SpruceRoots Magazine - February 2001

abalone sex

by Lynn Lee

The life of an abalone begins with conception. Unlike our familiar concepts, abalone conception doesn't begin inside a mature animal ­ it occurs in the seawater column! Abalone are broadcast spawners ­ adult abalone of separate sexes release millions of eggs and sperm into the water and there it must meet. A rather optimistic way to get together in all that fluid space! As you can imagine, many eggs and many sperm must be released at the same time in the same place for fertilization to succeed. To help the process along, when abalone spawn, mature adults congregate in the shallow subtidal, gathering in relatively high densities on the peak of the highest rock in the area - being higher when sperm is released gives eggs a greater chance of encountering it. Adding to the excitement, they sometimes balance on top of one another up to 6 abalone tall ­ all to get that extra high! In BC, spawning tends to occur between June and August.

Within several days, the eggs hatch, beginning the free-swimming larval phase of the abalone life cycle. The first stage is a very small larvae called a Trochophore, which does not need to eat because it is equipped with its own yolk sac for energy. The trochophore larva moves upwards in the water column, attracted to light, thus increasing its travel opportunities on ocean currents ­ this is really the abalone's only chance to visit wider parts of the neighbourhood. In a couple more days, a larval shell starts to form and the trochopore transforms into the second stage of larval life as a Veliger that is no longer attracted to the light. Within 5 to 8 days of hatch, the larva undergoes metamorphosis, turning into a very very small abalone ready to spend life roaming the rocks.

When juvenile abalone are ready, chemical signals emitted from particular types of algae, including encrusting red coralline algae, compel the larva to undergo metamorphosis and settle. Juvenile animals less than one or two year old tend to be found in crustose red algae gardens generally deeper than adult animals. It is thought that juveniles settle in deeper water ­ usually deeper than the kelp zone ­ down to 15m deep. As they grow, abalone migrate upward such that most adults are within 10m of the water surface.

As juveniles, abalone tend to be cryptic, hiding under rocks and in crevices for most of the day, as they get larger they tend to hide less such that by the time they reach 100mm in shell length, they are all "exposed". The story is that younger smaller abalone have to worry more about being eaten than their larger friends and therefore, the smaller animals need to hide to survive.

When they are very young and small, juvenile abalone must eat very tiny plants. Grazing first on diatoms (single-celled phytoplankton) and bacteria which collect on the surface layer of encrusting red coralline algae, the abalone moves on to scrap the surface layer of the coralline algae. By 6 to 13 weeks of age, crustose red algae dominate the abalone food palette. As they grow larger, abalone tend to move around less, preferring to graze less and spend more time trapping drift algae for food. But not just any food will do! Experiments show that as adults, abalone prefer bull kelp (Nereocystis) and giant kelp (Macrocystis) over other kelps.

When an abalone will fit into size 8 shoes is not as consistent as you might think. The growth rates of abalone are highly variable, depending on numerous environmental factors such as the quantity and quality of available food, the frequency and magnitude of storm conditions (exposure). At each extreme, abalone fortunate enough to settle in a protected kelp forests with plenty of high quality drift algae will tend to grow much faster and to a larger maximum size than those who settled in a high exposure kelp forest with little drift algae. Interestingly ­ like tomato plants taken from outdoors, placed in a greenhouse and fertilized - abalone will increase in growth rate and size when moved from difficult to more favorable habitat conditions.

Unlike the aging of trees, salmon and geoducks, the age of abalone cannot be determined by counting growth rings. Although abalone do have seasonal growth cycles ­ for example, little shell growth during periods of high egg and sperm production ­ annual growth ring formation is not consistent. Measuring, tagging and re-measuring of individual animals a year later has been the only reliable indicator of natural growth rates. From these data points, growth curves generated for abalone suggest that it takes at least 6 years, and more often 7 or 8 years, for an abalone to reach 100mm in shell length (significant because this was the minimum size limit of the commercial and recreational harvest). By 3 years of age (~50mm length), gonads are maturing and by 4 years of age, individuals are generally spawning. Like many long-lived slow-growing animals, though, the level of egg production increases rapidly with size after maturity, reach a climax age then decline in egg production beyond that. The largest northern abalone ever documented was 165mm in length, at least 15 years old and perhaps up to 50 years old!

photo - Bart DeFreitas

SpruceRoots Magazine - February 2001