The Accuracy of Archerfish

Front view into the mouth of an archerfish
Image: Sourced from
australianmuseum.net.au
[Accessed: 30 March 2014]

Toxotidae is a family of archerfish, specializing in hunting for insects and small creatures that are land based. There are seven different species in the family Toxotidae. They are found in a variety of environments, varying from estuaries, river systems, mangrove forests and even open ocean, and target prey that live in trees or on structures out of the water. They are known for their ability to capture their prey by spitting quick and accurate jets of water to knock their prey off their perch. Prey can vary from flies and beetles to larger prey such as small lizards.

This is an unusual ability is made possible due to the structure of the fish’s mouth, which has a narrow groove on the mouth roof. When the fish presses its tongue against the roof of its mouth, and closes its gills together, water is forced through and creates a long stream of water, which is used to dislodge prey, and bring them into the water for easy eating (Schlegel et al, 2006).

There are several challenges associated with targeting prey that sits at height, out of the water, when the predators are themselves under the water. The first issue is the optical properties of the water and air together. When firing a water jet, the fish must compensate for the refraction and angle of the target when moving between air and water, and the impact of gravity, as the image outside the water will appear at a different distance and angle (Temple et al, 2010).

Image: Photographer Andrew Lawson [Accessed 30 March 2014]

Image: Sourced from Caters News Agency
[Accessed 30 March 2014]

It has also recently been found that archerfish are able to precisely calculate the force at which they deliver their water jet, in response to the size of the prey target. This would inevitably be an energy conserving method, allowing their complex hunting method to be less costly. Archerfish were found to identify the size of larger targets, and deliver larger forces in their water jet, when compared to smaller targets where they delivered smaller forces in their stream of water (Schlegel et al, 2006).

Even fish, which had been raised in an artificial environment, where all advantages or force related adjustment were removed, were able to fine-tune their water stream to increase or decrease relative to prey size (Schlegel et al, 2006.

To gain further understanding of the amazing technique of this little fish, a BBC Earth video has been posted below.

Note: At 2:00 minutes onwards is a section on Velvet Worms, which is unrelated but still fascinating. Perhaps not recommended for those that aren’t keen on creepy insects with too many legs.

References

Schlegel, T., Shmid, C J., Schuster S., 2006, ‘Archerfish shots are evolutionarily matched to prey adhesion’, Current Biology [Online], 16(9), R836-R837. Available at: http://dx.doi.org.elibrary.jcu.edu.au/10.1016/j.cub.2006.08.082 [Accessed 31 March 2014]

Temple, S., Hart, N S., Marshall, N J., Collin S P., 2010, ‘A spitting image: specializations in archerfish eyes for vision at the interface between air and water’, Proceedings of the Royal Society of Biological Sciences [Online], 277, 2607 – 2615. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982040/

Doi: 10.1098/rspb.2010.0345 [Accessed 31 March 2014]

Mysterious Mantis Shrimps

Mantis shrimps are in the order Stomatopoda, and are marine crustaceans. They are roughly split into two different groups based on how they hunt for their prey: smashing, or spearing their prey to death.

Mantis shrimps are essentially the Swiss army knives of the sea. They have appendages that use a spring-loaded strike mechanism, which involves simultaneous contractions of muscles in the ‘merus’, which, through a series of complex steps, results in a final powerful release that pushes the appendages at force towards prey. Their raptorial appendages vary in structure between species, some specialized for spearing techniques and others specialized for smashing, with hammer-like clubs.

Top Image - Florida Fish and Wildlife Conservation Commision
Bottom Image – Adapted from a photo by Professor Roy Caldwell. [Both Accessed 21 March 2014]

“Spearers” utilize the very end of their raptorial appendages called the ‘dactyl’ and ‘propus’, and use it to slice and snare evasive prey. Spearing mantis shrimps are generally ambush predators, hiding in burrows dug out of the sand on the sea floor. Most of the time, only their eyes and antellules will be exposed. When an unfortunate creature such as a fish is to come within range of the mantis shrimp, it lunges from its burrow to snare its prey and drags it down to the safety of its burrow (deVries et al., 2012).

Image: Photographer ‘diverdk’ on Flickr.com [Accessed 22 March 2014]

“Smashers” tend to search away from their burrows and approach prey with harder shells, such as other crustaceans. They also have been recorded attacking cephalopods, including the Blue Ringed Octopus. They utilize both the spear tip and the club of the ‘dactyl’ to deliver high force blows over short periods of time, or to spear softer bodied prey (Patek, 2005). Due to their ability to deliver such forceful blows, the smashing mantis shrimps are able to prey upon marine creatures with shells, including clams, snails and crabs of various shapes and sizes.

It was originally thought that the spearing mantis shrimps struck at higher speeds than smashing mantis shrimps, as scientists thought that speed was the necessary component in catching quick, evasive prey. It was actually found that the speed was not the primary component needed in a strike, but accuracy.

Mantis shrimps are equipped with eyes that happen to be the most unusual in the animal kingdom. They are mounted on stalks, and move independently of one another. Mantis shrimps are able to see eight-channel colour vision, remarkable range finding, linear and circular polarization vision, and luminance and form vision (Chiou et al. 2008). These enable mantis shrimps not only to see everything in their predatory strike, but also communicate with each other using body colour signals. Colouring of the body in mantis shrimp species is thought to be involved in recognition of their own or other species by the individual. In mating rituals, it has also been seen that mantis shrimp fluoresce, which can be detected by the many wavelengths that they are able to see. One of the most colourful and well-known species is Odontodactylus scyllarus, the Peacock Mantis Shrimp.

Image: Photographer Gabriel Barathieu [Accessed 21 March 2014]

To fully understand the technique of the mantis shrimp, videos by the Smithsonian Channel and BBC Earth on Youtube have been posted below so you can fully enjoy watching a mantis shrimp grabbing or smashing down a meal.

[Note: Smithsonian Channel shows a smashing mantis, BBC Earth shows a spearing mantis]

References

deVries, M.S., Murphy, E.A.K., Patek, S.N. 2012. Strike mechanics of an ambush predator: the spearing mantis shrimp. The Journal of Experimental Biology [Online]. 215, 4374-84. Available at: http://www.ncbi.nlm.nih.gov/pubmed/23175528 doi: 10.1242/jeb.075317. [Accessed 22 March 2014]

Patek, S.N., Caldwell, R.L. 2005. Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarus. The Journal of Experimental Biology [Online]. 208, 3655 – 3664. Available at: http://jeb.biologists.org/content/208/19/3655.full doi: 10.1242/ jeb.01831 [Accessed 21 March 2014]

Chiou, T.H., Kleinlogel, S., Cronin, T., Caldwell, R. Loeffler, B., Siddiqi, A., Goldzien, A., Marshall, J. 2008. Circular Polarization Vision in a Stomatopodal Crustacean. Current Biology [Online]. 18, 428-434. http://dx.doi.org/10.1016/j.cub.2008.02.066 [Accessed 21 March 2014]