by Prachi Dadhich
Embryos of many species are often considered completely defenceless and highly vulnerable due to predation and pathogen infections. They are not able to run away or even protect themselves from egg predators. However, recent studies have shown that amphibian embryos are not completely helpless. They have successfully developed an antipredator defence. This means they are capable of defending themselves by hatching early in response to different environmental conditions. This process is known as environmentally cued hatching. This enables them to lower their mortality level.
One of the frequently studied species in this context is the Red-eyed tree frog. The embryos have developed behavioural antipredator defences against multiple predators. It inhabits wet forest stretching from the Yucatan to Panama. The species is covered in vibrant colours ranging from bright green, yellow, blue to red. Generally, frogs with bright colours are considered as poisonous. However, this species uses its colours for camouflage in order to protect themselves from different predators. They feed on different insects like flies, moths, crickets and many more to fill their belly. As arboreal frogs, they are known to lay their eggs on the vegetation overhanging a water source. The eggs are always found in a clutch glued together with help of a jelly. After hatching there is switching of habitat from trees to swamps and ponds.
Embryos are constantly surrounded and threatened by various aerial and arboreal predators. Most common ones include egg-eating snakes, Polybia– the social wasps, and egg killing fungi. Out of all these, snakes are considered as the primary predator. It has been observed that snakes consume half of the eggs at some particular sites. Even tadpoles face many aquatic predators like the freshwater shrimp, Macrobrachium americanumand the poeciliid fish Brachyraphis rhabdophora.
Depending on the threats by different predators they have adapted various approaches. According to a study in Costa Rica by Karen Warkentin it was noticed that normally eggs hatch in six to seven days. However, it will hatch as early as four days when they sense any threat. Therefore, this species provides a case of adaptive plasticity in hatching timing.
Several studies have been done to answer one question: how do these embryos sense danger and hatch early? K. Warkentin, a biology professor at Boston University studied the reaction of embryos in presence of different predators. In one of her studies, she noticed that eggs hatched in 16 seconds after first snake contact. Surprisingly, this response was not the result of chemical or visual cues. Indeed, only vibrations from the snake was enough to induce the behavioural response in hatching. The embryos were smart enough to differentiate between vibrations caused by benign disturbances like tropical storms and an egg eating predator. The author concluded that the eggs hatched 30% earlier and almost 80% escaped successfully when threatened by an egg eating predator.
Other studies covered the interaction between Polybia rejecta– the social wasp and the eggs. The little embryos have the same response in the presence of wasps as for snakes. But the timing and the way eggs hatch are different. Unlike snakes, the wasps attack only one embryo at a time leaving the jelly undisturbed. The wasp holds a single egg with help of its mouthparts and then pull it off from the clutch. If there is difficulty in removing the egg, the wasp will only break the vitelline membrane but leave the embryo inside. Whenever there is a physical disturbance, a wriggling movement in the embryo is noticed followed by the tearing of the egg membrane. Apparently, the entire clutch hatches when a snake attacks because it tends to eat a lot of eggs. Whereas a single egg hatches during a wasp attack as it targets one embryo. Therefore, the study claimed that the embryos respond in accordance with the scale of the risk.
In addition to wasps and snakes, different fungi and water moulds are also a threat to eggs. In one study, Warkentin noticed that just like other predators, fungi do not provide any vibrational cue. So, the question arises how do eggs hatch early when a fungus attacks the clutch? There are three different mechanisms explaining this process:
- Eggs sense the chemicals released from fungi or the already attacked embryos which leads to early hatching.
- The fungi cover the outer egg surface which is mainly used for oxygen diffusion. The low level of oxygen accelerates the hatching.
- The fungal hyphae tend to grow in the perivitelline space of the egg. The contact between hyphae and embryo causes irritation which leads to vigorous movement of the embryo and rupture of membrane.
A.callidryas embryos are vulnerable to at least one lethal fungus. The study revealed that 40% of eggs were killed by the fungus infection. Thereby, the clutches hatch in few days, seconds or immediately when attacked by fungus, snake or wasp respectively.
A high-speed video in a study by Kristina L. Cohen revealed the secret behind the mechanism of early hatching. It is a three-step process. First, pre-rupture shaking and gaping. Second, rupture of vitelline membrane near the snout. Third, thrashing of muscle to exit from the hole. A.callidryashas a unique and distinct hatching procedure that helps embryo to escape from the predator quickly. The reason for such unique mechanism lies in the snout of the embryo. This region is heavily concentrated with hatching glands filled with enzymes. Usually, anurans release this enzyme slowly over the whole development. This leads to weakening of the membrane gradually. But as soon as a predator approaches they release the enzyme in a quick motion. This leads to the formation of a hole in the egg layer creating an escape path for the embryo.
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