He Can't See Us If We Don't Move!
Updated: Apr 26
You might imagine that sharks are some of the toughest animals in the oceans. Whilst this is true for some species, sharks do not materialise fully formed; they are more vulnerable to predation when they are young and small. This is especially true for species which reproduce by "oviparity" - laying eggs. As the mother does not watch over the eggs before they hatch, the young are extremely vulnerable to predators... But do these tiny embryos have some awareness of the treats around them? And some way to defend themselves? New research shows us that they do!
The brownbanded bamboo shark (Chiloscyllium punctatum) is an oviparous species living in the tropical Indo Pacific from China to Australia. The female lays her eggs on the ocean floor (known as the "substrate"), in shallow regions, like coral reefs. Whilst the female will often try to find a sheltered spot for the eggs, where they might be more difficult for predators to spot, she leaves them to develop on their own, without her protection. They hatch when they reach approximately 10–12 cm in length (Compagno, 1984).
The embryos remain within their leathery egg case for up to five months, whilst they develop. As the juvenile sharks are unable to escape predators by swimming away at this point, they have evolved to be harder for predators to detect. The pigmentation patterns of the egg case offer it some camouflage and, as the egg case is sealed tight, there is no exchange of fluid which might allow predators to smell the embryos (Kempster et al, 2013).
However, during later embryonic stages, the bottom edge of the egg case starts to weaken and the seal opens. At this point predators are able to detect the embryos by "olfaction" (smell) and also by "mechanoreception" (detection of the displacement of caused by the near-term embryo moving). During the later stages of development, the embryos will begin to undulate their tail in order to circulate oxygenated sea water within the egg case. Clearly, this would massively increase their likelihood of detection by predators (Kempster et al, 2013).
However, recent research has shown us that these sharks are able to detect the presence of predators and protect themselves before they are even hatched! At the later stages of embryonic development, the tiny sharks will stop beating their tail and curl it up around them when a predator is nearby. They will also stop moving their gills so much. Basically, they stay very, very still and hold their breath so they don't get noticed. This kind of behaviour is known as a "freeze response" (Kempster et al, 2013).
A research team discovered this by studying the developing embryos in the lab. They set the embryos up in a tank, where they could watch their tiny movements through a video camera. They then simulated the presence of a predator by turning on electrodes; this tiny electrical current mimicked the natural electricity generated by predator (a beating heart, moving muscles and nerves firing all generate minuscule electrical fields). A stronger electrical stimulus would mean a predator, potentially a larger predator, is nearby. The scientists counted the tail beats and the gill movements, in order to determine how these changed when the shark embryos detected a predator was close (Kempster et al, 2013).
The discovered that embryos at an earlier stage of development did not show any response to the predator stimulus, but older embryos showed more and more of a response. Therefore, they concluded that late-term embryos must be capable of "electroreception" and have an innate behavioural response, that they adopt to reduce their likelihood of detection.
This means that these tiny sharks are much more aware than we previously thought and they have evolved advanced defensive behaviours to protect themselves from predators (Kempster et al, 2013).
Cute and smart!
If you would like to learn more about shark embryos (and watch more adorable footage of sharks before they are born), you can check out Swimming in the Womb.
Compagno LJV (1984). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO Fisheries Synopsis.
Kempster RM, Hart NS & Collin SP (2013). Survival of the stillest: Predator avoidance in shark embryos. PLoS ONE 8:1, e52551. Access online.