trinidad's diverse aquatic habitats provide an excellent natural laboratory for studying these fish.
Ecological Significance
Killifish play a crucial role in the aquatic ecosystems of Trinidad. They are omnivorous, feeding on algae, detritus, small invertebrates, and mosquito larvae, which helps control mosquito populations and thus potentially reduces the spread of mosquito-borne diseases.
Evolutionary Studies
Trinidadian killifish, particularly guppies, are famous for their rapid and observable evolutionary changes. Researchers study these fish to understand natural selection, sexual selection, and adaptive evolution. Key areas of research include:
- Predation Pressure:
- Guppies in high-predation areas exhibit different behaviors, coloration, and life histories compared to those in low-predation areas. For instance, guppies in high-predation environments tend to mature faster, reproduce earlier, and have more offspring, which is a strategy to ensure their genes are passed on before being eaten by predators.
- Sexual Selection:
- Male guppies are known for their bright colors and elaborate courtship displays, which are used to attract females. However, these traits also make them more visible to predators. Thus, there is a balance between attracting mates and avoiding predators. Female choice plays a significant role in shaping the evolution of male traits.
- Life History Traits:
- Studies have shown that guppies can rapidly evolve life history traits, such as growth rates, age at maturity, and reproductive strategies, in response to environmental changes. This makes them an excellent model for studying the principles of life history evolution.
Behavioral Studies
Behavioral ecology of killifish, including guppies, in Trinidad is another major research area. Studies focus on:
Conservation and Environmental Impact
- Schooling Behavior:
- Guppies exhibit schooling behavior as a defense mechanism against predators. The dynamics of group formation, leadership, and decision-making within schools are explored to understand social behaviors in animals.
- Mate Choice and Mating Systems:
- Researchers examine how females choose mates based on male coloration, courtship displays, and other factors. They also study how males compete for access to females and the impact of these behaviors on genetic diversity.
While Trinidad’s killifish are not currently endangered, they face threats from habitat destruction, pollution, and the introduction of non-native species. Conservation efforts focus on preserving their natural habitats and maintaining the ecological balance.
Research Contributions
Research on Trinidadian killifish has significantly contributed to the fields of evolutionary biology, ecology, and behavioral science. Some key findings include:
- Evidence of rapid evolutionary changes in natural populations.
- Insights into the balance between natural and sexual selection.
- Understanding of how environmental pressures shape life history traits.
Summary
Killifish on the island of Trinidad, particularly guppies, are a cornerstone of ecological and evolutionary research. Their diverse habitats, rapid evolutionary responses, and complex behaviors make them ideal for studying fundamental biological principles. Conservation efforts are essential to protect these natural laboratories and ensure that future research can continue to uncover the mysteries of evolution and ecology.
FISH ADJUST REPRODUCTION IN RESPONSE TO PREDATORS
UT Arlington research shows fish species evolve egg-laying habits when threatened
Some species of fish can evolve their egg-laying habits in response to predators in the area in order to survive, according to new research from The University of Texas at Arlington.
It has long been observed that organisms modify their traits, including reproductive patterns, in response to changes in their environment. This type of evolutionary plasticity has been observed in many types of animals in different habitats and with varying predators.
For his research, Dr. Walsh and biology lab technician Christopher Roden studied a small type of fish called the killifish that lives on the island of Trinidad in the Caribbean. Ranging in size from about 2 to 6 inches, the killifish are ideal for evolutionary studies because they are highly adaptable to their surroundings. Some types of killifish are even known to be amphibious, able to live on land to avoid predators.We knew that fish who laid their eggs externally often adapted depending on the predators in the area, but we did not know how quickly species could change to these externals pressures.
Professor Matthew R. Walsh, lead author. Department of Biology
University of Texas at Arlington, Arlington, TX, USA.
The researchers tested the differences in egg-hatching plasticity among killifish living in sites with and without predators. They then examined the reproductive habits of those two populations by measuring the rates of hatching when eggs were laid submerged in water versus outside water on the surface of moist peat moss. The timing, hatching and offspring growth rates between the two groups of fish were then compared.
Our study found striking differences in egg-hatching plasticity among killifish living in different habitats. This research provides new insights into how aquatic organisms adapt and evolve to changes in their environment. These findings may be particularly important in predicting how species are able to adapt to external pressures, such as those caused by climate change.
Professor Matthew R. Walsh.
AbstractWhat we can expect from the evidence of evolution such as this, as evolution is defined by science, is for creationists to pretend that evolution is something other than that defined by science, so they can triumphantly declare, "But they're still fish! None of them turned into something else!".
Externally laid eggs are often responsive to environmental cues; however, it is unclear how such plasticity evolves. In Trinidad, the killifish (Anablepsoides hartii) is found in communities with and without predators. Here, killifish inhabit shallower, ephemeral habitats in sites with predators. Such shifts may increase the exposure of eggs to air and lead to possible desiccation. We compared egg-hatching plasticity between communities by rearing eggs terrestrially on peat moss or in water. The timing of hatching did not differ between communities when eggs were reared in water. Eggs from sites with predators responded to terrestrial incubation by hatching significantly earlier compared with water-reared eggs. These responses were weaker in sites with no predators. Such divergent trends show that the presence of predators is associated with evolutionary shifts in hatching plasticity. Our results provide evidence for local adaptation in embryonic plasticity at the population scale.
1. Introduction
It has long been known that organisms exhibit the capacity to modify their traits in response to changes in environmental signals. This phenotypic plasticity is widespread across taxa and environmental stressors [1,2]. Research has also shown that eggs are responsive to environmental signals during development [3–6]. Egg developmental plasticity has been documented in a wide array of taxa including fish [7], reptiles [8], amphibians [9–12] and invertebrates [6,13] in response to stressors such as predators, pathogens, UV, desiccation and hypoxia. For instance, predator attacks induce faster rates of egg development and earlier hatching in red-eyed tree frogs (Agalychnis callidryas) [9]. Examples of embryonic plasticity that appear to be adaptive foreshadow that variation in environmental stressors has the potential to drive evolutionary changes in the responsiveness of eggs to the environment. In general, phenotypic plasticity is expected to evolve in spatially or temporally heterogeneous environments [14,15]. However, studies testing for evolutionary shifts in plasticity in egg development are lacking (but see [16]).
Many species of fish and amphibians lay eggs in terrestrial habitats that are intermittently exposed to air as they develop (reviewed in [3,5]). For instance, the mummichog (Fundulus heteroclitus) is a killifish that spawns in salt marshes during high tides by adhering its eggs to sea grasses [17,18]. These eggs are exposed to air during low-tide events and hatch upon re-immersion in water following predictable changes in tidal conditions. Plasticity in egg development and the timing of hatching is common in organisms that spawn in habitats where their eggs are exposed to air and may therefore desiccate. One strategy that has been documented is the ability to delay hatching upon air exposure (reviewed in [3,4]). Varela-Lasheras & Van Dooren [19] showed experimentally that the egg development rate of several species of non-annual killifish is slowed by exposure to air. Other studies have shown that some fish can accelerate rates of egg development upon experimental exposure to air [3,20].
Anablepsoides hartii is an amphibious species of killifish that is regularly observed outside water [21]. This species has been documented to emerge from water to avoid predators, aerially forage on terrestrial insects and disperse [22]. On the island of Trinidad, A. hartii (hereafter ‘killifish’) is found in communities that differ in the presence of predators [23,24]. This includes ‘high predation’ (HP) sites where they co-occur with several piscivorous predators (Crenicichla frenata and Hoplias malabaricus). In Trinidad, killifish are also observed upstream above barrier waterfalls in ‘killifish only’ (KO) sites, where they are the only fish species present. In HP sites, killifish experience increased mortality rates and are, in turn, found at lower densities with higher per capita resource availability [24,25]. Much research has shown that these ecological differences are associated with evolutionary changes in many phenotypic traits in killifish [24,26–29].
The presence of predators modifies the behaviour and habitat use of killifish in Trinidadian streams [30,31]. In HP sites, killifish are rare or absent from the open water of larger pools and instead occupy shallow, suboptimal and ephemeral habitats [30,31]. This includes shallow riffles, temporary side pools and rocky sections at the edge of streams, where the bodies of killifish are only partially submerged [30,31]. Furthermore, killifish lay eggs in terrestrial vegetation (leaves and roots) that is submerged in water at the edge of streams, and experiments have shown that they preferentially lay eggs in shallow water [25]. This is important because stream water levels are strongly influenced by precipitation that varies between wet and dry seasons each year. Therefore, any eggs laid in stream margins or side pools will potentially be exposed to air as water levels decline and such air-egg exposure will be increasingly probable during the dry season. More importantly, the shift towards shallower and ephemeral habitat in the presence of predators may, in turn, influence the frequency at which eggs are exposed to air and possible desiccation in HP sites. Such differences present the opportunity for divergent selection on embryonic plasticity.
Here, we tested for differences in egg-hatching plasticity in response to terrestrial incubation in killifish from HP and KO sites across two independent rivers. We reared eggs on the surface of moist peat moss or submerged in water. We then compared the timing of hatching and subsequent offspring performance (i.e. growth) between the fish communities. We had a priori expectations for how air exposure will influence hatching plasticity that is based upon the following background information: first, the volume of oxygen is higher on land versus in water and oxygen may diffuse faster across the egg membrane on land [32]. Second, similar to organisms that inhabit ephemeral habitats (i.e. annual fish and amphibians) [33], egg exposure to air represents a declining water level and, therefore, a habitat for killifish. Killifish eggs will rapidly desiccate under dry conditions but the likelihood that eggs will be re-submerged in water is unpredictable. Finally, killifish eggs need to be submerged in water to hatch. As a result, we expect that killifish eggs will respond to the terrestrial incubation treatment by hatching earlier versus eggs reared in water. If the influence of predators on the behaviour and habitat use of killifish leads to increased exposure of eggs to air and, in turn, modifies selection on egg-hatching plasticity, then we expect that the acceleration in the timing of hatching will be greater in HP fish.
[…]
5. Conclusions
We observed striking differences in egg-hatching plasticity between locally adapted populations of killifish (figure 1). Given the known impacts of predators on the behaviour and habitat use of killifish in the streams of Trinidad [25,30,31], our results provide new insights into how aquatic organisms adapt to increased exposure to terrestrial environments.
Walsh, Matthew R.; Roden, Christopher
Fish (eggs) out of water: evolutionary divergence in terrestrial embryonic plasticity in Trinidadian killifish Proceedings of the Royal Society B: Biological Sciences 2024, 291(2025) 20240083; DOI: 10.1098/rspb.2024.0083
Copyright: © 2024 The authors.
Published by The Royal Society. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
And there we have the difference between intellectually bankrupt and scientifically illiterate creationism, and the intellectual integrity of science. Scientists use carefully defined terms and test against them; creationists carefully avoid defining terms so they can avoid accepting contrary evidence and maintain their counter-factual superstition.
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