Tuesday, October 26, 2010

Northeast Egg Mass ID

Winter has ended (we think), and the salamanders are on the move! Reports of egg masses are beginning to trickle in here in Vermont and soon they'll be pouring in. Identifying the eggs, thankfully, is not very difficult once you learn how to tell them all apart. Here in Vermont we have only 8 species of amphibian that lay egg masses in the Spring, and that is true for most of New York and New England as well. Those species are the:

Wood Frog
Northern Leopard Frog
Pickerel Frog
American Toad
Fowler's Toad
Jefferson Salamander
Jefferson/Blue-spotted hybrid
Spotted Salamander

In this post I will not be dealing with mid-summer breeders such as Bullfrogs and Green Frogs, or species that do not lay noticeable egg masses such as Eastern Newts, Blue-spotted Salamanders, or Spring Peepers, etc, which all lay individual eggs or groups of 2-3 eggs attached under leaves and debris. Please note that old egg masses that have hatched out or are about to hatch may be tattered, torn, or completely separated into a film. First, I'll start by teaching you about the difference between frog and salamander eggs, then move on to identification of each species.

Frog vs. Salamander eggs

Let's start simple. Frog eggs vs. salamander eggs. Telling the difference between the two is quite easy. Frogs lay individual clear eggs with a visible embryo contained within each egg. With frogs, the outside edge of the egg mass is made up of the eggs themselves. Salamanders take it a step farther and coat the entire egg mass with an additional layer of jelly. These two photos demonstrate the difference clearly:

Frog eggs. Notice that you can easily see the contour of each individual egg on the outside of the mass.


Salamander eggs. Notice that there is a layer of gel surrounding the mass of eggs. This protective film around the egg mass is characteristic of all salamanders in the genus Ambystoma, which includes pretty much all the pond-breeding salamanders you're going to find the eggs of.

Notice that you can still see the outline of each egg within the mass.

Again, frog = no gel surrounding entire mass, you can see the contour of each individual egg.

Salamander = layer of gel surrounding the entire egg mass.

And side by side:


This extra layer of gel on salamander egg masses is thought to provide the eggss some protection against predators such as the dreaded Eastern Newt, which apparently just eats its way past the gel anyway:

Now we'll go down the list egg-mass laying species in Vermont one by one.


Wood Frog

Wood frog egg masses are pretty easy to identify. You would expect to find Wood Frogs breeding in ponds, vernal pools, and marsh edges in or near forested habitat at a wide range of elevations as soon as the snow melts and the ground thaws. A typical egg mass can have between 500 and 2000 eggs. The embryos start out black on top and white on the bottom, as do most open-water amphibian eggs, but as the embryo develops into a tadpole the white is lost. A fully-formed mass that has been in the water for a day or so is about the size of a softball and the clear space between the embryos and the margin of each egg is many times greater than the width of the embryo (see the first photo posted).


This, of course, is a much closer look than you'd normally get. Here's another example.


At a glance, Wood Frog egg masses underwater will look something like this:



Often, groups of Wood Frogs will lay their eggs in close proximity, usually on emergent vegetation or submerged tree branches:


Sometimes amphibian eggs can have an opaque hue to them.  I am not sure what causes this in Wood Frogs although in Spotted Salamanders it is caused by the genetics of the female.

When wood frog eggs are laid, however, the masses are much more compact. Obviously a golf ball-sized frog isn't going to lay a softball-sized egg mass. Directly out of the frog an egg mass is smaller than a golf ball, but swells to full size within hours.



Northern Leopard Frog

Leopard Frog eggs look a lot like Wood Frog eggs with a couple key differences. The embryos are about the same size (2-3mm), but the eggs themselves are much smaller and tighter. The clear space between the margin of the eggs and the embryo is usually the about the same thickness as the embryo itself (remember, the clear space in a wood frog is much bigger). Because Leopard Frogs lay more eggs per mass than Wood Frogs (2000-4000), but the eggs are much smaller, the entire egg masses end up being about the same size (think baseball-softball).


Notice how tight the egg mass is? Wood frogs have much more clear space between the embryos. Here's a picture of an undisturbed Northern Leopard Frog egg mass.


Leopard Frogs typically lay their eggs in lake and river flood planes where sedimentation and silt can build up pretty quickly. Remember that Wood Frogs tend to breed in ponds and vernal pools. Sure, there is some overlap in breeding habitat between Leopard and Wood Frogs, but knowing the typical breeding habitat for each species can help in many cases.

Pickerel Frogs have very similar egg masses compared to Leopard Frogs, but notice how the Leopard Frog eggs are black on top and white on bottom. Pickerel Frog eggs are brown on top and yellow on the bottom. Other than that the eggs are pretty much the same.


Pickerel Frog

If you can identify Leopard Frog eggs you can identify Pickerel Frog eggs. The egg masses are almost exactly the same except instead of the eggs being black on top and white on bottom, Pickerel Frog eggs are brown on top and yellow on bottom. Pickerel Frogs are usually found higher in elevation than Leopard Frogs, usually found breeding in upland ponds or small lakes, compared to Northern Leopard Frogs which most often breed in lowland flood planes. Habitat counts!


You might read that Pickerel Frog egg masses are plinth-shaped. I didn't know what a plinth was and had to look it up. Other sources say the egg masses are spherical, which is more consistent with my observations. Anyway, for what it's worth, this is a plinth:


American Toad/Folwer's Toad

Where I live, the American Toad is the only frog that lays its eggs in a long string. A single strand could have between 2,000 and 20,000 eggs depending on the size of the female. Breeding occurs in the warmer months (mid-summer in Vermont). As far as I know, there's no telling the difference between American or Fowler's Toad eggs. If you live in a place with both species then you can use habitat as a clue, but not a very reliable one. American Toads TEND to be found in hardwood forests with loamy soils. Fowler's Toads TEND to be found in coastal or flood plane habitat with sandy soils.


Once sediments settle on the strands actually recognizing them as eggs can be difficult to someone unfamiliar with them.

I've never seen Fowlers Toad eggs and don't know if there is a good way to tell them apart. At least I'm being honest. If you have photos or ID tips, post 'em! My understanding is that the eggs are indistinguishable from one another. Interestingly, these toads are the only frogs in the northeast that will lay eggs alongside flowing streams and you can often see schools of their tadpoles moving around in brooks, especially in the pools adjacent to brooks where the current is a little slower.



Spotted Salamander

Spotted Salamander egg masses are made up of about 50-250 eggs, can be as large as a grapefruit, and are very dense/firm. Usually they are laid in ponds, vernal pools, and marsh edges without fish, but you'll find them in pond with fish too. If you pick up a Spotted egg mass it will usually hold its shape in your hand. The eggs are usually attached to sticks, branches, and vegetation below the surface of the water.





Even as the egg mass ages and the embryos develop you can see that it is firm and continues to hold its shape when pulled from the water.

Just like Wood Frogs, you'll often fine huge numbers of egg masses all in one spot. Interestingly, some Spotted Salamander egg masses are a grayish opaque color. This is caused by a genetic trait of the mother and is common in some places.



And again, Spotted Salamander egg masses expand after being laid. Obviously the eggs don't take up the volume of a softball while still inside a hot dog-sized salamander.



Jefferson Salamander

Pure Jefferson Salamander eggs are laid in masses of 20-30 eggs but females usually lay multiple masses. Sometimes masses are laid in a line down a single stick and, once they swell with water, may fuse into one another and appear to make up a single mass. The masses closely-resemble those of Spotted Salamanders but, in addition to being much smaller, are not firm. If you pick up a Jefferson mass the eggs will run through your fingers or break off the stick before even making it into your hand.





And here's a freshly-laid Jefferson Salamander egg mass next to the salamander who laid them:



Blue-spotted Salamander

Blue-spotted Salamanders do not lay egg masses. Their eggs are attached individually or in groups of two or three on the underside of leaves. Hardly anybody ever sees them, I don't have any photos of them, don't bother looking for them.

Jefferson/Blue-spotted Salamander hybrids

Hybrids between Jefferson and Blue-spotted Salamanders exist. The two pure species cannot breed with one another however a hybrid line of almost entirely parthenogenic females can breed with either pure species. It's a very complicated subject that I'd be happy to talk about in more detail another time. Hybrids more closely-related to Blue-spotted Salamanders will lay individual eggs or small clusters of eggs under leaf matter. Those closer to the Jefferson Salamanders will lay egg masses that basically look like those of pure Jefferson Salamanders. Hybrid egg masses, however, usually have a high proportion of nonviable eggs that do not develop. The dud eggs are usually gray and swell up quickly. The gray swelling is caused by the water mold saprolegnia. Saprolegnia can be seen in amphibian eggs of any kind but it is very common in Jeff/Blue-spotted hybrids.

Here is a good photo of a hybrid egg mass. Observe how a bunch of infertile (whitish) eggs are in the center of the mass. You can't determine with 100% certainty this is a hybrid egg mass, but any egg mass that looks like it is from a Jefferson Salamander, is within a zone of hybridization, and has more then one or two infertile eggs should be considered most likely that of a hybrid salamander.



Bryozoans

These are not amphibian eggs. They are colonies of microscopic animals similar, but not related, to corals. They are firm, have these weird crusty things on the outside, and no embryos on the inside. People confuse them for amphibian eggs a lot, but now you know better.


That is all. Have fun herping! And remember to submit reports to your local herp atlas. For Vermont that would be this one: VT Reptile and Amphibian Atlas Project. Use google to find your own if you don't already know.

In addition to that, or if your state doesn't have a herp atlas, report your findings to Herp Mapper :)