Mermaid purses are unique structures found in marine environments. These fascinating objects are the egg cases produced by certain marine animals, serving as protective nurseries for developing young.
Understanding the Mermaid Purse
A mermaid purse is a tough, leathery pouch that encases a developing embryo. These capsules are typically dark brown or black and vary significantly in size and shape depending on the species. They are primarily composed of collagen protein strands, which give them their distinctive texture. Many feature tendrils or horns at their corners, which aid in anchoring the case to underwater structures.
These egg cases frequently wash ashore, often found along the high tide line tangled in seaweed. While the term “mermaid purse” is broadly applied, distinctions exist. Skate egg cases commonly appear rectangular with prominent horns at each corner, designed to grip soft sediments. In contrast, shark egg cases often have a more rounded or tubular shape with curly tendrils, which help them attach to seaweed.
Sharks That Lay Them
Approximately 30% to 40% of shark species are oviparous, meaning they reproduce by laying eggs in these protective cases. These egg-laying sharks typically inhabit bottom-dwelling environments. Several prominent families and species are known for producing mermaid purses.
Catsharks, a diverse group, are well-known oviparous sharks. Species like the small-spotted catshark and nursehound lay egg cases that are generally rounded with long, curly tendrils for anchorage in seaweed. Horn sharks, including the Port Jackson and California horn shark, produce distinctively spiral or corkscrew-shaped cases designed to be wedged securely into rocky seafloor crevices. Swellsharks also produce egg cases with curly tendrils. Other oviparous sharks include bamboo sharks and carpet sharks.
The Biology of Oviparous Sharks
Oviparous reproduction begins with internal fertilization, after which the female forms the egg case around the fertilized egg. This case is secreted by a gland and is initially soft, gradually hardening and darkening over a few hours. The female then deposits these cases in safe, sheltered locations on the seafloor, such as among seagrass, in rocky crevices, or attached to seaweed. The tendrils or spiral shapes of the cases are crucial for anchoring them against ocean currents.
Once laid, the developing embryo inside the mermaid purse relies entirely on a yolk sac for nourishment. The duration of embryonic development varies significantly among species and is influenced by environmental factors like water temperature. This period can range from a few months, such as 6 to 9 months for some catsharks, to over a year for other species like horn sharks, which typically take 9 to 12 months; warmer waters can accelerate the development process. When the young shark is fully developed, it emerges from an opening in the egg case, ready to live independently, as there is no parental care after hatching. This strategy protects the developing embryo, enhancing offspring survival.