Anatomy - Abalone (Haliotis Sp.)

Abalone are marine, gastropod mollusc's (sea snails) that inhabit inshore reef areas of most temperate oceans.
They are members of the family Haliotidae. The genus name is Haliotis which means "sea ear" and refers to the flattened spiral shape of the shell.
The shell, which protects the body of the abalone, has a row of respiratory pores along the spiral that allows water to bathe the gills for oxygen absorption and for flushing away bodily wastes.

Abalone attach themselves firmly to rocks with a muscular foot. The foot is surrounded by an epipodium (or frill) - extensions of the foot containing small, tentacles and sensory structures. The epipodium, which extends beyond the shell edge in living abalone, is the most reliable structure for identifying abalone species. Depending on species characteristics, the epipodium surface may vary from smooth to pebbly in appearance and its edge may be frilly or scalloped.
The animal itself has a head, a pair of eyes, a mouth and a large pair of tentacles. Inside the mouth is a file-like tongue which scrapes algal matter to a size that can be swallowed. The internal organs of abalone are arranged spirally under the shell.

The most conspicuous organ in adult abalone, just prior to spawning, is the crescent shaped gonad, grey-green in females and cream coloured in males. It extends around the side, opposite the pores and to the rear of the abalone (spiral end).

Spawning occurs mostly during the summer months and multiple events during one season are possible.

Triggers to spawning may include;

• elevated water temperature,
• high wave action or extreme weather conditions,
• photoperiod (day length),
• lunar cycle, as well as the presence of abalone gametes.

Some species (H. scalaris, H. roei) spawn throughout the year.
Females expel eggs into water via the respiratory pores, this triggers males to eject sperm in the same way and fertilisation takes place in the water column.

At the completion of the breeding season sexing abalone can be difficult, the same difficulties are experienced when trying to determine the sex of immature abalone.

The gill chamber is under the respiratory pores and next to the mouth. Water is drawn in through the pores and under the outer edge of the shell, flows past the gills and out the pores. Waste products are carried out in this flow of water, as are the eggs and sperm during breeding. Abalone blood flows through arteries, veins and sinuses assisted by movement of a heart and surrounding muscles and tissues. Abalone have no obvious brain structure, but a complex of nerves in the head region called ganglia travel down the centre of the foot branching out serially towards the epipodium.

Abalone mature at between 4 and 6 years old and can live up to 50 years of age. Several million eggs are released in each spawning by a mature female and many times this amount of sperm is released by males.

After fertilisation, larvae are initially planktonic for several days. Although larvae can survive dispersal over long distances, survival is dependant on finding suitable habitat upon which to settle. Some evidence indicates that abalone released from reefs exposed to currents are flushed away from suitable habitat, whereas those released from more sheltered reefs are likely to complete their larval life near parent stock. The dense growths of seaweed and complex topography typical of many abalone reefs may create water movement patterns that cause localised larval dispersal/capture and settlement. Successful settlement may depend on environmental stimulus provided by encrusting coralline algae, once on suitable bottom larva lose ability to swim so start to crawl and graze. Slime trails of adult abalone and/or habitat modifications caused by the activities of adult stocks may also play a role in successful settlement.
Among those abalone that settle few survive the first few months, however in ideal conditions with plentiful food supply fast growing blacklip abalone may reach a size of 120mm in 4 years.

MOVEMENT

There is little understanding of how far planktonic abalone can travel prior to settlement at 2-4 days of age. There is some genetic evidence to suggest that recruitment is generally restricted to the locality of parent stocks with discrete subpopulation differences occurring over relatively small distances. Although planktonic abalone are dispersed to float freely in the water column during the earliest part of their life, abalone are generally distributed locally and are most likely to be concentrated in areas where current flow causes drift seaweed to accumulate.

Young abalone from 2-10 mm in length are coral pink in colour mottled with grey or white and they blend inconspicuously on the sides and upper surfaces of rocks and boulders. At about 10mm the young abalone move off to a cryptic site under boulders.

At about 50 mm the abalone can be found in narrow crevices and deep fissures. From about 75-100mm blacklip abalone emerge to be found in larger crevices, caves or vertical rock faces whereas greenlip abalone are most likely to be found on rock and boulders in more open situations. There is some tendency for abalone to be found in greater depths as they grow larger and change their habitat. Adult abalone tend to stay in one location, waiting for food to be brought to them by water currents.

Abalone expend a large amount of energy when moving from one location to another so movement is limited. As with all snails they must lay down a trail of slime as they move about.
In the presence of a good supply of drift algae, abalone may only move a few meters within their lifetime, often returning to the same "home-site" after short nocturnal forays. If food becomes scarce for a long period, however, abalone will search for it over greater distances and migrations of up to 1 km have been recorded.

GROWTH

Growth rates differ from site to site according to environmental factors such as water flow and wave activity. Abalone from sheltered waters grow more slowly than those from exposed reef areas where higher water exchange provides more food in the form of drifting seaweeds. Large brown algae such as kelps are the main food source of adults. Juveniles feed mainly on rock encrusting coralline algae, diatoms and bacterial films.
Fastest growing individuals may reach 120 mm or more in 4 years but individuals from some sheltered reefs may take many more years to reach that size (if at all). Harvestable individuals from fast-growing populations may be reproductively immature whereas individuals from the slow-growing (stunted) populations may produce eggs for many years before reaching legally harvestable size.

PREDATORS AND COMPETITORS

Throughout their life cycle abalone must contend with a variety of predators. Eggs and larvae are eaten by filter feeding animals. Settled juveniles are eaten by crabs, whelks, lobsters, octopus, starfish, fish and predatory snails. Large abalone are less threatened by those predators of their earlier life, but larger, and often more efficient predators still pose a threat to them. Larger fish and rays can dislodge abalone when they find them in exposed positions and larger octopus can seek abalone out under stones and amongst reef crevices.
Observations of populations also indicate that on some reefs where abalone have been removed, sea urchins become prolific, creating reef barrens without seaweed cover or soft sponges or animal colonies displace pink encrusting coralline. This habitat alteration may inhibit subsequent natural recolonisation by abalone.

Abalone Habitat

Abalone are common in all Australian temperate waters from NSW through Victoria, South Australia and Western Australia including Tasmania.
They occur on rocky reefs from low tide down to about 40 metres in both exposed and sheltered waters, living in crevices and fissures amongst large boulders. Juveniles remain hidden beneath boulders and in deep crevices until they approach maturity, when they emerge to occupy more prominent sites where algae is readily available. The abalone graze on broken pieces of algae set adrift by currents and surge. Areas where large quantities of algae accumulate are often areas of rapid abalone growth.

Greenlip abalone are found in clusters living in more sheltered waters, often coupled with strong tidal movement, from Wilson's Promontory, Victoria and Tasmania through to Cape Naturaliste, West Australia.

Haliotis Rubra  (blacklip abalone)

Blacklip abalone is the main commercial species in Victoria representing about 80% of the commercial catch nationally. Adult blacklip grow to between 10 and 20 cm long, producing an oval shell with spiral ridges under laid by radial wrinkles.

The shell exterior is often masked by encrusting organisms, as are most species of abalone, which makes distinguishing its shell features difficult. The shell is reddish brown in colour with green tinges or patches throughout, and it has a rounded spire.

There are 6-7 small raised holes, called tubercules, along the spiral margin of the shell, and the zone between the holes and the shell perimeter is concave. Older holes close over as the shell grows. Inside, the shell has a lining of obliquely corrugated mother of pearl.

The side of the foot of the animal has a black mantle which gives H. rubra its common name. Blacklip Abalone are found along the rocky coastline of NSW, Victoria, Tasmania, South Australia and Western Australia.

Haliotis laevigata  (greenlip abalone)

Greenlip abalone are relatively easy to distinguish from blacklip, firstly in terms of larger size as they can grow to be as long as 22 cm (though 12-16 cm is more common), secondly, the shell is quite rounded and smooth, is pale in colour and has a chalky texture.

There are fine, radial, growth lines on the surface and the spire is towards the edge of the shell and usually quite low. The species has 6-8 holes that are only slightly raised and the zone between the holes and the margin is quite convex.

A smooth layer of mother of pearl covers the inside. The side of the foot is a distinctive and unmistakable, bright green.

Greenlip abalone are found in clusters living in sheltered waters often with strong tidal movement and nearby sandy areas, from Wilson's Promontory, Victoria and Tasmania through to Cape Naturaliste, Western Australia.