The strange escape strategy of the sea urchin
Paracentrotus lividus, more popularly known as the sea chestnut, is a herbivorous sea urchin with long and robust spines that abounds in the rocky bottoms and the seagrass meadows of the Mediterranean Sea and the Atlantic Ocean. It belongs to the echinoids, a class of echinoderms without arms and very slow movements, among whose characteristics stands out their ambulacral system – a set of tubes and chambers that run through the interior of the echinoderm and flow outward in the form of thousands of feet– of radial symmetry and with five rows of these little feet with suction cups.
These animals are endowed with an ancient and quite limited sensory system. When moving, in addition, sea urchins show a slow and somewhat erratic movement, however, despite what was previously believed, these beings related to the stars, the stars, the feathers and the sea cucumbers, can show a much more complex behavior than would be expected of them when fleeing from a predator.
When sea urchins smell a predator, they escape the threat by following a ballistic trajectory: straight, fast and directional
This is how at least one article describes it under the title The scent of fear makes sea urchins go ballistic is published this week in the magazine BCMMovement Ecology. According to its authors, led by the researcher from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona, Jordi Pagès, When sea urchins smell a predator, they escape threat by following a ballistic trajectory: straight, fast, and directional.
Blind, but not blind
To carry out their study, the team has the movement patterns of sea chestnuts compared in the laboratory depending on whether or not they are exposed to the smell of one of their main predators: the sea snail Hexaplex trunculus. Unlike other research, focused on the immediate response to the stimulus, the work focused on analyze the movement pattern of the hedgehogs for minutes, and even hours, after these perceive, or not, the presence of a predator.
“Until now, we knew that Paracentrotus lividus it could react to different chemical and light stimuli in the marine environment. For example, when they smell a predator or another wounded sea chestnut, they reduce activity and practically do not move from their shelters: holes or cracks in the rocks, “explains Pagès.” I knew how fast the reaction to a chemical stimulus could be, nor how it would translate into changes in the trajectory of movements if there were no shelters, “he adds.
“It is surprising that such ancient and simple invertebrates, without visual organs or central nervous system, show such diverse and complex movement patterns”
The work reveals that, in absence of stimuli, that is, without the smell of predators, sea chestnuts move very differently: from random and twisted trajectories without direction – known as Brownian motion-, up to displacements called super diffusive, which involve much more complexity, both in the trajectory and in the behavior of the animal. “It is surprising that such ancient and simple invertebrates; without visual organs or central nervous system, they show such diverse and complex movement patterns “, continues Pagès. “The ability to move with this variety of trajectories in the absence of the predator’s smell would surely facilitate its survival in an evolutionary way”, the author says. “In fact, there are fossil traces of echinoids in marine sediments that suggest that sea chestnuts already used these complex movements more than 50 million years ago. Therefore, it is believed that this ancient and well-preserved behavior must have been an effective strategy to find resources, such as food or shelter, scattered and sparsely distributed, as is often the case in the marine environment. “
A quick getaway in slow motion
Nevertheless, this all changes when hedgehogs smell a predator, the researchers explain. It is then that the wide range of movement patterns disappears and a single response is imposed; the ballistic movement: straight, fast and directional, typical of leakage reactions.
As an example, the work highlights that, in the absence of a predator smell, the average speed of sea chestnuts is around 8 centimeters per minute, while When subjected to the chemical stimulus of the predator, the echinoids reached a top speed of up to 15 centimeters per minute, with an average of 11.
The fast and straight-line escape is a fairly common pattern in prey that wants to flee from a predator that moves with a similar speed. “In the case of sea chestnuts, they can perceive the chemistry of their predators and react instantly by changing movement patterns. That is, they fear their predators and react accordingly. This response, coherent and consistent in all individuals, it has a clearly adaptive sense “, Pagès details. “Which makes us think that, just as there are optimal movement patterns for foraging, it also seems that hay optimized movements to escape predators “, Add.
A waterfall effect motivated by fear
Although studying this peculiar behavior of this animal may seem like a simple curiosity, the work is important because it provides new points of view to understand how this behavior can influence large-scale ecological processes. In ecology, cascading effects have often been described due to the changes that fear of predators generates in the behavior of prey. This is the case that Pagès explains about the Medes Islands, found in previous studies: “In protected areas such as the Medes Islands, where the predator population is higher and, therefore, the risk of being predated, also, sea chestnuts practically they do not dare to leave their refuges, which causes that the populations of algae are very abundant.
On the other hand, a kilometer further, on the Montgrí coast, where predator populations are much less abundant, sea chestnuts venture further out of the shelters and can completely consume the algae populations, leaving behind areas rocks completely empty, the so-called “white”.
The classical models to understand the interactions between predators and prey –the Lotka and Volterra equations– assume that the individuals of both populations move like the molecules of an ideal gas, with a Brownian pattern of motion, until they meet by chance. It is then that the predator consumes the prey. Recently, several studies have reviewed these models to add the option for predators to move in a more realistic, and therefore complex, way, but the focus has not yet been put on the fact that prey may also have the ability to modify their patterns. movement and adopt complex behaviors like this evasion of predation, “says the researcher.
“Our study shows that if a species as simple as the sea chestnut is able to react so clearly to a predator, it is worth incorporating this complexity into predator-prey models. Only then can we predict the coexistence capacity of predator and prey populations and understand their population dynamics in nature “, concludes Jordi Pagès.
- You enjoy history? Are you a photography lover? Do you want to keep up to date with the latest scientific advances? Do you love to travel? ¡Apúntate gratis a nuestras newsletter National Geographic!