Who would have thought the noblest of fish may well have a lot in common with the tapeworm? In recent years a symbiotic relationship between the yellowtail kingfish and the southern stingray has been readily assumed in the shallow waters of New Zealand. As catching a kingfish for many anglers is the pinnacle of fishing, what is better to aid the angler than a large black undulating target, carrying several of the kingfish. An interesting question arises, why pelagic predators such as these accompany the southern stingrays?
Globally, certain ray-finned fish, like kingfish, belonging to the class Actinopterygii, are observed travelling with sharks, skates and rays, of the class Chondrichthyes. The ray-finned fish travel alongside, on top of and even briefly below the Chondrichthyes, and feed, or appear to feed in close proximity. This relationship in New Zealand between Dasyatis thetidis, our southern stingray, and Seriola lanadi, the New Zealand yellowtail kingfish, among others, is no exception. But how does the southern stingray assist an already top predator?
A phenomenal drone shot of two ray-riding kingfish by friend Tom Basset-Eason. In this instance, both kingfish swimming precisely in tune with that of the stingray.
One of the main characteristics that Chondrichthyes boast and Actinopterygians lack, is the possession of electroreceptors. Electroreceptors are composed of a series of minute, canal-like, pore openings, situated around the mouth of Chondrichthyes. Each pore is comprised of a gelatinous fluid leading to a rounded structure, called an ampullae; originally referring to a Roman Flask. Ampullae have a concentration of synaptic nerves and the complete structure is called the Ampullae of Lorenzini.
With some chemistry in mind, we can explain how these ampullae work in Chondrichthyes and why they indeed aid kingfish. This is only a brief explanation, for a more in-depth understanding, a detailed molecular path should be pursued.
The first thing to remember is that saltwater is conductive. Unlike freshwater, the dissociation of NaCl (salt) to Na+ and Cl- ions by water molecules permits an electrical current to be carried. These ions are mobile, they are free to accelerate and move in the presence of an electric field and so participate in an electrical current. Ions are atoms that have lost or gained electrons to stabilise. Consequently, for a charge to carry, all that is required is that an object has an electrical field so that a second object may receive the conducted current from this field.
All organisms produce these weak electric fields, called bioelectrical fields. Electric fields are more noticeable in a saltwater environment. Particles in the air, relatively speaking, are more spread out than those in a denser medium, such as a fluid. Thus, in air, for a charge to travel from one particle to the next is often not possible unless it is strong and highly concentrated, a rare occurrence in nature. Furthermore, the conductivity of salt water, due to the presence of mobile ions, allows a weak current to travel.
The production of these bioelectric fields is due to muscle contractions, respiration, osmoregulation, and even brain activity. For example, when you run from a lion, a signal is sent from your brain to relevant cells by means of an electrical current. This current is a series of electric explosions along your neurons, known as an action potential, again done by the manipulation of ions. Taking into consideration we are thought to have around 100 billion neurons within the human body, there is no shortage of electricity. With Professor Theagarten Lingham-Soliar stating “In behavioural experiments with sharks and rays, sensitivity to changes of 0.01 microvolt per centimetre (one microvolt = 1/1,000,000 of a volt) along the body surface has been found for the ampullae of Lorenzini.”
Of course, baitfish too emit these weak bioelectric fields. Many are agile, rapid in movement and display shoaling behaviour to combat predators. Consequently, their capture by a predator, such as a kingfish, is often made difficult. Unless the pack of predators work together (a sight to be witnessed), energy expenditure for one fish is too high and often not worthwhile.
The New Zealand flounder (Rhombosolea plebeia) is different. Its morphology is not consistent with that of agile bait fish, it does no shoaling to evade predators and nor is it built to outrun them. Rather, it solely relies on camouflage. By blending in with surroundings and scuttering up sand to conceal beneath, it hides its presence and thus appears to be perfectly adapted to live in an environment patrolled by vision reliant predators.
The remnants of a mornings hunt by a kingfish left in the net of Alex Waller. The image illustrating the near iridescent and well camouflaged juvenile flounder morphology.
If that sounds too good to be true, it is. The well-adapted flounder, a species who evolutionarily has done well to combat visual hunters, is readily disclosed to another species. Dasyatis thetidis, New Zealand’s southern stingray possesses the above-mentioned Ampullae of Lorenzini. With these electrical receptors, it scours the bottom in search of flounders, crustaceans and other potential prey who unwittingly emit a bioelectric field. Once compromised, the prey may attempt to flee from below the stingray, only to be exposed to the accompanying kingfish. It is no longer camouflaged on the bottom, nor hidden beneath the sand, and due to its shape stands no chance to out dash the accompanying predatory fish. A kingfish, by moving at a slow pace alongside the scouring stingray and allowing it to uncover its next potential meal, expends very little energy when contrasted to it actively hunting baitfish, and so this forms an ideal way of gathering food.
A kingfish in the net giving an insight into what it has been feasting on. The hand of Alex Waller, from company trippinontrout, holding numerous juvenile flounder (Rhombosolea plebeia).
A symbiosis in which one organism (called the parasite) benefits at the expense of another organism, usually of a different species (called the host) is a parasitic relationship. Consequently, the kingfish capitalizing on the feeding behaviour of the southern stingray, for the benefit of itself, similar to a tapeworm, is thus parasitic, if it as at the expense of the stingray. However, it is likely to be weak parasitism as the stingrays appear not to attempt to get rid of the kingfish, suggesting they are not prejudiced and still receiving an adequate amount of food.
It is further possible that the composition of their diets may well not completely overlap, the stingray perhaps eats a higher concentration of crabs and the kingfish flounder. The stingray’s diet, therefore, is largely unaltered by the presence of the kingfish. This could suggest that the relationship can also be classed as one of commensalism: in which one species gains and the other is neither hurt nor disadvantaged.
In my view, it is more likely that the relationship between kingfish and stingray in this regard is one of weak parasitism. I consider it likely that the kingfish do eat some of the organisms that would have been eaten by the stingray, were it not for the behaviour of the kingfish.
However, to get a better indication of the nature of the relationship, more evidence is needed. For example, an analysis of the gut contents of stingray and kingfish would need to be contrasted. Also, perhaps a comparison is made of the stomach contents of various stingray, some who have fed without the presence of kingfish and others with, in the same geographical area. This may allow a better indication of how similar the kingfish’s diet is to that of a stingray, and if it is to a point where the stingray may be disadvantaged.
It is to be noted that this type of symbiotic relationship between kingfish and stingrays is not unique. Chondrichthyan class members and Actinopterygian class members are observed together throughout the world, some common examples are trevally, permit, cobia and bonefish, all swimming with rays. In New Zealand specifically, trevally and kahawai too have been seen accompanying stingrays. Interestingly, kingfish also are observed with bronze whalers.
For the fisherman, this symbiotic relationship has unintended benefits. Southern stingrays are often large and dark in colour, thus a target that can be seen from a great distance. Furthermore, in comparison to predatory fish, stingrays are slow travelling, allowing fishermen to pursue the ray, and remain with the ray for a prolonged time. Moreover, stingrays congregate in our shallow environments, estuaries and harbours, giving anglers an opportunity to catch large, ocean-going fish within shallow, often very shallow, environments.
The purpose of the article is to hopefully give insight into a wonderful phenomenon observed in New Zealand. It serves to show that patterns, relationships and obscure events on the water can be explained. Moreover, may this article inspire you to do so with your own fishing conundrums, as the reward for understanding why a fish has been caught far exceeds that of catching it by chance.
Ichthyologist - Dr. Kirill Kuzishchin
“I do not distinguish fishing from research because it is a path. I am sure a good angler is a good scientist.”