Jellyfish are different, at least some of them. The comb jellyfish “Pacific sea gooseberry” (Pleurobrachia bachei) hunts for food. It is a predator, paddling through the sea, and grasping its prey with sticky tentacles.
|Pacific sea gooseberry|
(from Wikipedia Commons)
Recent analysis of the Pacific sea gooseberry revealed a fascinating detail, making them much of an alien . Their nervous system miss the habitual set of components that are found in most other animals. The chemical signals (neurotransmitter) of the Pacific sea gooseberry, which make the nerves work are different. The Pacific sea gooseberry does not use the set of chemical signals that we know from most animals.
Without question, the Pacific sea gooseberry has a fully developed nervous system. It consist of a network with a ring of nerves around the mouth. The nervous systems senses light, detects prey and coordinates moves of muscles. However, the nervous system performing these operations is using different chemical signals. That is less than a detail. It indicates that nervous systems evolved on Earth twice, at least. The deciphered gene code of the Pacific sea gooseberry shows this.
Other differences between Pacific sea gooseberry and other animals are found for immune and development genes, and respectively for the related physiological processes.
Whether comb jellies descend from ancient organisms that lived 580 million years ago (in the Ediacaran [*] ), as some speculate, is a provocative hypothesis. If that is the case, then the Pacific sea gooseberry may trace that ancient world to present times and indicate how complex this ancient world was already. Ecosystems with prey-predator relationship existing more than half a billion years ago!
|A comb jellyfish |
(from Wikipedia Commons)
A less precocious hypothesis would be that the evolution of Pacific sea gooseberry included the replacement of the chemical signals that we know to be common in nearly all animals.
What should be noted, beyond any speculation, that the sea is full of treasures. Many we only do not know; one more was discovered recently.
Understanding that nervous systems, immune systems or development process can be constructed from different building blocks is deep insight with possibly far-reaching consequences, be it for regenerative medicine or synthetic biology .
Comb jellyfish [**] are classified as a sister group to jellyfish and sea anemones. This kind of animals are ancient and lived in the ocean since very long time, even if the modern species evolved more recently. These animals have in common that the distinction between head and rears does not apply to them as it applies for slugs, fish and humans. This feature makes them simpler and they are put at the bottom of the tree of life, comb jellies among them. Thus it is useful to assume that similar functions evolved several times along parallel paths.
[*] The Ediacaran (635 - 542 million years) is the geological period preceding the Cambrian Period. The Ediacaran biota have little resemblance to modern lifeforms and include the oldest organisms with tissues; hard-shelled animals had yet to evolve.
 Ewen Callaway (2014) Jelly genome mystery, Nature (509) p. 411
 Leonid L. Moroz et al. (2014), The ctenophore genome and the evolutionary origins of neural systems, NATURE, doi:10.1038/nature13400