Fish are the oldest known vertebrates, with the earliest fossil evidence dating back to the lower Cambrian period almost 530 million years ago (Shu et al., 1999). Since their inception, fish have exhibited a variety of different physical and behavioural traits to best exploit their environments. Over time, the effectiveness of these traits will be tested through competitive pressures or environmental factors.
This raises a rather silly but nonetheless interesting question; if we could design a ‘frankenfish’ using features from other fish, what would the best combination of traits be for our modern oceans? Will older trends still work today? Is there a fish now that is already perfect?
To help us answer this question, we will need to set a few ground rules:
The idea of a ‘perfect’ animal is incredibly subjective and does not follow any known ecological frameworks. For this thought experiment, our ‘frankenfish’ will need to be able to manage the impacts of climate change and global fisheries.
We will assume that the frankenfish must compete with existing species in the ocean. We can choose where we initially release our fish.
Other than a rapidly warming ocean, we will assume no catastrophic extinction level event.
We will assume that our frankenfish will survive long enough to reproduce at least once, ensuring the initial population is allowed to grow in size.
Considerations
Thermal tolerance
With mean ocean sea surface temperatures predicted to increase by 1-2 degrees Celsius in the next century (Mimura, 2013), we should first design our fish after more tropical or temperate species. If sea surface temperatures become too high, our new fish could move towards the poles. This phenomenon is known as a range shift (Rubenstein et al., 2023) and has already been performed by many different marine species in recent years. When looking at the larval stages of different marine organisms, those that live in higher temperatures are generally better-equipped to deal with changes in the surrounding temperature (Marshall & Alvarez-Noriega, 2020).
Trophic position
Although it would be fun to simply create a new apex predator, we will need to think of
trade-offs between energy expenditure, energy requirements and food availability. As a general rule of thumb, only 10% of caloric energy is transferred through each trophic level (Lindeman, 1942). Essentially, this means an organism at the top of the food chain will need to consume thousands of different organisms over its lifetime. Likewise, a lower-order organism will likely be a food source for a higher one but require less total energy to grow and reproduce over its lifetime. Essentially, there will be more room in the environment for lower-order fish, meaning more individuals can be placed, increasing the chance of successful future reproductive events.
Life history and reproductive strategy
In the world of ecology, species can broadly be categorised into 2 groups based on life history strategies: r-selected and k-selected species (Pianka, 1970). R-selected species tend to produce large numbers of offspring, develop quickly, and have higher rates of offspring mortality. Likewise, k-selected species develop slower, have less offspring but have higher rates of offspring survivorship.
Group behaviours
Fish often display group behaviours known as schooling and shoaling. Shoaling refers to a congregation of fish, whilst schooling requires coordinated movement of fish in the same direction.
By grouping together, fish have less individual risk of being eaten by a predator and the group’s ability to sense danger is also heightened. Furthermore, schooling behaviour can reduce the energy an individual fish spends whilst swimming by 20% (Marras et al., 2014). Group behaviour may also lead to confusing an inexperienced predator (Magurran, 1990), though many modern predator species have adaptations to take advantage of shoals and schools.
There are some drawbacks to group behaviour. Firstly, fish will have access to less food individually as enough food will need to be distributed across the group. Secondly, groups which grow too large attract large numbers of predators and lead to ‘bait balls’, which is essentially a floating buffet for any larger animal.
Group behaviour is incredibly common in lower-order fish but is also exhibited in higher order predators such as Tuna and some shark species. It is estimated that almost half of all fish species will partake in group behaviour at some point in their lifecycle.
Scales, Plates and Skin
The structure of skin has implications for the hydrodynamics of an organism, influencing the level of lift and drag. The type of skin will also influence protection from parasites and predators. We will briefly discuss two types of scales, but other specialised scales exist.
The skin of cartilaginous fish (sharks and rays) is composed of microscopic interlocking teeth-like structures known as placoid scales. The unique design of placoid scales facilitates the formation of small whorls whilst moving, reducing the drag experienced by the fish (Helfman et al., 2009, pp. 23–41). Placoid scales also act as a parasite deterrent, comparable to antifouling designs in modern cargo ships.
Alternatively, many teleosts (bony fish) are covered in larger (non-microscopic), thinner scales known as leptoid scales (Helfman et al., 2009, pp. 23–41). These are further differentiated into circular and toothed scales (Helfman et al., 2009, pp. 23–41). Circular scales are smoother and uniformed, whilst toothed scales are rougher. Similar to placoid scales, leptoid scales reduce drag experienced by the fish (Roberts, 1993). Additionally, leptoid scales can be highly reflective, allowing for a unique form of camouflage known as silvering (Herring, 2001).
Another thing to consider is colour. Red light is almost invisible past 40 metres of depth (National Oceanic and Atmospheric Association, n.d.), whilst blues and greys can. provide better camouflage from predators above and below you through countershading (Ruxton et al., 2004).
Extra features – toxins, slime and light
These are niche defence mechanisms which reduce the risk of predation.
When agitated, Hagfish are able to release a thick, quickly expanding mucus from their skin, blocking the gills of an attacking fish (Zeng et al., 2023). Hagfish are only able to remove excess mucus on their skin by creating a knot with their own body (Böni et al., 2016), which is possible thanks to their eel-like shape. This design may not translate well when creating our own perfect fish, as the elongated shape limits it to the bottom of the ocean (Friedman et al., 2020).
Other fish, such as some species of pufferfish, house bacteria in various organs that produce toxins which pool in livers and ovaries. A downside with toxins is that they only work if an attacker is already aware of their effect, meaning at least 1 pufferfish was consumed in the past. Furthermore, some fish species can ignore the effect of certain toxins. Toxin-producing bacteria is acquired through diet, which could limit the dietary range of our frankenfish. Other species of fish such as lionfish, stonefish and
some catfish contain specialised venom glands which release toxins along the spines of their fins, which is considered a more efficient delivery method. Even without toxins, sharper fins can act as a deterrent for predators from swallowing you whole.
Fish living in deeper waters tend to display bioluminescence, which causes them to produce light with the help of bacteria. This has numerous benefits including startling predators, camouflage, attracting food, and in unique cases allows an animal to see red pigments deep underwater (Young & Roper, 1976; Herring & Cope, 2005). As a downside, humans tend to exploit bioluminescence and use it to find large groups of fish and squid.
Past and current champions
The armoured fish
The armoured fish, known as Placodermi, were a widespread group of fish who were prominent during the Devonian period (419 – 359 mya). The Placoderms are subdivided into 8 orders based on body shape characteristics, the most successful of which was known as Arthrodira.
Species in Arthrodira occupied a variety of different niches from apex predators to detrital feeders, but all shared the common feature of jointed armour plates near the neck and face.
The Placoderms were never outcompeted in their 60-million-year run. Instead, their time on Earth was cut short by multiple catastrophic events associated with the Late Devonian extinction. This could suggest that without random chance, the Placoderms would never have been dethroned.
Sharks
Sharks emerged at a similar time to the Placoderms but managed to survive the Late Devonian extinction events. Sharks have a cartilaginous skeleton as well as electromagnetic receptors known as Ampullae of Lorenzini, which are used to detect prey activity. The body plan of sharks has stayed relatively consistent over the last 400 million years, and they’ve managed to survive various extinction level events. The only issue with sharks is their value to humans, leading to millions of sharks being harvested for fins each year.
Sharks are a k-selected species and produce only a handful of young. Most sharks deposit a handful of eggs which are protected by a casing and filled with yolk,
increasing the fitness of a successful juvenile but also increasing the chance of predation removing it from the gene pool. Smaller egg clutches also mean the loss of a young shark has a higher relative impact on a population compared to a mass spawning species.
Bristlemouths and Lanternfish
These are similar families of fish and are some of the most abundant vertebrates on the planet. Unlike sharks, these fish are R-selected. Otolith (fish ear bone) samples suggest both families rose to prominence at least 5 million years ago (Přikryl & Carnevale, 2017; Schwarzhans & Carnevale, 2021) due to a massive bloom in phytoplankton.
Out of these 2 groups, the Bristlemouths are the most abundant. Although survey data from the deep ocean is rare, prior studies revealed between 70-80% of all deep-sea fish were a variation of a Bristlemouth (Sutton et al., 2010). Despite their abundance, not too much is known about the Bristlemouth due to the depths they inhabit; 1000- 2000 metres.
Meanwhile, Lanternfish are responsible for displaying a rising and falling ‘false sea floor’ in early sonar technology, known as the Deep Scattering Layer (Carson et al., 1951/1991). Movement of the layer is attributed to Diel Vertical Migration, a phenomenon where fish will move up and down the water column at certain times of day to avoid predation (Ritz et al., 2011).
Constructing our fish
Despite the historical success of the Placoderms, current trends in prey behaviours and morphology means armoured jaws are unlikely to be very useful in modern oceans (Bellwood et al., 2015). Furthermore, armoured plates will be heavier compared to scales or cartilage, meaning excess energy will have to be gathered via predation.
Given that the oceans are abundant in second-order consumers such as zooplankton and planktotrophic fish, it may be worthwhile to make our new fish a third-order consumer. The sheer abundance of bristlemouths and lanternfish should make up for the inefficiencies of higher trophic levels. Habitat-wise, our new fish should adopt a pelagic (open ocean) lifestyle to best take advantage of the abundant smaller prey animals.
When thinking of behaviours, our fish taking a nocturnal approach would work best to exploit the previously mentioned diel vertical migration behaviours seen in
bristlemouths and lanternfish. This also allows for daytime predator avoidance, providing our fish the best possible chance to grow in numbers and proliferate.
Given the trophic position of our fish, it is reasonable to also give it the capability to form schools and shoals. The group energy costs can be offset by the abundance of prey species, which also exhibit group behaviour.
The best place to release our new fish would be somewhere in the mid-latitudes. This would make it more tolerant to higher temperatures and the percentage of global ocean area is only expected to increase in the near future (unless humans can somehow revert anthropogenic climate change).
Our fish should be relatively slender and be red in colour. In theory, when combined with the depth of habitat, this will make our frankenfish almost invisible to organisms without additional specialised adaptations. Taking a page from the squid playbook, small bioluminescent regions along the top half of the fish would provide some further camouflage from predators looking down.
The spines on our fish’s fins should be longer and sharper than average. For fun, we can also give our fish a venomous gland. Combining long spines with venom could dissuade some predators from eating our fish, through either awkward positioning or risk of poisoning.
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