Marine Science Chapters


Abalone Reproduction and Growth

Abalone are separate sexed. The sex of a live abalone can be determined by holding the abalone out of the water with the holes along the bottom. The abalone will usually get tired and fall to the side so that the reproductive organ (on its right side) can be seen tucked between the mantle and the shell. If you look through the mantle tissue you can tell if it is a female (green reproductive organ) or male (beige reproductive organ).

Abalone broadcast spawn, releasing their reproductive products into the ocean. The eggs or sperm are released into the same area where the gills and anus are located. There is always a gentle flow of water out of this area and the eggs or sperm are washed out through the holes in the shell by this respiratory/sanitation current. If the mantle cavity, under the holes, gets too full of eggs or sperm the abalone may raise its shell up then quickly pull down, creating a squirt of water out of the holes. This may happen a couple of times to clear the eggs or sperm from the gill area. The exact reason that starts an abalone to spawn in nature is not known.

Fertilization occurs in the ocean water. In general when one abalone spawns in an area the others usually start as well. This ensures a large number of eggs and sperm in the same water mass which increases the probability of fertilization. But, fertilization is still left to chance.

The fertilized egg (zygote) develops to a typical gastropod larval form called a veliger. These microscopic veligers 'sink and swim' in the ocean and are ready to take up benthic (bottom dwelling) life in about 7-14 days. They do not require any food during this time, making them an ideal animal for mariculture.

When competent the veligers will sink and test the substrate. When the proper substrate is encountered they will stay and stop their 'sink and swim' planktonic existence. For most microscopic planktonic marine larvae (of benthic, bottom dwelling, adults) there is some chemical cue that causes them to stop swimming and stay on the bottom. For abalone this chemical is GABA (gamma-aminobutyric acid), a chemical found in coralline algae.

Coralline algae
Coralline algae growing on an abalone shell.

Coralline algae is often the first food source for baby abalone. They graze on the thin layer of diatoms that are usually found on the coralline algae. Coralline algae grow as a thin, pink, encrustacean on shells and rocks in the ocean. As the abalone grow they begin to feed on seaweed, primarily kelp. Some species of seaweed cause the baby abalone to put on various colors of shell. As the abalone grows it adds new shell to the outer rim causing the shell to increase in diameter and a change in food can be seen in the resulting shell.

Black abalone with brown growth rings due to feeding Red abalone with red and white growth rings due to feeding
Black abalone with brown growth rings due to feeding (left). Red abalone with red and white growth rings due to feeding (right).

Green growth ring due to feeding
Green growth ring due to feeding.

Abalone growth rings due to feeding are concentric rings laid down all along the edge of the shell. These circular banding patterns can be used as a clue to the types of food available to the abalone when it was young. As the abalone grows it adds new open holes in the area just to the left of its head, as needed. As a new hole is made the abalone will plug up an older hole, at the end of the line, that no longer is above the respiratory area. Each species of abalone has a range of open holes that can be found in each shell.

Red abalone repair to broken shell Black abalone repair to broken shell
Red abalone repair to broken shell (left). Black abalone repair to broken shell (right).

Green abalone repair to broken shell Threaded abalone repair to broken shell
Green abalone repair to broken shell (left). Threaded abalone repair to broken shell (right).

Abalone can repair minor damage to their shell. The repair is done by the mantle as it lays down new shell and increases the thickness of the shell. Abalone can be damaged by humans trying to pluck them from the rocks, and by sea otters and crabs trying the same thing. If the damage is not too severe and the animal’s tissue has not been cut, the abalone may repair its shell and continue living. This leaves a tell-tale irregularity in the shell.

Abalone shell layers
Abalone shell layers shown on a broken shell.

Abalone grow by adding new layers to their shell. These layers are laid down along the edge of the shell (increasing the shell diameter) as well as along the entire inside of the shell (increasing the shell thickness). The inside layers are mostly ‘mother-of-pearl’ and present the beautiful color patterns found in abalone shells. It is the mantle that lays down the shell. Some environmental conditions may change the way that the shell is laid down.

Rounded and flat abalone shells
Rounded (left) and flat (right) abalone shells. Both shells are the same diameter but differ greatly in depth.

Abalone may grow rounded or flat shells shells depending on their environment. An abalone living in a thin crack may stay there and grow up to an adult with a very flat shell. Flat shells also may occur in areas with strong currents as a highly rounded shell is harder to keep attached to the environment – the flat shell presents a low profile to the currents. In open areas with little current abalones tend to grow highly rounded shells.

Completely clean and highly fouled abalone shells
Completely clean (left) and highly fouled (right) abalone shells.

Abalone shells may be completely clean or highly fouled. Some of this depends on the species. Black abalone tend to have little fouling on their shells. Abalone who move into a crack and scrape the back of their shell may have abrasion marks and little fouling. Other abalone species may be covered with a whole community of marine organisms. Sometimes the fouling is so heavy that the characteristics of the shell cannot be seen. Common California fouling organisms found on abalone shells are shown here:

Highly fouled abalone shell
Highly fouled abalone shell.

Scaled tube snail, <i>Serpulorbis</i> Encrusting bryozoa, <i>Membranipora</i>
Scaled tube snail, Serpulorbis (left). Encrusting bryozoa, Membranipora (right).

Boring sponge, <i>Cliona</i>, may weaken the abalone shell Barnacles, <i>Balanus</i>
Boring sponge, Cliona may weaken the abalone shell by leaving it pitted with tiny pin-holes. Barnacles, Balanus (right).

Boring date mussel, <i>Lithophaga</i>
Boring date mussel, <i>Lithophaga</i> blisters inside abalone shell pieces

Spiral tube worms, <i>Spirorbis</i>
Boring date mussel, Lithophaga, may be hard to see from the outside (left) but this bivalve bores through the shell, creating an irritation on the inside of the shell. Note the two shells in one of the holes (left). The abalone mantle responds by creating a blister and laying down blackish mother-of-pearl inside the shell in the area of the boring date mussel (top right). This top right image has three shell pieces demonstrating a beginning blister, a black blister, and then when the date mussel breaks through the blister (left to right). Spiral tube worms, Spirorbis are spiral white tubes only a few millimeters across (bottom right).

Unusually large jewely-grade abalone pearl made into a necklace Abalone pearls
Unusually large jewely-grade abalone pearl made into a necklace (left). Abalone pearls (right).

Abalone pearls are beautiful but rare. The pearls are formed of the mother-of-pearl part of the shell and can be found nestled along the edge of the shell (near the apex) or in the gut. It is only about one in several hundred abalone that may have a pearl and the pearl may or may not be pretty. A number of abalone pearls have sharp edges and forms that are not useful for jewelry. But, the rare abalone pearl that has a rounded shape and beautiful colors can be used for a unique, one-of-a-kind piece of jewelry.

Abalone jewelry
Abalone jewelry.

Jewelry is made from the shell of the abalone. Most often it is the mother of pearl that is used but sometimes the entire shell is used. Abalone have been a source for decoration as well as food for centuries.

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(Revised 27 October 2003)
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