While beachcombing it's not uncommon to come across the strange thing pictured above mixed in among the various beach debris. This is the empty egg case, or "Mermaid's Purse", of a Skate.
Fish are divided into two main groups: the cartilaginous fish, which include sharks, skates and rays, and the bony fish, which include nearly all of the rest of the fishes found in both fresh and salt water. The bony fish have a skeleton made of calcium, like those of birds and mammals. The cartilaginous fish have a skeleton made of cartilage.
Unlike most bony fish, which shed hundreds or even thousands of unprotected eggs into the water, cartilaginous fish produce only a few young and protect them inside a leathery egg case. Most female sharks keep the egg case inside their bodies and give birth to live young. Skates, rays and some types of sharks, however, lay a couple of egg cases on the ocean floor and leave them to develop alone.
The Little Skate (Raja erinacea)
The egg case is created around each embryo inside the mother's reproductive tract by a special collagen-secreting gland. This gland lays down layers of protein, alternating directions as each layer is formed. This strengthens the case in much the same way that alternating fiber layers of plywood strengthens the board. Once the case is completed the female skate lays the egg on the sand. A thin, sticky mesh of threads on either side of the egg case trap sand granules and act as an anchor, weighing the egg case down to prevent it from being cast onto shore before the embryo has finished developing.
Females usually lay eggs in pairs throughout the year. Notice the horns on the four corners of the purse, as well as the sticky sand-trapping mesh attached to the sides of the case.
Before diving into the details of the development of the eggs, I'd like to mention a little bit about beach debris. This stuff is found at the strandline, that is the point where the incoming tide stopped and reversed direction as the ocean began to ebb again. Beach debris can be lumped into two general catagories: Flotsam, which includes naturally made materials such as driftwood, uprooted seaweed and mermaid's purses, and jetsam, which includes man-made objects such as sea glass, bouys and bits of plastic. Flotsam and Jetsam were also the names of the two nasty moray eels in the Disney pic "The Little Mermaid".
Sharks and skates reproduce internally. The males have a pair of claspers which are used to penetrate the female and insert packets of sperm. These packets are kept within the mother and stored until needed. Currently I have two adult female skates, caught over a year ago. There is no male in their tank, yet even now they continue to lay eggs that develop and hatch. At least a year after they could have possibly last mated, the sperm packets are still viable.
Ventral claspers on a male skate.
It takes nearly a full year for the young skate to hatch from its egg case. This is a crucial period during the growth of any fish as it is the time of their highest mortality rate. By protecting a single young within the protective case the parent is nearly assured of the young surviving this critical stage. During this time it is nourished by a yolk sac which is attached to the embryo by an umbilical cord. As the embro grows and uses up its yolk, the yolk sac shrinks. Even after it is fully developed and ready to hatch the young will not emerge until the yolk is fully absorbed.
Newly laid skate egg with only the yolk visible.
While in the egg the young skate obviously needs oxygen. In the earliest stages the little bit of oxygen required is simply absorbed through the walls of the egg case, but as the animal grows it depletes its supply faster and faster and what little oxygen passes through the tough fibers of the purse is no longer sufficient. There is a small hole about halfway up each of the four "horns" of the egg case. The embryo must continuously pump new seawater into the case. It does this by inserting its tail into one of the corners and wagging it vigorously. The water pumped in one side forces old water (now polluted with wastes and CO2) out the opposite side. If one of these holes gets plugged with debris the skate will rotate its body around and use one of the other horns. If two or more of these horns is plugged up the embryo will perish.
Shark embryoes do not need to pump oxygen manually as skates do (in fact, because most shark egg cases do not have horns, they are unable to pump water in). They simply rely on water pressure differences between the inside and outside of the case. Although this saves a great deal of energy on the embyo's part, this only works if the purse is laid in areas with proper current flow. Unlike skates, shark mothers must be very careful where the eggs are placed.
Shark mermaid's purse. Note the lack of horns.
Occasionally a Mermaid's Purse will contain a set of twins (I've seen this only once out of the over four hundred eggs I've hatched). As far as I know these do not normally survive. Once the embryoes reach about the six-month stage there simply isn't enough room inside the case to support them both.
Egg case, and newly hatched young. Size proportional.
Notice that the egg case is not symmetrical. One side has short horns and the other has long horns, with a flattened piece of casing between them. This long-horned side is where the skate hatches from. It will emerge headfirst with its wings (actually pectoral fins) folded. It then unfolds its wings and propels itself out of the case.
Although I have successfully hatched hundreds of mermaid's purses, I have never actually seen one emerge. It happens very quickly and usually at night. I end up coming in some morning to find a new addition to the skate aquarium, usually clinging to the glass sides. Click the image below to watch a very short movie my son shot of some of my newly hatched babies. These are all between two days and four weeks old. At this age most of the young are still subsisting on the last internal remnants of the egg yolk and it's usually about two or three weeks before they start hunting for food. By the way, observing a baby learning to hunt is very funny as you watch them try to pounce on their "prey", which in the case of the little guys in this film is simply dead, frozen brine shrimp.
In the photo of one of my newly hatched skates above (don't forget to click for the movie) the internal organs can be clearly seen. The two red areas are the gills. The red spot between the gills is the heart, which can be seen beating for several weeks until the skin thickens as the fish grows. The yellow area is the liver and below that is the blue intestinal tract. This one is a female. Even when young, tiny claspers can be seen on the males.
After hatching I keep the young for a couple of months to be sure they are eating and healthy and then I release them into Narragansett Bay where they grow up and probably end up netted and sadly chopped up as bait for lobster traps.
Other diaries in this series can be found here.