May 2005

 

The 'golden' jellyfish, Mastigias, recently evolved
from the 'spotted' jellyfish (also Mastigias, see figure below)
Photograph: M. N Dawson

How the jellyfish lost its spots

If we told you everything we know about how the jellyfish lost its spots, this would be a very short feature. We don't know how or why the golden jellyfish lost its spots, in part because we don't know why their ancestors, the spotted jellyfish, have them in the first place. What we do know is that this is just one aspect of a fascinating new story in evolutionary biology (see below) and that the jellyfish lost their spots in the marine lakes in Palau—so at least we know where to start looking for an answer!

We have heard several ideas about why the golden jellyfish lost its spots, each based on different assumptions about their function. For example, if the spots act as camouflage, breaking up the shape of the jellyfish and making them less visible to predators such as turtles in the ocean, then the jellyfish in the lakes may have lost their spots because there are no visual predators in the lakes or because there are visual predators but the spots no longer function as camouflage (the lakes are visually very different places from the ocean). Alternatively, if the spots are dumps of metabolic waste, changes in the diet or physiology of the jellyfish in the lakes may mean there is less waste, or it may just be easier to get rid of waste in the lakes. Finally, if the spots reflect sunlight, either providing some protection from too much sunlight or re-radiating sunlight at different wavelengths that can be absorbed by zooxanthellae, then medusae may not need the spots in the lakes if they can control their exposure to sunlight in some other way. It is possible that some, all, or none of these ideas are right. For example, there are no large visual predators of Mastigias in the marine lakes, but small gobies will swim several feet up into the water column to take bites out of the oral arms of passing medusae. The diet of lake medusae does differ from that of ocean medusae (Muscatine & Marian 1982; McCloskey et al. 1994) but the effects on waste production are not known. Medusae can alter their exposure to sunlight by migrating vertically, as described in more detail below.

Whatever the reason for the loss of spots, it is just one facet of a general reduction in pigmentation (see figure below). Ancestral spotted jellyfish which bask in the sun at the very surface of the ocean often are also pigmented with a translucent blue, particularly the smallest medusae. The blue pigment in these medusae is similar to 'cassio blue' which occurs in another rhizostome, Cassiopea, and is thought to be photoprotective (Arai 1997). In contrast, the golden jellyfish in the marine lakes almost invariably lack any blue colouration and, interestingly, these medusae move deeper into murky water as sunlight gets brighter. This suggests golden Mastigias may have traded morphological for behavioral protection from too much sunlight (see also Hairston 1980); one might say that, rather than applying sunblock, Mastigias in the lakes use an adjustable filter (like photochromic sunglasses) to avoid being sunburned. The relative benefits of morphological and behavioral photoprotection in Mastigias have not been investigated, but the latter potentially allows finer, more flexible, more immediate control of exposure and, conceivably, may have evolved through positive natural selection. However, this interpretation rests on the assumption that 'cassio blue' and similar pigments do provide photoprotection, which has been questioned (Blanquet & Phelan 1987). An alternative is that, as has been hypothesized for the spots, the blue colouration acts as camouflage. However, as for the spots, the cause of reduced blue pigmentation in marine lake Mastigias is not known, it could alsoconceivably be related to changes in food, other resources, or metabolism that preceded, co-occurred with, or followed, the modification of vertical migratory behavior (see more information on vertical migration).

Colour is not the only thing that Mastigias in the marine lakes have lost. Golden Mastigias also have reduced or no terminal clubs (glossary). The competing hypotheses in this case are (1) that the clubs are antipredatory devices, autotomizing when disturbed by a predator which then eats the club but not the medusa, (2) that the heavily pigmented clubs are dumps for metabolic waste, and (3) that they make ocean medusae more hydrodynamic so they can swim faster against the stronger tidal currents in the ocean. Again, more than one of these is possible.

One thing that the golden jellyfish did not lose in the lakes is their sting. The golden Mastigias still use cnidae to capture small prey in the lakes, as do the ancestral populations in the ocean. The common misunderstanding that these jellyfish lost their sting seems to come from the fact that their sting is just not very strong, barely perceptible by people except on their most sensitive skin. So, if you go to visit the jellyfish in Palau, there's no need to worry about their sting, you probably won't feel a thing.

 

Mastigias medusae from five populations in Palau show stages of gradual loss of spots as 'spotted'
medusae (left) that became isolated in marine lakes for between 5000 (centre-left) to 15000 (right) years
evolved new colours, patterns, and shapes. Blue pigmentation and terminal clubs were also reduced and lost.

Iconography and jellyfish evolution

Evolutionary iconography, such as the ascent of man parodied in the figure above, holds a particular place in the public understanding of science. It simplifies a complex process and, in doing so, presents an incredibly powerful message: evolution happens and it can have amazing results. The message embodied in a study of ancestral ocean (left) and derived marine lake (centre-left to right) populations of Mastigias medusae in Palau, is that dramatic evolution can be exceedingly rapid in marine taxa. This has important implications for the study or marine species world-wide. The diversity of Mastigias medusae shown in the image evolved in between 5000 and 15000 years. Moreover, the morphological radiation was accompanied by dramatic behavioural and likely also physiological adaptations to differing selective regimes. Feeding—energy and nutrient acquisition via symbioses with zooxanthellae, eating plankton, and uptake of dissolved organic matter—had to be balanced with each other and also with avoiding predation—for example by the jellyfish-eating sea anemone, Entacmaea medusivora, endemic to only a subset of lakes. The result is the evolution of five subspecies of Mastigias, each in a separate lake, during the Holocene in Palau. This radiation is analogous to other icons of evolution such as the three-spine sticklebacks of freshwater lakes in British Columbia, the cichlids of African rift lakes, the finches of the Galapagos Islands, Anolis lizards in the Antilles, and Drosophila in Hawaii. Moreover, it presents a forum for unifying theory on the evolution of 'island' species whether they are freshwater, marine, or terrestrial.

 

New news on jellyfish

The preceding short article was reprinted with permission from the most recent edition of the new current-marine-affairs magazine JMBA Global Marine Environment (GME; published by the Marine Biological Association of the United Kingdom). This, the second, edition also contains several other topical stories about scyphozoan jellyfishes:
"Temperature effects on moon jellyfish"
"Jellyfish invade power stations"
"Human jellyfish interactions" and
"Jellyfish may benefit from global changes"

This issue of Global Marine Environment precedes a special issue of the Journal of the Marine Biological Association of the UK (a venerable scientific journal first published in 1887) that focuses on the latest research on jellyfishes. Articles in the journal will cover a wide range of topics, some providing the scientific detail behind the topical stories published in Global Marine Environment. There are both original research articles and new reviews on key topics, including:
"Climate effects on jellyfish and ctenophore blooms"
"Predation on pelagic coelenterates"
"Gelatinous animals and physiology" and
"Renaissance taxonomy: integrative evolutionary analyses in the classification of Scyphozoa"

Copies of GME are available from the
Marine Biological Association (MBA) of the United Kingdom
by emailing jmba@mba.eclipse.co.uk
(£2.50 per copy plus postage)

 

Old news on jellyfish renewed

In addition to these new articles, three classic publications on scyphozoan jellyfishes have recently been given a new lease on life in digital form.

Two offerings can be found online at the Marine Biological Association of the UK:
»Kramp, P.L. (1961) Synopsis of the medusae of the world. JMBA 60. here
»Russell, F.S. (1970) The Medusae of the British Isles. Vol.II: Pelagic Scyphozoa, with a supplement to Vol. I. Cambridge University Press. here

The third is brought to you online by The Scyphozoan:
»Mayer, A.G. 1910. Medusae of the World, III: the Scyphomedusae. Carnegie Institute, Washington. here

The Scyphozoan bibliography also has links to reprints of some of the other older journal literature on scyphozoans.