by Matthew Norton
When we think of box jellyfish it is their deadly sting, easily fatal to humans if not treated quickly, that comes to mind. What is less well known is their remarkably complex visual system, for such a simple organism, with a grand total of 24 eyes arranged among four sensory clubs, called rhopalia.
Many organisms are able to detect light, even when they can’t actually ‘see’ the world around them, when it is absorbed by light sensitive pigments embedded. The chemical reaction that is triggered produces an electrical signal that tells the organism that light has been detected.
But within each rhopalium of a box jellyfish, there are distinct eye types that include a pair of eye pits and a pair of eye slits. Both of which contain light sensitive pigments, and an upper and lower camera lens eye. The simple structure of the pit and slit eyes suggest that their primary function is detecting and monitoring the level of light in their surroundings. However the discovery of a lens-like structure in the slit eyes may suggest that they may have some limited ability to interpret images of the world around them.
Both camera lens eyes are similar in structure to the eyes of many vertebrate animals (including humans). Theoretically we could say that box jellyfish can ‘see’ the world around them, although the ‘quality’ of their sight is likely to be very limited compared to our own, for two very good reasons. Firstly, the lens is too close to the retina, thus inhibiting their ability to focus images, and secondly, it is doubtful that the box jellyfish’s simple nervous system has the processing power to produce a coherent image.
Despite these limitations the complex eye arrangement is still put to good use, helping box jellyfish perform visually guided behaviours such as avoiding obstacles and altering swimming speed and direction in response to changes in light intensity. Each rhopalium helps to guide these behaviours by transmitting electrical signals, nicknamed “swim pacemaker signals”, which directly control swimming contractions.
Furthermore observations from some species suggest that box jellyfish could also see in colour. For example the camera lens eyes of Tripedalia cystophora are especially sensitive to blue-green light, and Chironex ﬂeckeri has shown a curious ‘figure eight’ swimming pattern in response to blue light. In theory colour vision would also be very useful to box jellyfish in shallow water as glare from the water surface could confuse any creature that relies on changes in light intensity to see, whereas the contrast between different colours would be unaffected.
However, there are convincing counterarguments. In most species the camera lens eyes only possess one type of light sensitive pigment which, despite being sensitive to a particular colour of light, still leaves them colour blind. Also, the idea that it helps to overcome surface water glare is a very general theory on the evolution of colour vision and may be more applicable to other animals. Furthermore, the close proximity of the lens to the retina in the camera lens eyes has been speculated as a possible alternative to overcoming surface water glare.
From a human perspective
The arrangement and capabilities of the many eyes that box jellyfish possess is definitely fascinating, but there are at least two ways in which an understanding of how box jellyfish see the world may benefit us.
Firstly, existing measures to reduce contact with human swimmers (eg. protective clothing, beach warning signs) could be improved by exploiting the visually guided behaviours of box jellyfish. For example the use of dark coloured material in protective wetsuits may increase the likelihood of a box jellyfish perceiving the wearer as an obstacle, and actively avoid them.
Secondly the similarities between the human eye and the camera lens eyes of box jellyfish could, in theory, be used to develop treatments for malformations in the former. Some such malformations, such as displaced pupils, have been linked to mutations in a ‘master control gene’ in eye development, Pax6. PaxB, a gene which performs similar functions to Pax6, has been found in the camera lens eye of Tripedalia cystophora, which makes it a potentially useful ‘model species’ to test possible treatments. However, there are still some structural and developmental differences between human and box jellyfish eyes. Therefore Tripedalia cystophora should probably only be used in the early testing stages of such treatments.
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All other images are public domain and do not require attribution
One thought on “Box Jellyfish- For your eyes only”
Brilliant collection of marine life- so good to have a blog like this if you’ve never studied it before and especially in light of the destruction going on under the waves. This just shows us what treasures there are down there and how essential it is to protect “this Life”.