Saturday, March 24, 2012

Flight is the new black

There are several kinds of animals that leave the water by jumping. A few have adaptations that allow them to increase the distance that they travel in a jump. For instance, flying fish and flying squid use their fins (and tentacles in the squid) to help them glide above the surface.

A flying fish in flight. Note the enlarged pectoral and pelvic fins to assist in gliding (photo Danielandmelora).
It's long been thought that jumping and gliding are an adaptations to aid in predator avoidance.  By leaving the water it is harder for predators to track their prey. And, prey that glide can change direction in the air to make it even harder for predators, because it becomes more difficult to determine where the prey will re-enter the water. 

A squid (probably Ommastrephes bartramii) in flight. Note the way the fins and tentacles (with membranes between the arms) are held for gliding (photo Geoff Jones). 
Recently, though, some researchers have suggested that squid might use aerial gliding to reduce the energetic cost of migration. They were able to collect data on the acceleration and velocity of squid in air from analysing photographs of jumping squid taken in rapid sequence. They found that travel in air was five times as fast as any measurements of squid movement in water. I'm a little skeptical of this claim. Mostly because speed is not a good way to measure the energetics of movement. 

Squid have two ways of moving. They can use their fins or they can use a jet propulsion system. The jet propulsion system is the primary form of movement in most squid, but often it's used in concert with the fins. And it's the jet propulsion system that squid use to exit the water. To move this way, the squid takes up water into its mantle cavity and then forces it out a narrow funnel beneath its head.  

The funnel of Illex illecebrosus, which is used in jet propulsion. The arrows in the image point to the lateral adductor muscles that support the attachment of the funnel to the head (photo M. Vecchione).

To help direct the water through the funnel, rather than back out the mantle opening, squid have a specialised apparatus to 'lock' the mantle shut. Cartilaginous pegs (one on each side) on the  inside rim of the of the mantle slot in to clips on the upper margin of the funnel. These lock the mantle closed as it contracts, which forces the water to squirt out through the funnel in a jet.

The funnel-locking apparatus of a flying squid, O. bartramii. The peg on the right is found on the inside rim of the mantle and fits inside the clip on the left, which is found at the top of the funnel. They clip together to lock the mantle opening shut and help to direct water through the funnel. The tissue in the image has been stained blue (photo R. Young).
Squid must achieve quite high speeds when using their jet propulsion system to exit the water. And the squid continue to shoot water out through the funnel to rocket through the air. Travelling at this speed is likely to have a metabolic cost because oxygen demands increase exponentially with increasing swimming speeds1. I suspect that, although jetting through the air is more efficient than putting the same effort into swimming, it’ll be more energetically efficient to travel at speeds that are too slow for flight. The effort required for flight may explain why squid are rarely seen jumping and have never been observed making repeated jumps. 

Jet propelled squid (probably Sthenoteuthis pteropus). Note the trails of water behind the squid, squirted from their funnels (photo Bob Hulse).

Another paper2 that's just come out shows a whole new group of swimmers are able to fly too. The animal part of the plankton, or zooplankton, is made up in large part by tiny crustaceans called copepods. Copepods swim by beating elongated antennae like oars, which moves them through the water in a typically jerky fashion. With just a single swimming stroke some copepods are able to exit the water and travel up to 17 centimetres through the air. Not bad for an animal that's only a couple of millimetres long. 

Copepod crustaceans make up a large part of the zooplankton. They swim by beating their elongated antennae producing a typically jerky swimming motion.

Just like flying squid and flying fish, copepods can travel further through the air for the same effort than they can travel through the water. But, the swimming speeds required to exit the water are extremely high. Indeed, the study reports that before they leave the water the velocity of the copepods was higher than the previously documented maximum swimming speeds of similar sized copepods.

The really neat thing about this paper is that the authors were able to document why the copepods were jumping. And it was not because it’s a more efficient way of travelling. It’s a way of avoiding predators. The authors observed the jumping copepods in the wild and found that they jumped in response to approaching predatory fish. And it seems to be a pretty successful escape mechanism too. Of the 89 escape jumps they observed only one individual was eaten.

You can read more about the jumping copepods and see a video of them jumping on the ScienceNow website.

1. Webber, D. M. & D'Or, R. K. (1986) Monitoring the metabolic rate and activity of free-swimming squid with telemetered jet pressure. Journal of Experimental Biology 126, 205 - 224

2. Gemmell, B. J., Jiang, H., Strickler, R. J., & Buskey, E. J. (2012) Plankton reach new heights in effort to avoid predators. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2012.0163

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