Western rock lobsters, like all crustaceans, display incremental growth through the moulting of their exoskeleton (shell). Moulting, or ecdysis, occurs when the underlying muscle and tissue have grown too large for the exoskeleton. At this time, the lobster sheds its exoskeleton by exiting through the space between the carapace and the tail, on the dorsal (top) side.
Observations of lobsters in the laboratory indicate that moulting occurs at night, and it is believed among fishers that moulting occurs on or close to the new moon. While behavioural changes associated with the moult can last several hours, the withdrawal from the old shell only takes between 8 – 10 minutes. See Thomas (1966) for a detailed description of a moult event.
After moulting, while the outer layer of skin (epidermis) that will form the new shell is still soft, the lobster will rapidly absorb water and expand its epidermis to ensure that, as the new shell hardens, there is room for subsequent tissue growth. According to Dall and Barclay (1977) the new shell takes around two days to begin hardening, and around 19 days to complete. Under warmer water conditions the shell hardens more rapidly.
Moulting serves several purposes in addition to growth. It allows for the regeneration of lost or damaged limbs or antennae, repairs to the exoskeleton from disease or damage, as well as allowing for the development of reproductive structures at maturity and prior to the breeding season.
Moult stages
The moult cycle (from hard to moulted and back to hard) has been classified into several stages commonly referred to as; pre-moult, moult (ecdysis or exuviation), post-moult, and inter-moult. Western rock lobsters do not exhibit any obvious external indicators of moult condition, although a softening along the branchiostegites (sides of the carapace) on and along the ecdysial line (where the carapace breaks in two), indicates the pre-moult condition several days before moulting (Thomas 1966). Moult stages are more accurately identified through microscopic examination of the distal tip (the tip located furthest from the point of attachment to the body) of the pleopod or uropod, and is based on the retraction of the epidermis (future shell) from the outer cuticle (current shell), and the development of the structures in the new shell. The criteria for the classification of each stage follow the methods of Drach & Tchernigovtzeff (1967) and are described specifically for P. cygnus in Dall and Barclay (1977). These methods outline 12 stages from A-E (A, B, C1, C2, C3, C4, D0, D1, D2, D3, D4, E), with A and B being post-moult, C inter-moult, and D pre-moult conditions, and E moulting.
Moult cycle
Juveniles
The moult cycle of juveniles has been studied in aquaria (Dall & Barclay 1977). Recently settled post puerulus have a moult cycle of approximately 10 days, increasing to around three weeks over the next three to four months. During this time the premoult period occupies approximately 45-50% of the moult cycle: i.e. 4 to 10 days.
By around three to four years post-settlement the moult cycle has lengthened to every 4 to 6 months (dependent on water temperature). Once the moult cycle is greater than around three months the premoult period appears to consistently take around one month (Dall & Barclay 1977).
“Whites”
At approximately 4 years of age, just prior to the onset of maturity, lobsters enter the “whites phase”, during which time they experience two moults per year. In early summer, (Late October/ early November) these lobsters undergo a synchronised moult, which results in a shell colour change from red to pale pink. Many of these lobsters then undertake an extensive, synchronised migration offshore.
At the conclusion of this migration, around February/March, the white lobsters moult again, returning to their usual red colouration. Anecdotal evidence from fishers indicates that most lobsters have returned to their deep red colour by early March, indicating that the moult occurs in February or early March. This pattern is also supported by tagging field studies, which found that lobsters 60 – 80 mm in carapace length moult twice per year, once between February and April, and again in November (Melville-Smith et al. 1997).
Adult females
Because adult females moult into and out of a reproductively mature state (the presence of setae) their moult cycle is easier to track than adult males. Most females will moult twice per year; once in February/March, transitioning from a setose to a non-setose state, and again in May, when they transition into a setose state in preparation for the following spawning season (Chittleborough 1976, de Lestang & Melville-Smith 2006). The first moult in February/March may line up with the aforementioned white to red moult or may be linked to when females have finished incubating their eggs.
Water temperature
These synchronised moults appear to be affected by water temperatures. Evidence indicates that the whites migration, and the associated moult, occur earlier in warmer years and later in cooler years. Modelling of commercial fishers’ behaviour has indicated a strong correlation between the average water temperature in August/September, and the day in November that fishers move their gear offshore as they “chase the whites” (Figure 2). Because the timing of fishers moving offshore is indicative of the timing of the white’s migration, and therefore the associated whites’ moult, we can conclude that in warmer years the white’s moult occurs earlier in November (or potentially even in late October), while in cooler years, the moult occurs later in November.
Similarly, the synchronised February/March moults of the whites and adult females have also been correlated with water temperatures. The peak in catches following this moult is thought to be related to these moults, as 1) the white-red moult results in many lobsters increasing in size from below to above the minimum legal limit, 2) females moult out of their legally protected reproductive state (carrying eggs, tarred, and, until recently, setose), and therefore contribute to catches, and 3) lobsters are more catchable directly after moulting as they are hungry. In warmer years this peak occurs earlier than in cooler years, as indicated by the ratio of March to April catches in Figure 3. Therefore, it is believed that these moults occur earlier in warmer years compared with cooler years.
What about males?
The moult cycle in adult males is much harder to track as there are no obvious indicators of moult stage. Anecdotal evidence from adults held in aquaria (personal observations DPIRD 2017) indicates there may be a synchronised moult in September, potentially after mating, however, future research is required to confirm this.
Additional moults
Moulting may occur throughout the year in addition to these synchronised moults, as a result of limb loss, exoskeleton damage, or disease. While studies have demonstrated limb loss can cause precocious moulting in juveniles (Chittleborough 1975), no such studies exist for adult females or males.
Moulting affects behaviour
In the lead up to a moult, while in a late-stage pre-moult condition, food consumption and movement rates drop dramatically in western rock lobster (Chittleborough 1970, Morgan 1978, Joll & Phillips 1984, Jernakoff & Phillips 1993), with feeding in juveniles observed to completely cease from between two (Chittleborough 1975), and 21 days (Dall & Barclay 1977) prior to moulting. This causes lobsters in a premoult condition to be less catchable than those in an intermoult or post moult (Morgan 1974). Part of the reason behind this is that lobsters (like most crustaceans) also moult their stomach at ecdysis, so having it empty makes the moulting process far easier. We know from other spiny lobster species, that during this time large amounts of calcium are absorbed from the exoskeleton into the lobster’s blood, causing the exoskeleton to become more brittle. Post moult, this calcium will be redeposited into the new exoskeleton, to help it calcify. Feeding re-commences after around 2 days (Chittleborough 1975, Dall & Barclay 1977). According to one study, food consumption rates rise rapidly, peaking on the fourth or fifth-day post moult, and follow a downward trend thereafter (Chittleborough 1975). Catchability is therefore considered to peak while lobsters are in a post-moult condition.