The Reproductive Cycle
The reproductive cycle, and its timing, is summarised in Figure 1 below. The cycle begins in May when sexually mature females moult into a setose state; they develop fine hairs, called setae, on the endopods (the inner branch of the pleopod) (images 1 – 3 of the reproductive cycle slideshow below). These reproductive females begin mating in June/July and are identified by the presence of the external spermatophore, or “tar spot”, attached to her thoracic plate (images 4 and 5). Females then extrude and fertilise their eggs some months later, around September/October. Females carry the eggs under their tails (image 6) until fully developed (image 7) at which time the larvae hatch (image 8). Females then moult back into a non-setose state around February/March. This cycle, and its variations, are discussed in more detail below.
Figure 1. Timing of the reproductive cycle of the western rock lobster. Percentage = the percentage of the catch of mature females that is either setose (blue), mated (red), egg bearing (black).
Reproductive Cycle Slide Show
Mating
Mating (deposition of the spermatophore) begins in June, peaking in September/October. While little is known of western rock lobster mating behaviour in the wild, according to Gray (1992):
“In the laboratory male western rock lobsters have been observed confronting females head-on, trying with elongated legs to withdraw her from her shelter and into mating position. These attempts fail repeatedly, but the male persists, and his increasing agitation is matched by decreasing resistance from the female. He pulls her into an upright stance, and copulation (mating) is a tangle of legs in a head to head and belly to belly affair.”
A pair were also captured on film mating in the tanks of DPIRD’s Western Australian Fisheries and Marine Research Laboratories. As can be seen from the video, copulation lasts less than a minute.
Spermatophore
Western rock lobster spermatozoa (sperm mass) are packaged into a spermatophore, a central sperm mass encapsulated by several layers of protective walls. The male releases the spermatophore through the genital openings at the base of his last pair of legs and deposits it onto the sternum of the female. Initially, the spermatophore is a white sticky substance, however, once deposited it darkens and hardens (probably over the course of a day or two) to form the black, butterfly-shaped “tar spot”. At this point, the female is totally protected from fishing and allowed to complete her reproductive journey until she moults into a clean shell again.
Fertilisation
The sperm are long-lived, capable of fertilising eggs up to 69 days after mating in aquaria (Chittleborough 1976). Males are capable of mating with multiple females and with females larger than themselves (Chittleborough 1976). In the wild, polyandry (mating with multiple partners) appears common, with one study finding that 45% of females captured at Rottnest island had attached spermatophores arising from at least two males (Loo et al. 2018). Despite the high level of polyandry, the same study found that most eggs were fertilised by only one male despite the presence of two spermatophores. Because fertilisation takes place under the female’s tail aided by the beating of the swimmerets (pleopods), there is no need for the sperm to travel long distances. Therefore the sperm of spiny lobsters lack tails and are non-motile, and are instead star-shaped, with multiple appendages or arms that aid in attaching to the eggs.
Eggs
The female’s eggs develop internally until they fill the cephalothorax with a tightly packed, bright orange, caviar-like mass that can be seen through the dorso-thoracic musculature (between the carapace and the tail) (Melville‐Smith & de Lestang 2005). When the female is ready to fertilise her eggs, around September/October (Figure 1), she extrudes the eggs from her oviducts (small pores at the base of the third pair of walking legs). According to Gray (1992), the eggs flow for up to an hour, in a steady stream towards the tail, where the female has spread her tail fan and swimmerets to form a “basket” to catch eggs. As she does so, she uses the claws on the fifth pair of walking legs to scratch the spermatophore releasing the encapsulated sperm and fertilising the eggs. The eggs then attach to the setae (the fine hairs on the endopodites) and are carried there, under the tail, until they are ready to hatch. Fertilisation rates in the wild are high, with one study finding that 95% of the eggs in female clutches are successfully fertilised (Morgan 1972). Unfertilised eggs also attach to the setae and are either preened out by the female or carried until the fertilised eggs hatch, at which time they are cleaned away with the remaining egg capsules (Chittleborough 1976). The number of eggs produced per female is positively related to size, with larger females producing relatively more eggs (Chubb 1991). The greater production of eggs by larger females is further enhanced by larger females undertaking two egg batches (double breeding) during a season
Incubation
Incubation and hatching
Egg incubation periods (the time it takes for the eggs to fully develop and hatch) have been shown to vary between 19 – 68 days dependent on water temperature, with incubation times reducing with warmer water temperatures (Figure 3) (Chittleborough 1976).
In the wild, hatching occurs from November to February, with warmer waters associated with earlier hatching (Rimmer 1980). The timing of hatching does not appear to be influenced by the lunar cycle (Rimmer 1980). All eggs have effectively hatched by the end of February, with very few to zero berried females captured in March (Figure 1).
Egg stage
Microscopic examination of lobster eggs has generally classified them from stage one to five according to the description outlined for Jasus lalandii by (Silberbauer 1971). On a macroscopic level, eggs are assessed visually as early, mid or late-stage (Melville‐Smith & de Lestang 2005). Recent unpublished work comparing microscopic development with macroscopic staging as used by DPIRD is summarised below.
Early-stage
Stages 1 and 2 (Silberbauer 1971), freshly spawned and early embryonic development.
Macro-observations: The eggs are round and filled with bright yellow to orange yolk with no visible eyespots.
Micro-observations: Splitting of the egg (cleavage) commences and a dark pigmented median eye develops between two cephalic lobes, with initially unpigmented eye capsules, that begin to pigment, but remain very small.
Mid-stage
Stages 3 and 4 (Silberbauer 1971), a well-formed to advanced embryo.
Macro-observations: Eggs become dark or dull orange in colour and eyespots become visible to the naked eye.
Micro-observations: Yolk is reduced as the embryo develops. Eye bulbs become crescent-shaped and filled with black pigment. Red chromatophores appear in the extremities of appendages.
Late-stage
Stage 5 (Silberbauer 1971), about to hatch.
Macro-observations: Eggs are dark purple/grey with visible eyespots.
Micro-observations: Yolk is reduced to a small mass, with the embryo taking up most of the egg capsule. Chromatophores extend along the length of all appendages. Embryonic movements are visible.
Double Breeders
Large females can produce two batches of eggs in one breeding season. These animals are known as “double breeders”. The carapace length at which females become double breeders shows a similar spatial trend to female size at maturity, declining from south to north along the coast, from 96.6 mm in Fremantle to 84.1 mm in Dongara, and further declining to 78.7 mm at the offshore Abrolhos Islands (Figure 4).Size at double breeding is on average around 10 mm larger than the female size at maturity at the same location (de Lestang & Melville-Smith 2006). Based on the moult cycle of females, twice per year, and the mean growth increment per moult of newly mature females of between 2-3 mm, this indicates that females require a further two years of growth after becoming sexually mature before they are larger enough to double breed.
Whether double breeders are capable of more than two batches in a season has been hard to investigate in the field. However, Phillips et al. (1983) showed that when kept at a constant temperature and fed to excess, large females were still only capable of a maximum of two or three spawnings per moult event. One reason for this may be the damage caused to the setae by preening and removing the used egg shells after the larvae have hatched. After the second batch of eggs, the setae are frayed with many having been snapped. In this condition, they would not be able to successfully hold very many eggs if they were to be used again. By moulting, however, the female is able to replenish her setae for use in the following breeding season.
Philips et al. (1983) also found that another mating was not necessary between additional spawnings and that one tar-spot was capable of fertilising up to three batches of eggs.
Moult “clean”
Signalling the end of the breeding season, females moult in February/March returning to their non-setose state, colloquially known as “clean” (de Lestang & Melville-Smith 2006). On average, 80% of mated females will undergo this moult, while around 20% will bypass this moult, instead moulting from setose to setose in the May/June moult. Research has demonstrated that the number of females skipping this moult can vary from 1 to 40%. Skipping this moult is more common among large females, and is correlated with water temperature, with more females skipping the moult in cooler years (de Lestang & Melville-Smith 2006). Females who become ovigerous later in the breeding season, most commonly large double breeders spawning their second clutch, may not have time to develop and hatch their eggs before the synchronous moult in February/March. In addition, because the embryonic incubation period is related to water temperature, this time restraint is exacerbated in years of cooler water temperatures and the number of females missing the February/March moult increases (de Lestang & Melville-Smith 2006).
Interestingly, females held at a constant temperature and fed to excess in aquaria, have been shown to continuously spawn throughout the year, even in the absence of mating activity (Chittleborough 1976; Phillips et al. 1983). These females did not moult clean, instead, they moulted on average 3 times per year, from setose to setose, spawning on average twice per moult cycle. While we are unsure what exactly triggers the end of the reproductive cycle for females in the wild, these results suggest external factors, potentially temperature or food availability, may be responsible.