DIVE LOG Australasia

Hale’s Ornate Wobbegong shark at Black Rock, SW Rocks showing clasper with two types of skin.

It is wise to remember that sharks are fish. Fish fall into two main groups bony fish and cartilaginous fish. Sharks and Rays are cartilaginous. Everyone knows that fish lay eggs. Snapper produce millions of eggs which are spawned into the water, fertilised and drift off into the plankton to fend for themselves. This technique is inefficient and very wasteful in terms of energy demands on the mother. The vast majority of the eggs simply provide food for countless hungry plankton feeding creatures. Fish parents generally have little to

develops within the egg without any help from the mother. Port Jackson sharks produce the renowned spiral corkscrew shaped egg. The egg case is an extremely tough cellulose and keratin spiral and is impenetrable to most predators. Imagine a Wobbegong that is trying to eat a Port Jackson shark egg. It faces a very serious and most probably life threatening problem. Spiral shaped egg cases are designed to wedge themselves very firmly into narrow spaces. The spiral egg would plug up the throat of the shark. Apart from being uncomfortable, it could indigestible. If it did get swallowed, it would plug up the digestive canal, killing the shark. Port Jackson sharks migrate up to eight hundred kilometres during late winter to join mating aggregations. These Port Jackson ‘parties’ last for over a month. I have counted 140 sharks on a single dive at Jervis Bay one August. They were stacked up two and three deep resting after another heavy night of PJ activity. What exactly is a Port Jackson party really like? What is their fore play and mating ritual? It usually happens under cover of darkness. It is probable that few people has ever witnessed exactly what they do. What a great opportunity for someone to document the nocturnal behaviour of PJ’s. eventually starve. Wobbegongs would quickly vomit out the egg and never try to eat one again! I also suspect that the tough egg case is quite

40% OF SHARKS ARE OVIPAROUS

do with caring for the eggs. One exception is the mouth brooding Cardinal fish. Male fish scoop up the eggs, protect them by holding them inside their mouths. Many sharks have more advanced

reproductive strategies! They invest a lot of time and energy in producing offspring. Some sharks even caring for their offspring, nurturing them for a long time and teaching them how to hunt and survive. The more advanced reproductive techniques produce fewer offspring which are born much larger and are often fully developed. Grey Nurse shark babies are born at over one metre in length. FORTY PERCENT OF SHARKS ARE OVIPAROUS Forty per cent of sharks lay eggs which develop in the typical fashion. The embryo feeds on the yolk and

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DIVE LOG Australasia #408 - October ‘24