ASFB Home > The effects of spatial and temporal factors on the abundance of seven key finfish species along south-western Australia.

Development of the Port Jackson shark embryo.

Kate Rodda and Roger Seymour

South Australian R&D Institute, PORT LINCOLN, AUSTRALIA

THEME: ASFB

The Port Jackson shark (Heterodontus portusjacksoni) is an oviparous shark that breeds once annually, during spring. Each egg contains one embryo and is encased in a thick (1-2mm) protective capsule. The egg capsule has two respiratory slits, one located at each end of the capsule, however during the first four months of incubation, the respiratory slits are plugged with a thick wedge of albumen, sealing the embryo from the external environment. During this time, the embryo undergoes substantial changes in its' morphology in preparation for survival in potentially difficult conditions. The most substantial changes involve the production of external gill filaments and vascularisation of the yolk sac which serve to increase the surface area available for gaseous exchange. After four months, the plugs dissolve and the embryo is in direct association with the external seawater environment. The embryo actively ventilates the open egg capsule with vigorous scooping movements of the tail that draws water into and out of the egg capsule through the respiratory slits, ensuring a continual supply of oxygen, thus stimulating growth. The embryo grows exponentially until reaching a short plateau phase near hatching. Embryos hatch at 180mm long and 55g. This relates to between 77 - 85 % of the initial dry mass of yolk being transferred into hatchling mass at 18-22oC. Water temperatures around South Australia vary between 12 and 22oC, resulting in long incubation periods of over 12 months. Incubation period in the laboratory was, however, strongly influenced by temperature, with embryos hatching after 400 days at 18oC and after 310 days at 22oC.