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www.thevillagenews.co.za
4 November 2020
MY ENVIRONMENT
The ghost of Walker Bay By Dr Anina Lee
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trollers on Grotto Beach in Hermanus have no doubt come across ‘mermaids' purses’ washed up on the sand. They are actually shark egg cases from which the little sharks have (usually) emerged. Does that mean that sharks lay eggs like birds? Well, some of them do 'shed' eggs. About a quarter of all shark species shed eggs into the sea. We call them 'oviparous' sharks and they comprise mainly the small shysharks and skates. Unlike birds, they don’t care for their young. Inside the egg is a shark embryo with a large yolk sac which it feeds off while growing. Often there are tendrils or hooks on the egg case which secures the egg onto a plant or rock. When the yolk is used up by the growing embryo and the embryo is ready, it hatches from the egg case, which then often washes out onto the beach. The most commonly found egg cases on Grotto are those of the St Joseph shark, also known as a Cape ghost shark or Cape elephantfish (Callorhinchus capensis). Other types of shark eggs commonly found are puffadder shyshark, dark shyshark and spearnose skate. St Joseph (Cape ghost) sharks are really weird looking. They have a floppy, trunk-like nose which is a sensory organ used to fish tiny creatures from the sand – creatures like small crustaceans, worms, urchins, and other tasty morsels. Although they look a bit scary, they are actually not dangerous, unless you are a shrimp. The end of their snout has highly specialised pores that can sense movement and electrical fields, making it easy for them to find almost anything. Those little sand critters don't stand a chance. St Joseph ghost sharks are smallish (between
0.5 and 1.2m in length), silver and scale-less – and, like all sharks, they have cartilage instead of bones. Their bodies are soft, their heads are large, and they only have a single gill opening on each side. The reason why there are no other fishes that look like them is that they have evolved separately from other cartilaginous fishes for about 400 million years. They are a member of the subclass Holocephali, commonly known as chimaeras – the only member of this group commonly found in South African waters. In Greek mythology, a chimaera was a beast composed of the parts of different animals, many of which you may have heard of – minotaurs, gryphons and Cerberus, the multi-headed dog that guarded the gates of hell. Unlike these mythical creatures, the St Joseph ghost shark is as close to a real-life chimaera as we can get – a combination of traits commonly associated with sharks and those associated with bony, ray-finned fish. You can think of a St Joseph shark as being halfway between a typical fish and a typical shark.
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is unique, as other chimaeras mainly live in deep water. St Joseph ghost sharks reach sexual maturity at the age of three or four years when they are just over half a metre long. During the breeding season, females move closer to the shore to lay their eggs, and juveniles will remain in these shallow waters for the first few years of their lives. These nursery areas are prime targets for overfishing and a significant reason why this species has been classified as Red on the WWF SASSI List. The eggs are sort of leaf-shaped, and they are laid directly onto the sea bed. The frilly edges of the 'leaf' are thought to anchor them into the
sand or surrounding rocks. Inside the egg, the baby shark develops, feeding off the yolk of the egg. After about nine months, the baby shark emerges from the egg. But in rough weather, when the autumn winds start to blow, many of them are washed out on the beach. Some are unhatched with embryos still inside, or already hatched. This is when we get to see them. When next you go down to Grotto Beach, try to spot some egg cases of the ghost that may be living in Walker Bay. If you would like to identify all the egg cases you find, you can check the board on the wall of the Grotto Beach building. Happy shark egg-case hunting! 1
According to the Two Oceans Aquarium, some of the key differences are:
• Elasmobranchs have multiple gill openings, but Holocephali like St Joseph sharks only have one on each side – a trait they share with bony fish. • In addition to having only one set of gills, these are covered by operculums (gill covers) – structures only seen in bony fish. • Their upper jaw is fused with their skull, whereas the upper jaw moves freely in true sharks. • Like other sharks, St Joseph sharks have external claspers that are used for reproduction. However, they also have three tentacle-like structures, two that can be extended from the pelvis and the other from their heads, to help clasp the female during mating. • Unlike sharks, their teeth are not replaceable, and they only have three pairs of large grinding teeth that must last their entire lives. Cape ghost sharks live in shallow coastal waters, predominantly along the southern and western Cape, which is why we see many of their empty egg cases on Grotto Beach. This shark's habitat
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1. St Joseph (Cape ghost) sharks are really weird looking. They have a floppy, trunk-like nose which is a sensory organ used to fish tiny creatures from the sand. PHOTO: Wikimedia Commons 2. Inside the shark egg is an embryo with a large yolk sac which it feeds off while growing. Often there are tendrils or hooks on the egg case which secures the egg onto seaweed or corals. When the embryo is ready, it hatches from the egg case, which then often washes out onto the beach. PHOTO: Sheraine van Wyk 3. The most commonly found egg cases or ‘mermaids’ purses’on Grotto are those of the St Joseph shark, also known as a Cape ghost shark or Cape elephantfish (Callorhinchus capensis). PHOTO: Ispotnature.org 4. A Pyjama shyshark that was caught in a fishing net and sadly died. PHOTO: Nicola Stelluto, sharkwatchsa.com
