A spotted salamander metamorph (Ambystoma maculatum) rests on a bright patch of mushrooms (possibly jack-o’lantern mushrooms; Omphalotus illudens) extending over the pond’s surface. Although still a bit too early to wander a ways onto land, this metamorph has at least developed lungs, allowing it to breathe in open air through its nose and semi-permeable skin. Behind its jaw you can see remnants of the receding gills. It can sometimes be difficult to diagnose species during transitional stages like this, but fortunately Arianne (our local salamander expert!) is well-versed in the life stages of various Ambystomatids through her research.
Ringed salamanders (Ambystoma annulatum) are endemic to the Ozark and Ouachita Mountains, including parts of southern and eastern Missouri. Despite being relatively large with bright yellow rings, they are secretive and nocturnal, migrating in the fall during rainy conditions to find suitable ponds for breeding and laying eggs. Like most amphibians, the highest mortality rates are during early development. Being a small free-swimming larva, almost anything that eclipses you in size becomes a potential predator. This includes carnivorous invertebrates, fishes, tadpoles, newts, and even other ringed salamander larvae. To alleviate the predation pressure from newts, larval salamanders are able to detect the newts’ chemical signature and decrease their activity accordingly. Behavioral adaptations to avoid fish are yet to be determined in this species, though other amphibians are known to increase their use of refuges when presented with the chemical cues of fish. Because two introduced fishes (fathead minnows & mosquitofish) are together responsible for the greatest detriment to larval ringed salamander survival, cryptic behaviors in the presence of fish would be especially pertinent in Ambystoma.
After being so active and vulnerable in the murky waters, salamanders metamorphose into a reclusive and fossorial lifestyle on land. But only one out of every one thousand salamanders is able to make it this far. Hiding underground, salamanders are thought to have a much greater probability of survival. However, estimating juvenile survival rates is notoriously difficult due to their subterranean habits. Arianne’s dissertation work has recently filled this gap for several Missouri salamander species, including A. annulatum. Using years of mark-recapture data from outdoor enclosures, she arrived at a mean annual survival probability for terrestrial juveniles of approximately 0.50. Vital rate estimates such as these are useful for informing land management practices that involve ringed salamander habitat. Moreover, the results from intensive studies such as these can be applied to other species that are similar in ecology but data deficient.
Many ambystomatid salamanders lay their eggs submerged in stagnant pools of water, anchored in aquatic vegetation. Underwater development prevents desiccation and provides protection from predators, but creates a major challenge for oxygen delivery to embryos. Remarkably, studies suggest oxygen uptake is facilitated by algae that grows within the jelly envelopes. In a putative symbiotic relationship, algae provide an endogenous source of oxygen, while the embryos increase levels of carbon dioxide and ammonia. Experimental work corroborates this idea, with (1) eggs having a greater probability of hatching and developing more quickly in the presence of algae, and (2) algal growth increasing in the presence of embryos. Oxygen delivery to embryos via diffusion from the surrounding aquatic environment to the anoxic egg mass is likely only sufficient during early stages of embryo development. Late-stage embryos, on the other hand, depend heavily on algal oxygen production: capable of surviving anoxic conditions in darkness, but necessitating daily fluctuations into hyperoxic conditions when exposed to light.
Though pond-dwelling, marbled salamanders (Ambystoma opacum) have an unusual oviposition behavior for ambystomatid salamanders. Instead of laying their eggs underwater, they deposit egg masses in mats of leaf litter and debris along the edges of temporary pools. To prevent the eggs from desiccating, females will attend their egg masses for days or even weeks until the ponds fill up from rain. A few studies on oviposition site preference suggest that females avoid laying eggs in deeper areas of ponds, which are more susceptible to premature drying after females abandon the inundated eggs during initial rains. Experimental manipulation of nest sites also found that females preferentially laid eggs in grass clumps, possibly due to a combination of moisture requirements and an inclination for cryptic and cluttered spots.
Eastern newts (Notophthalmus viridescens) are a prolific inhabitant of stagnant water bodies throughout the eastern U.S.— from the frigid winters of the northeast to a tropical Floridian climate. Newts have fascinating life histories: hatching as aquatic larvae with external gills, metamorphosing into a terrestrial eft with rough skin and fully developed lungs, then returning to the water as a fully aquatic adult. As larvae, newts experience predation mostly from fish, aquatic insects (e.g. immature dragonflies & beetles), and conspecifics. But after emerging from the murky waters, they are exposed to an entirely different assemblage of predators. Cumbersome on land, efts can no longer rely on mobility and speed to escape predators, but they have another trick up their sleeves.
Many newts contain potent tetrodoxins in their skin membranes, in some species surpassing most poison frogs in toxicity. Being potentially deadly prey items, they effectively advertise this quality to predators via a bright aposematic coloration. Whether it be innate avoidance or a learned association, birds and mammals are less likely to pursue a brightly-colored newt.. and the ones that do try to eat one are in for a miserable experience. So, efts can be found strutting out in the open by the dozens with little hesitation, as they typically do. This behavior is also concordant with efts tending to be much more toxic than their larval and adult counterparts.
Dragonfly larvae are common predators of salamander and newt larvae. Their most unusual anatomical feature is a hinged “labial mask” that extends outward to clasp prey. The structure ends in two moveable hooks, which may appear to be mandibles but they are actually modified labial palps. The mandibles lie above, serrated like a blade for cutting up prey into smaller parts before ingestion. In the above photo you can also see a network of white tubes permeating through the translucent mask. These branching tubes are part of the tracheal system which is responsible for oxygen transport throughout the insect body. In aquatic larvae, oxygen uptake occurs within gills in the anus, and after metamorphosing into adults, they use spiracles along the sides of the abdomen.