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