Posted on: August 21, 2015
Western rock lobsters reach sexual maturity at 5-6 years of age. By this stage they are generally living in deeper water (> 20 m) and are about 80 mm in carapace length, which is a size that can be legally landed (kept).
Each year in May /June, mature female lobsters moult and produce long hairs (setae) attached to the pleopods (paddle-like swimming appendages) under their tails. Fertilised eggs attach to these hairs later that season.
Once females have hardened their shells and water temperatures have bottomed out (July/August), females begin to become receptive to male lobsters. From the little we know, male lobsters approach females with the aim of mating. Only when a female is receptive will she allow the male to copulate with her. In this vision, it seems the female not only allows an approach from the male, but actively participates and appears to be the one approaching the male and climbing on top of him.
As the female slides off the male you can just see the dark spermatophoric mass (or tarspot, so called due to its colour) stuck to her abdomen. You can also see the enlarged right gonapod (the male has two of these reproductive organs, left and right) of the male which is used for the application of the spermatophoric mass. The mass is a combination of sperm and a cementing material. This material will protect the sperm until the female wishes to fertilise her eggs.
When the female feels it is the correct time to fertilise her eggs (this can be a few months after mating, usually in October) she will bend her tail in a similar fashion as in the video and use special hooks on the ends of her last pair of legs to scratch open the spermatophoric mass. The sperm are motile (can swim) in water and swim about under her closed tail while she extrudes sticky eggs onto the hairs (setae) under her tail.
Once fertilised, the eggs will remain attached to the setae for about 6–10 weeks (depending on water temperature – hotter means faster growth and shorter gestation). There, they will progress from orange in colour, as they are full of yolk, to dark grey, when the yolk is used up and the lobsters larvae (phyllosoma) can be seen through the egg case.
When ready the female shakes her tail and the eggs split open and the larvae drift off on a 9–11 month journey of the ocean, traveling up to 1500 km offshore, before they return as puerulus and settle along our coast-line to start their lives as lobsters.
Females can mate and spawn multiple times within a spawning season (September – January), with smaller females appearing to only spawn once, while larger females can spawn up to three times. Females can re-mate with a male only a few days after fertilising a batch of eggs.
Depending on size, a female can produce over 1 million eggs in a single batch.