Venomous Fish

In the penultimate instalment of our venture into marine environments, the evolution of venom in fish will be discussed. Venom exists in both bony fish (belonging to the class osteicthyes) and cartilaginous fish (belonging to the class chondrichthyes). Some examples of venomous bony fish include the lionfish, stonefish, bullrout and catfish, while venomous cartilaginous fish consist only of rays. 

Surprisingly, venom in fish is actually more common than in reptiles, with 1200 species utilising venom.  The method of delivery can differ between some species. Most common delivery tool is the spine. In this context, a spine is a sharp appendage on a fish, not the backbone. Nearly all fish have spines of some sort. The benefit of these spines is fairly self-explanatory. They are a defence mechanism which make the fish harder to ingest by predators. 

The addition of venom to the spines of a fish is increasing the effectiveness of the defence mechanism by a huge amount. Being pierced by a non-venomous spine is painful of course, but it is nothing compared to the pain experienced by a victim of the stonefish, which is said to have the most painful sting of any animal, and it is claimed by some to be the worst pain known to man.

The evolutionary story of spines in bony fish is fairly simple. They are a skeletal supporting structure of the fins, and can be used to raise and lower them at will. The advantage of spines as discussed previously gave the individuals with the sharper spines an increased ability to evade predation, which increased the fitness of the fish.  The complication arises when we try to puzzle out how the fish developed venom glands to accompany the spines.  For this section, I will use the stonefish as an example, as it is well known, and possesses highly effective venomous spines. One theory is that venom arose in bottom-dwelling fish as an adaptation to kill bacteria, which can often infect fish that live in a detritus-rich environment. It is thought that the venom covered the skin and prevented bacterial infection. This could have led to larger venom glands being developed at the base of the spines. The theory bridges the gap between non-venomous and venomous species like the stonefish in a very logical way. Since the spines of fish are mostly hollow, the only adaptations needed to complete the journey from ancestral non-venomous fish to modern venomous fish are the joining of the gland and spine, the opening of the spine tip and the development of muscles around the venom gland. 

The skeletal structure of a fish, showing the location and configuration of the spines.

Venom is also present in some freshwater fish. The Bullrout can be found in fresh and brackish waters along the eastern coast of Australia. It is very similar to the stonefish in appearance, and has the same method of injecting venom.  The sting is said to be extremely painful, but not dangerous. Several species of catfish are also venomous. In freshwater, both the eel-tailed and fork-tailed catfish have venomous spines, and the striped catfish in saltwater is the same. Catfish spines differ from the bullrout and stonefish spines in that there are three (one dorsal, and two pelvic), and they do not inject venom. The spines are instead covered in venom secreted from glands alongside the spine. The venom is thought to have originated from skin secretions to prevent bacterial infections as mentioned earlier, and in some species, the venom is remarkably similar to the present secretions of the skin.

Much like the bullrout, the catfish is capable of inflicting a painful sting, but not a fatal one. As a clichéd anecdotal tangent, my grandfather once told me of a time he was netting catfish with a friend, and received so many catfish stings that all they could do was collapse on the riverbank and lie there in agony until the pain subsided hours later. Since a huge part of my childhood consisted of fishing in the same freshwater rivers of southern Queensland, I have always been wary of catfish. Anecdotes aside, stings from catfish are rare, and no fatalities have been recorded.

Bullrout: its 7 venomous spines are located on its back, much like a stonefish.

The eel-tailed catfish has 3 venomous spines, which are located at the front of the dorsl and pectoral fins.

We now move back to salt water, and into the class chondrichthyes, more commonly known as cartilaginous fish. The venomous fish to be investigated now is the stingray. Stingrays belong to the order Myliobatiformes, and the majority of families within this order are venomous. Stingrays deliver their venom using a barbed spine located at the base of their tail.  This spine is for self-defence only, and can be whipped forward much like a scorpion’s tail in order to strike its attacker. The spine is actually a modified “scale” of the ray. While the term “scale” was used for simplicity, the skin of cartilaginous fish is actually made up of placoid scales, which are small pointed spikes containing dentine, much like teeth. This gives rise to their other name of “dermal denticles”.  These types of scales are a superb adaptation for hydrodynamics, as they create tiny vortices in the water around them, which lowers drag considerably. In the case of the stingray, however, they have been modified over time, and have been developed into potentially deadly weapons.

The barb of a stingray.

The spine of a stingray is covered in a venomous skin, which is easily scraped off during defensive strikes.  This skin is called the integumentary sheath. The venom glands are located within two grooves running up the sides of the barb, called the ventrolateral grooves. The venom is released when the integumentary sheath is damaged or ruptured. Stings can cause quite severe local pain, as well as lacerations and tears from the barbed spine. The thrashing of the ray can exacerbate the damage done to the victim. Stings from rays are not uncommon, and are usually caused by stepping on them or handling them when caught on a fishing line. Stings to the extremities are not fatal, however many deaths have been recorded from stings to the body which damage vital organs.

The venom of stingrays is thought to have emerged in the same fashion as venom in other fish. As a secretion to prevent bacterial infection. It then became localised to the tail spine and refined into the venom it is today.

This image shows the barb in more detail, with the integumentary sheath easily visible. It is peeling off the tip, and some is congealing on the thumb of the gloved hand.

In this post we have investigated the different types of venomous fish in both freshwater and saltwater. The next post will be the final one to deal with venomous animals in a marine environment. 

 SOURCES

TEXTS:
Survivor of Stingray Injury to The Heart
Beatrix Weiss MBBS FRACS & Hugh Wolfenden MBBS FRACS (from MJA 2001; 175: 33-34)

The Diversity of Fishes : Biology, Evolution, and Ecology
Helfman, Gene; Collette, Bruce B.; Facey, Douglas E.; Bowen, Brian W.

Venom Evolution Widespread in Fishes: A Phylogenetic Road Map for the Bioprospecting of Piscine Venoms
William Leo Smith, Ward C. Wheeler

Survivor of Stingray Injury to The Heart
Beatrix Weiss MBBS FRACS & Hugh Wolfenden MBBS FRACS (from MJA 2001; 175: 33-34)


WEBSITES:
http://www.elasmodiver.com/Stingray_Barb_Pictures.htm - Accessed 15 May 2014
http://en.wikipedia.org/wiki/Placoid_scale#Placoid_scales- Accessed 15 May 2014
http://animals.nationalgeographic.com.au/animals/fish/stingray/- Accessed 15 May 2014
http://www.potamotrygon.de/fremdes/stingray%20article.htm- Accessed 15 May 2014
http://en.wikipedia.org/wiki/Placoid_scale#Placoid_scales- Accessed 19 May 2014
http://www.livescience.com/996-venomous-fish-outnumber-snakes.html- Accessed 28 May 2014
http://www.nytimes.com/2006/08/22/science/22fish.html?ei=5094&en=3d2f666379306107&hp=&ex=
1156219200&partner=homepage&pagewanted=print&_r=0- Accessed 28 May 2014
http://ns.umich.edu/new/releases/7453- Accessed 28 May 2014
http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2010/Nassar/pagereferences.htm- Accessed 28 May 2014


IMAGES:
http://www.infovisual.info/02/034_en.html
http://www.qm.qld.gov.au/~/media/Images/Find%20out%20about/Animals/Fishes/Venomous%20fishes/notesthes-robusta.jpg?mw=671
http://www.fishesofaustralia.net.au/images/image/N_pseudospinosus_hero.jpg
http://www.tankterrors.com/wp-content/uploads/2013/04/stingray-barb.jpg
http://www.oceanwideimages.com/images/16921/large/common-stingray-24M2755-13D.jpg
 

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

Marine Stingers And Their Deadly Touch

In this blog post I will be covering a mechanism of delivering venom that is a lot different than what has been covered so far. The animals that utilise this method are found in a marine environment, where venomous animals are quite common. There are over 1300 species of venomous fish that occur in the world’s oceans, as well as sea snakes and venomous molluscs such as the cone shell and the blue-ringed octopus. These organisms utilise mostly conventional methods of delivering venom such as spines in fish, fangs in sea snakes, or simply through the saliva in the case of a blue-ringed octopus.

The animals that will be covered in this post, however, are the Jellyfish. They are members of the phylum Cnidaria, which also includes organisms such as hydra and sea anemones. Almost everyone who lives in a coastal area and frequents the beach would be familiar with the sting of a jellyfish, although the severity of the sting depends on the both the species involved and the length of tentacle that has contacted the skin. That being said, many people will stop swimming when they are known to be present. I can say from personal experience that diving under a wave and surfacing with a bluebottle completely wrapped around your face and neck is a fast way to kill a good mood, not to mention the bright red tattoo left behind for a few days. 

While species such as the bluebottle are somewhat painful, they are not dangerous to humans. Venture into the warmer waters of tropical Queensland, however, and swimming during the summer months becomes more dangerous. Chironex fleckeri, or the Box Jellyfish, and species of irukandji make swimming during the summer months a dangerous venture without a stinger suit, or within a stinger net. The box jellyfish can kill a human being faster than any other venomous animal on the planet. A large enough sting can stop a person’s heart in under two minutes. In terms of venomous animals, there are none more potent than the box jellyfish. It is difficult to see in the water, and it is not very big, reaching sizes of up to 25cm across the bell. Symptoms of a box jellyfish sting include immediate severe pain, which may radiate up the affected limb, as well as red or purple tracks where the tentacle touched the skin, and cardiac arrest can occur within minutes from a large enough sting. There is an effective antivenin for the box jellyfish, however, which is more that can be said for the next species of dangerous jellyfish. 

The whip-like tracks of a box jellyfish sting.

Irukandji jellyfish, of which there are four species, also inhabits the waters of tropical Queensland. The species that will be covered in this blog post is Carukia barnesi. It is tiny, and almost invisible in the water, making it far more difficult to detect in comparison with the box jellyfish. It can also make its way into stinger nets, moving unseen through the gaps in the netting. The sting of an irukandji cannot kill as quickly as the box jellyfish, however the aftermath is far more painful.  The sting does not cause undue pain immediately, however after 20-30 minutes, what’s known as irukandji syndrome sets in. Symptoms of the irukandji include nausea, severe headaches, cramps and lower back pain, chest and abdominal pain, anxiety, and can cause tachycardia and a pulmonary edema. These symptoms can last up to 30 hours, and may not completely go away for 1-2 weeks. There is no antidote to the venom of an irukandji, and painkillers do not seem to work with victims. The only option left is to deal with the pain until it abates.

The size of the Irukandji makes it almost impossible to see in the water.

 

So how can these seemingly simple animals be so deadly? We know they sting using their tentacles, but they aren’t like any other venom delivery system we've covered so far. Being able to see something with the naked eye makes it more tangible, and less frightening. The unknown element is always the scariest, and the way that jellyfish deliver their potent venom seems to play on this fear, especially since all it takes is to be brushed by a tentacle for the venom to enter your body. The venom is delivered through specialised cells known as nematocysts, often simply referred to as stinging cells. They consist of a capsule containing a coiled up hollow tube under great tension, surrounded by a stiff membrane. When the cell fires, the coiled tube inverts and springs out of the cell at high speed. It then penetrates the skin of the victim and delivers venom through the hollow tube. Nematocysts are a defining characteristic of Cnidarians, and are only present in this phylum.

Each tentacle of the box jellyfish has up to 5000 nematocysts, so it is easy to see how being brushed by a tentacle can be so dangerous. While touching a tentacle will cause the nematocysts to fire, they are not in fact triggered by touch. Instead, they are triggered by chemicals present on the skin of the victim. This is an adaptation to preserve energy by preventing unnecessary firing of the cells, as nematocysts can only be fired once, and are energy-expensive to create. The actual mechanism of activation involves the sudden release of calcium ions into the cell. This creates substantial osmotic pressure, at which point water rushes into the cell, inverts the coiled tube and the cell fires. This all takes place in a few microseconds.

This diagram shows the pahses of the firing nematocyst.

We have now read about the characteristics of two of the most dangerous jellyfish, but we have not yet learned how it is that nematocysts actually evolved in cnidarians. This is exactly what will be covered in the next blog post.

 SOURCES:

 Mini-Collagens in Hydra Nematocytes 1991
EvaM.Kurz, ThomasW.Holstein, BarbaraM.Petri, JiirgenEngel and Charles N. David - Accessed 15 April 2014


http://animals.nationalgeographic.com.au/animals/invertebrates/box-jellyfish/ - Accessed 15 April 2014

http://www.biosci.ohio-state.edu/~eeob/daly/nematevolution.htm- Accessed 15 April 2014

http://health.nt.gov.au/library/scripts/objectifyMedia.aspx?file=pdf/26/02.pdf&siteID=1&str_title=Box%20Jellyfish.pdf- Accessed 27 April 2014



Images:
http://en.wikipedia.org/wiki/Irukandji_jellyfish  - Accessed 2 May 2014
http://local.brookings.k12.sd.us/krscience/zoology/webpage%20projects/sp11webprojects /boxjellyfish/boxjellyfish.htm - Accessed 2 May 2014
http://www.csulb.edu/~zedmason/emprojects/charlie/moffet.html - Accessed 2 May 2014