Cnidarians and the Evolution of Nematocysts

In the last post we discussed the venom of jellyfish and the toxicity of different species. This post will cover the evolution of the nematocyst, and explore some other cnidarians which also possess these unique venom delivery tools.

Since all cnidarians have nematocysts, it is likely that they were a very early adaptation. It is understood that they are a result of a “post-Golgi vacuole”. The Golgi body is an organelle found in both animal and plant cells. It is responsible for packaging and processing proteins for secretion, storage or breakdown. A vacuole in animal cells can be used for protein transport.  So the term “post-Golgi vacuole” simply means a vacuole of proteins that has been processed by the Golgi body of a cell. The majority of the proteins in a nematocyst are mini-collagens, which are just very short collagen molecules. This vacuole is secreted, where it becomes a nematocyst. It is very difficult to be more specific on the evolution of nematocysts, and like many unique adaptations in the animal kingdom, a complete step by step evolutionary story is absent. Keep in mind that cnidarians are an extremely old phylum, and have had hundreds of millions of years to develop this adaptation.   

The age of the phylum Cnidaria also explains how some species have evolved to become so potent.  Jellyfish have been identified from periods of time as far back as 500 million years, so it is not hard to imagine how some species have developed such potent stings. The box jellyfish venom, which we know can stop a human heart in under two minutes, is one of the results of the immense time frame the jellyfish have had to hone their weapons.  

The oldest jellyfish fossil found, next to a modern jellyfish.

This brings about the argument of over-excessive venom potency. Why are some species so venomous? A dead fish is a dead fish, surely the toxicity of the box jellyfish venom is excessive? One answer is that jellyfish, in keeping with their name, are fragile.  The bell of a jellyfish can easily be damaged by the frantic movements of a fish it is ingesting, not to mention other creatures that view the jellyfish as potential prey. It is in the best interests of a jellyfish to kill or at least completely paralyse their prey as fast as possible.

A Jellyfish with two captured fish in its bell.

It’s time to investigate some other members of the phylum Cnidaria, which have been neglected so far. Corals, Anemones, Hydra and two sub-phyla of jellyfish are all cnidarians. Hydra and some species we view has jellyfish such as the blue bottle (or Portuguese Man o’ War) belong to the sub-phylum Hydrozoa. Box Jellyfish are in the sub-phylum Cubozoa, while jellyfish like the massive Lion’s mane jellyfish belong to the sub-phylum Scyphozoa, and are considered “true jellyfish”. The main differences between the two is that box jellyfish are unsurprisingly box-shaped, and have four main tentacles at each corner. Box jellyfish are also fast and agile swimmers compared to true jellyfish, and are able to see using eye-like organs in their bell. The final sub-phylum of cnidarians is Anthozoa. This contains the corals and anemones.  As we’ve discussed, all these cnidarians use nematocysts to capture prey. On a slightly less venom-related subject, one species of sea anemones actually have specialised tentacles used exclusively for fighting one another for the best real estate. Here is an excerpt from a documentary about anemones, showing both the function of a nematocyst, as well as two fighting anemones. 

  In the next blog post, we will continue exploring marine environments for venomous creatures.

SOURCES:

TEXTS:
Evolution of complex structures: minicollagens shape the cnidarian nematocyst 2008
Charles N. David, Suat Özbek, Patrizia Adamczyk, Sebastian Meier, Barbara Pauly, Jarrod Chapman, Jung Shan Hwang, Takashi Gojobori, Thomas W. Holstein

Venom Proteome of the Box Jellyfish Chironex fleckeri 2012
Diane L. Brinkman, Ammar Aziz, Alex Loukas, Jeremy Potriquet, Jamie Seymour, Jason Mulvenna


WEBSITES:

http://www.youtube.com/watch?v=WQEiYWGitKs - Accessed 12 May 2014

http://www.livescience.com/1971-oldest-jellyfish-fossils.html  - Accessed 12 May 2014

http://tolweb.org/Cnidaria - Accessed 18 May 2014
http://mesa.edu.au/cnidaria/default.asp - Accessed 22 May 2014
http://www.eoearth.org/view/article/151566/ - Accessed 28 May 2014

IMAGES:

http://cdn.c.photoshelter.com/img-get/I00002fW3noEXhfc/s/750/750/1018375.jpg

http://i.livescience.com/images/i/000/001/853/i02/071030-jellyfish-fossil-02.jpg?1296071467