Sharks that live in tropical areas are exposed to extreme seasonal weather events called hurricanes (aka typhoons or cyclones), which form over oceans near the equator. You have probably seen on the news, the incredible damage that tropical storms cause when they make landfall, but what you may not know is that hurricanes can also cause massive damage underwater, when they strike coastal habitats such as coral reefs and mangroves. So, if you are a shark living in the path of a storm, you could be seriously at risk... Unless you can predict it and get out of the way...
Why Do Hurricanes Form?
Hurricanes are formed when warm, moist air rises off the surface of the ocean. Rising air leaves an area of low pressure near the surface, which draws air in from surrounding areas. As this air swirls in, it too becomes warmer and more moist... and rises, creating a self-feeding system. The system eventually begins to spin faster and faster and an area of low barometric pressure (called the eye of the storm) is formed at the centre.
When hurricanes enter shallower coastal waters, extreme winds and associated wave action can cause damage to underwater structures; even smashing entire coral reefs to pieces sometimes. But how do marine animals, like sharks, survive a hurricane unscathed?
How Do You Study an Animal During a Hurricane?
It has been very difficult to study how sharks respond to tropical storms because it is too dangerous to observe them directly throughout extreme weather events. However, the development of "acoustic telemetry" technology has allowed us to track shark movements remotely (Heupel et al, 2003; Llerena et al, 2015).
This method involves tagging sharks with transmitters, which can be detected by an array of receivers that are placed throughout the shark's environment. As a shark swims around, it will ping off different receivers and we can map how it moved around (Llerena et al, 2015).
Acoustic technology is normally used to study the movement ecology of different species of sharks, but whilst researchers were studying juvenile blacktip sharks (Carcharhinus limbatus) in their nursery habitat in the Gulf of Mexico, Tropical Storm Gabrielle formed. This gave the scientists a unique opportunity to track how the sharks reacted to an oncoming tropical storm (Heupel et al, 2003).
A Storm's a'Brewin
Tropical Strom Gabrielle made landfall at 07:00h, 52km south of the nursery habitat. Acoustic monitoring data revealed that in the 12h period before the storm made landfall, the tagged sharks all had increased rates of movement within the nursery area, and between 01:30 and 05:20h, all 13 of the tagged sharks vacated the area entirely. After the storm had passed, all of the shark remained outside the acoustic array for 5 - 13 days, before returning and resuming their normal movement patterns (Heupel et al, 2003).
The researchers hypothesised that this flight behaviour had been triggered by a "cue" related to the storm that the sharks had detected. After investigating tidal height, wide speed and rainfall, they concluded that dropping barometric pressure, causing an associated drop in hydrostatic pressure was responsible; triggering the sharks to leave the nursery habitat (Heupel et al, 2003).
A Sixth Sense
Sharks are able to detect changes in atmospheric pressure via their "lateral line". This sensory system consists of a series of pores running along each side of the shark from head to tail, which are connected to two tubules lying under the skin. Water can flow into the tubules through the pores and strike sensory hair cells. The displacement of the sensory hairs causes nerve firing, which can be interpreted by the brain as a change in water movement. This allows sharks to detect prey, as they can actually sense animals swimming in the water. It also allows them to detect changes in hydrostatic pressure which occur as a result of changing air pressure (Ebert et al, 2021).
When the barometric pressure around the nursery area began to drop and then continued to drop at an increasing rate, as the hurricane approached, this acted as a cue to the sharks. Despite having never experienced a hurricane before and having rarely left the nursery habitat before (and never for an extended period), the juvenile sharks instinctively knew to move out of the path of the storm; seeking out deeper waters, where they would be safe from harm (Heupel et al, 2003). Remarkable!
References
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Heupel MR, Simpfendorfer CA & Hueter RE (2003). Running before the storm: blacktip sharks respond to falling barometric pressure associated with Tropical Storm Gabrielle. Journal of Fish Biology, 63, 1357–1363. Access online.
Llerena Y, C Peñaherrera, E Espinoza, M Hirschfeld, M Wolff and L Vinueza (2015). Nursery grounds of blacktip sharks (Carcharhinus limbatus) in mangrove-fringed bays in the central part of the Galapagos Archipelago. Pp. 103-110. In: Galapagos Report 2013-2014. GNPD, GCREG, CDF and GC. Puerto Ayora, Galapagos, Ecuador. Access online.
By Sophie A. Maycock for SharkSpeak
