Instant Body Plans: The Case of Jellyfish
Jellyfish (phylum Cnidaria, including jellyfish, corals and sea anemones) may look like the "simplest" of animal phyla after sponges, but there's a lot more complexity than appears on the surface. These animals can move, migrate, and see their world. They have muscles, nerves, sexual organs and digestive systems. They build and carry stinging cells of complex design and function. An article in Current Biology by Katsuki and Greenspan from the University of California, San Diego, describes them:
The term jellyfish generally refers to the umbrella-shaped gelatinous zooplanktons that belong to Scyphozoa (true jellyfish), Staurozoa (stalked jellyfish), Cubozoa (box jellyfish), and Hydrozoa of Phylum Cnidaria. Their sizes, shapes, and habitats are diverse. Sexually mature jellyfish range from millimeters to meters in diameter, and they can be found almost anywhere in the ocean from the arctic to the tropics and from the deep sea to the shoreline (some even live in freshwater). Despite the substantial variety of their morphologies, some features are shared in common. First, they possess cnidocytes or stinging cells with which they capture prey and protect themselves from predators. Second, they have only one external opening for food intake, waste disposal, and gamete discharge. Third, they are radially symmetric around the single oral/aboral body axis, typically with a symmetry order of four or more. (Emphasis added.)
It should be noted that jellyfish are lumped in with sea anemones and corals in phylum Cnidaria -- even though their body plans and habits look different -- primarily on the basis of their all possessing intrinsic stinging cells. The assumption is that those stinging cells arose in a common ancestor. And although they look like comb jellies in some respects, Stephen Meyer in Darwin's Doubt shows that phylum Ctenophora (comb jellies) is very distant from Cnidaria according to molecular data (p. 125).
The Current Biology article describes the following tissue types in jellyfish:
- Muscles: "Contraction of the subumbrella muscles squeezes the bell and generates the thrust for swimming."
- Epithelium: "In some parts of the ring nerve, neurons are divided into compartments by extensions from epithelial cells, which may provide electrical insulation between neurites."
- Reproductive cells: "Mature medusae, usually gonochoristic (being either male or female), produce eggs or sperms for sexual reproduction."
- Nerves: "...they have radially distributed nervous systems that are adapted to their unique body plan... most jellyfish species show some degree of neuronal condensation that serves as an integrative nervous system."
The nervous system is particularly interesting. Not just an undifferentiated network, it has at least three "physiologically and histologically distinct" parts: the rhopalia, the motor nerve net, and the diffuse nerve net. These can be further subdivided:
The rhopalium is the sensory structure that contains ocelli (pigmented photosensitive structures), statocysts (gravity receptors), and pacemaker neurons that set the basic swim rhythm. The neurons in a rhopalium form several morphologically and molecularly distinct populations, some of which are interconnected via commissures, locally exhibiting a bilaterally symmetrical organization.... Most scyphozoans have eight or more rhopalia, the number of which is typically in multiples of four, around the bell margin....
The motor nerve net of scyphomedusae is the network of neurons that directly activates muscle contraction in response to the signals from the pacemakers....
The diffuse nerve net is believed to relay sensory information to the musculature indirectly by affecting the pacemaker activities, and directly by providing modulatory input to the musculature.... One Hydrozoa species, Aglantha digitale, has been shown to have at least 14 physiologically distinct pathways in the nerve rings....
The jellyfish nervous system is also capable of generating behaviors. Most of the time, jellyfish swim slowly, but they also have an "escape" mode when threatened. They can sense gravity, depth, sunlight, salinity, and the presence of other animals. They can use a "sun compass" for migration. They can see predators and evade them.
Cubomedusae (box jellyfish) are particularly interesting. They have eyes that are almost human-like!
As the name depicts, Cubozoans have a squarish shape with four tentacles and four rhopalia. Each rhopalium contains six eyes of four different types, two of which (the upper lens eye and the lower lens eye) are highly developed image-forming eyes with cornea, pupil, lens, and retina, much like our own....
Cubomedusae demonstrate a richer repertoire of vision-based behaviors, as expected from their state-of-the-art light-sensing organs. Tripedalia cystophora look up through the water surface with their upper lens eyes, and direct themselves to the mangrove canopy, their preferred habitat. Laboratory studies have shown that they use visual cues most likely through their lower lens eyes to locate light shafts and to avoid obstacles as they swim....
These behaviors require not only accurate vision but also precise control of speed and direction of swimming. This is achieved by modulating pulse frequency, creating an asymmetry in the opening of the bell, and delaying contraction on one side of the bell in response to light changes.
There's a lot going on in a "primitive" jellyfish! These are not simple blobs of jelly, but complex animals with multiple systems working together, from the molecular level to the whole organism.
So how did the jellyfish body plan evolve? The authors say almost nothing about that. They only offer it as a topic for discussion, presumably somewhere else by others:
Being an outgroup to Bilateria with complete nervous systems, jellyfish have been attractive models for studying the evolution of nervous systems. For example, by comparing the expression of genes involved in the development of sense organs with that of Bilaterians, one could ask the evolutionary origin of sense organs.
One could indeed ask. What is the evolutionary origin of sense organs? Anyone? What would be the function of a sense organ if there were not the nervous system to take in the signal and induce behavior? All the systems -- sensory, nervous, muscular -- have to be in place together, or else there is no sense in having a sense organ.
It would seem that intelligent design theory would be better placed to understand jellyfish. After their brief mention of evolution, the authors list interesting questions that we think could be explored within a design paradigm, without evolutionary assumptions:
From a physiological point of view, the relatively primitive architecture and behavior of jellyfish provide an opportunity to address how sensory inputs and internal information are integrated to produce coordinated motor outputs. Intriguingly, despite the simplicity of their neuronal organization, jellyfish appear to have the full battery of molecular machinery for neurotransmission and neuromodulation (for example, ion channels, traditional neurotransmitters, peptides, amino acids, small molecules, and their receptors). It remains largely unknown how this molecular variety contributes to the function of jellyfish nervous systems.
In Darwin's Doubt, Meyer mentions phylum Cnidaria as part of the Cambrian explosion, with one possible caveat: some paleontologists believe some are found in Precambrian strata. The diagram on page 32 shows Porifera (sponges) in the Precambrian, but Mollusca and Cnidaria are listed with question marks. If this phylum preceded the explosion, it had an explosion of its own: Wikipedia says, "The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around 580 million years ago, although fossils from the Doushantuo Formation can only be dated approximately." So it's not clear that the dates are right, but even if they precede the main explosion by 40 million years, they are already "modern-looking."
As for subphylum Medusae, Science Daily noted in 2007 the discovery of fossil jellyfish at least 500 million years old, and possibly older:
Using recently discovered "fossil snapshots" found in rocks more than 500 million years old, three University of Kansas researchers have described the oldest definitive jellyfish ever found....
With the discovery of the four different types of jellyfish in the Cambrian, however, the researchers said that there is enough detail to assert that the types can be related to the modern orders and families of jellyfish. The specimens show the same complexity. That means that either the complexity of modern jellyfish developed rapidly roughly 500 million years ago, or that the group is even older and existed long before then.
That's another way of saying that either jellyfish complexity exploded onto the scene 500 million years ago, or it exploded earlier "during the Cambrian radiation, a time when most animal groups appear in the fossil record, beginning roughly 540 million years ago." So either way, they just "appear."
No putative ancestors of jellyfish are mentioned in any of the articles. The fossil representatives are all "modern-looking," right from the start. As Meyer explains on pages 92-96 and 111-113 of Darwin's Doubt, even if a putative ancestor were found, it would create a deeper problem. An ancestor with cnidarian-like features would not serve as an ancestor of arthropod-like features. The common ancestor of all the Cambrian animals would have to be a kind of "shmoo" (an amorphous, blob-like cartoony thing):
On the one hand, to be plausible as a common ancestor of all the animal phyla, a hypothetical ur-metazoan must have few characteristics of later metazoan forms. Indeed, the more plausible the hypothetical ancestor, the simpler it must be, meaning it will lack more of the specific distinguishing features of the individual animal phyla. But that means any evolutionary scenario for the origin of the animals that postulates such a "stripped down" animal form as its starting point will need to envision those distinguishing characteristics arising later....
On the other hand, proposing a more complex (and more anatomically differentiated) common ancestor closer in its affinities to some Cambrian animal forms, would eliminate the need for so deep a divergence point. Nevertheless, it would also diminish the plausibility of such a hypothetical ancestor as an ur-metazoan common to all the other Cambrian animals.
So the fossil record shows jellyfish and other cnidarians appearing abruptly, in all their complexity from the start. This is just one example of at least 20 animal phyla that burst upon the scene in the Cambrian explosion.
Image credit: Wikipedia.