Mermaid Purses. An Example of Reproductive Strategy in Evolution.

Mermaid purses are an example of a reproductive strategy in evolution. In this article, we will explain all about them. Let’s get started!

Reproduction is a common denominator in living beings, through which different species on our planet can replicate themselves from a single cell or groups of cells and thus perpetuate themselves over time.

Basically, there are two modes of reproduction: asexual reproduction and sexual reproduction, each with its advantages and disadvantages. Asexual reproduction is much simpler, as it typically occurs from a single cell, though it can also happen from groups of cells that, when separated from their progenitor, can generate a new individual. Examples of asexual reproduction include binary fission, budding, or fragmentation.

On the other hand, sexual reproduction is much more complex and involves specialized cells for the reproductive process called gametes. The union of these gametes is necessary to generate a new individual, where genetic material is combined, which is a significant source of genetic variability.

Reproduction in Evolution

There were key events in the evolutionary process of life that are considered of paramount importance and are related to reproductive processes. A clear example of this is the appearance of the amniotes, which are tetrapods that acquired an egg with a hardened outer shell, allowing oviparous reproduction independent of the aquatic environment. This was crucial for the terrestrial conquest of dominant fauna.

Another highly important evolutionary event was the appearance of placental mammals, which, along with marsupials, generalized another form of reproduction: viviparity. In viviparity, the embryo is nourished by the mother via a placenta, a temporary organ that provides oxygen and nutrients to the embryo while it develops in the mother’s uterus.

However, long before these events occurred, when life was still limited to the marine environment, the fauna had already adopted a series of reproductive strategies, with oviparism being the dominant form of reproduction. Most fish species exhibit oviparous reproduction with external fertilization, meaning eggs are expelled into the environment, and males release sperm to fertilize the eggs. The number of eggs released varies by species, with some releasing massive amounts. It’s important to note that once the eggs are expelled, they are left to their own devices, sometimes resulting in high mortality rates.

However, other fish vertebrates adopted different reproductive strategies, such as ovoviviparity without placentation. In this mode, eggs are incubated within the female for a certain period, which varies by species. Once fertilized, the egg develops a membrane that surrounds and protects the embryo, which feeds on the yolk sac. Once the embryo has fully developed, it is expelled from the mother through contractions.

Lastly, viviparity also exists in fish. Here, the embryo initially feeds on the yolk sac, but as the yolk is depleted, the sac becomes vascularized and eventually fuses with the uterine wall, allowing the embryo to absorb nutrients from the mother. This is quite similar to what happens in placental animals.

Reproductive Strategies in Chondrichthyans

Chondrichthyans are a class of vertebrates known as cartilaginous fish, which includes animals like sharks, rays, and chimaeras. These creatures are a clear example of how various reproductive strategies have existed for millions of years.

There are viviparous species, such as the blue shark and hammerhead shark; ovoviviparous species like the shortfin mako or the dogfish; and oviparous species such as the small-spotted catshark, the thornback ray, or some species of rays.

Viviparism has gained ground over other reproductive strategies, as 60% of these vertebrates are viviparous, which has contributed to their evolutionary success and diversification. In fact, viviparism in sharks is even older than previously thought, as recent fossils of extinct primitive fish called placoderms have been discovered, showing embryos inside and even displaying an umbilical cord.

Mermaid Purses

The term “mermaid purses” refers to the empty shark egg cases often found on the shores of beaches, particularly after storms. These ovigerous sacs have structures like horns or tendrils that allow them to attach to structures like corals or marine vegetation, preventing them from being swept away by currents. These eggs will hatch between 5 and 11 months after fertilization, depending on factors such as species and water temperature, and will give rise to a new juvenile about ten centimeters long.

In the case of rays, we typically find eggs that are black and more or less rectangular in shape, with two protruding tendrils on either side. The morphology of these eggs varies by species, and it is possible to identify the species just by examining the shape of the egg. Naturally, this requires much deeper knowledge.

In some shark species, such as the small-spotted catshark or the thornback ray, we find eggs that are ovoid in shape and have tendrils on both sides.

As we can see, the morphology of these unique eggs has been well studied, and there are experts who can determine the species just by looking at certain morphological features like egg size, color, or the length of the tendrils mentioned.

If you want to learn more about these fascinating animals, mermaid purses, and other “cool things” you might find on the beach, here are a couple of links to my YouTube channel, where I go into more detail on this topic.