Growth
Western rock lobsters, like all crustaceans, display incremental growth through the moulting of their exoskeleton. Because lobsters continue to moult throughout their life, growth continues, and lobsters never stop growing, although increases in size do slow in adulthood. Growth in lobsters is governed by the amount they grow each time they moult (growth increment) and how often they moult (moult frequency).
Growth increment per moult
While growth rates vary greatly between individuals, affected by age, sex, water temperature, food availability and diet, several studies have sought to estimate the mean growth increment per moulting event. One study found juveniles, three to five years post settlement, to grow an average of 3 – 5.4 mm per moult (Chittleborough 1976). This study found large differences in growth increments between locations, with the greatest moult increments obtained in aquaria by lobsters reared with minimal environmental stress and an abundant food supply. Another study estimated the mean growth increment per moult of two size classes of males and females, at Rat Island in the Abrolhos Island, the results of which are shown in Table 1 below (Morgan 1977).
| Sex | Carapace length | Growth increment |
| Males | 60 – 76 mm | 5.3 mm |
| Males | 77 – 90 mm | 3.8 mm |
| Females | 60 – 70 mm | 5.3 mm |
| Females | 70 – 90 mm | 2.3 mm |
Sexual Maturity
Growth in both male and female western rock lobster is categorised by rapid juvenile growth, followed by a reduction in growth rate after sexual maturity, (Figure 1.A) (de Lestang 2018). In juveniles, growth rates are inversely related to carapace length, i.e. growth rate decreases as carapace length increases (de Lestang 2018). Juvenile growth rates do not differ between sexes (Chittleborough 1975). However, the reduction in growth rates with sexual maturity is more pronounced in females than males, and as a result, adult males have higher growth rates than adult females and therefore attain larger maximum sizes (de Lestang 2018). This difference between the sexes is likely related to a greater reallocation of resources from somatic (body) growth into reproduction (e.g. development of eggs) with the attainment of maturity in female lobsters.
Increased growth in older males?
There is some evidence from lobsters in captivity that males over 8 years of age cease mating and experience an increase in growth rate (Phillips et al. 1983).
However, this seems unlikely as the presence of an increased growth rate at ~ 8 years of age is not supported by the over 30,000 tagged lobsters that have been returned to the department since the mid-1990s. All of this information indicates that as lobsters (both male and female) increase in age, their annual increase in body size decreases (de Lestang 2018).
Water temperature
Warmer water temperatures are associated with greater growth rates in juvenile and adult western rock lobster (Chittleborough 1975, Johnston et al. 2008). One aquaria study found that mean annual juvenile growth (in carapace length) was almost linearly related to temperature up to a peak around 26°C, above which growth was depressed (Chittleborough 1975). In the wild, increased juvenile and adult growth rates have been correlated with warmer waters throughout the western rock lobster distribution, (Figure 1.D) (de Lestang 2018). Under very warm conditions, juvenile lobsters experience precocious maturation, reaching sexual maturity much sooner, at a younger age and smaller size, than in cooler climates (Johnston et al. 2008, Melville-Smith et al. 2010, de Lestang 2018). As a result, growth in warmer waters is characterised by rapid juvenile growth, which slows down sooner (earlier sexual maturity) than in colder waters, where juvenile growth rates are slower, but persist for longer, therefore allowing lobsters to reach larger sizes (Figure 1.D).
Diet & Density
Food supply and nutritional value have been demonstrated to impact growth rates in juveniles in both the wild and in laboratory experiments (Chittleborough 1975, 1976, Edgar 1990). Limiting food supply has demonstrated that food shortage can lead to decreased frequency of moulting, and therefore reduced growth rates, and a more severe shortage of food, causes both further decreased moulting, and also depressed growth increments (Chittleborough 1976). Other studies have demonstrated that in the wild, growth rates can be correlated with the availability of higher quality food sources, such as the availability of preferred molluscan prey (Edgar 1990). Similarly, comparisons of fresh mussels to commercial food pellets found post-puerulus grew faster when fed mussels (Tsvetnenko et al. 1999). Growth rates also vary with density, with lower growth rates associated with greater lobster densities, potentially as a result of intraspecific competition for food or other resources (Figure 1.C) (Chittleborough 1976, Jernakoff et al. 1994, de Lestang 2018). Lower densities do not always result in increased growth however, as animals held in isolation display longer intermoult periods and therefore reduced growth rates, compared with those in groups (Chittleborough 1975).
Limb loss
Limb loss and other damage to the exoskeleton can also cause precocious (early) moulting. The loss of two limbs is to be sufficient to cause a reduction in moult interval without impacting growth increment, therefore increasing growth rate, while the loss of four limbs both shortened the moult interval and reduced the growth increment, therefore reducing growth rates (Chittleborough 1975).
Day length
Research has indicated that varying day length is unlikely to affect lobster growth, although continuous darkness has been shown to inhibit growth (Chittleborough 1975).
Length-weight relationship
The relationship between the carapace length and the weight of western rock lobsters is illustrated in Figure 2 below and is described by the following equations:
Females: weight (g) = 0.003 * length (mm) 2.7287
Males: weight (g) = 0.005* length (mm) 2.6367
The slope of this relationship is similar for males and females, indicating a similar weight increase for each growth increment.
As this data is obtained from recreationally caught lobsters, no length-weight relationship exists for animals below legal size (76 mm).
Ageing
Western rock lobster age is most commonly estimated from growth curves, using data collected from tag-recapture and/or laboratory studies (Chittleborough 1976, Joll & Phillips 1984, Caputi & Brown 1986, de Lestang 2018). Unlike finfish, where ageing via an otolith is the standard (akin to counting rings in a tree), there is no single preferred method for directly ageing crustaceans. Several methods for direct ageing of the western rock lobster have been trialled, with varying degrees of success. Lipofuscin, also known as fluorescent age-pigment (FAP), is deposited in a range of animal tissues throughout life, with concentrations, therefore, being highly correlated with age. Crossland et al. (1988) provided a preliminary assessment describing techniques for extracting and analysing lipofuscin in the eyes and tails of western rock lobster to determine age. Unfortunately, there was extreme variation in the results, which likely reflected both differences in the rate of FAP accumulation and issues with using carapace length as a proxy for age. However, through modal analysis of a lipofuscin concentration–frequency histogram, and comparison to tag studies, Sheehy et al. (1998) demonstrated that analysis of FAP was able to distinguish age cohorts of up to 7+ years within a juvenile habitat site, while length-frequency analysis could only distinguish up to 4+ years. Another method that has been applied to ageing lobsters is counting marks in cross-sectioned gastric ossicles (Leland & Bucher 2017). This method is similar to the use of otoliths to age finfish. While there is evidence that this method somewhat estimates age, the technique appears to be no more precise than non-invasive methods, such as modelling age based on carapace length alone.
Natural Mortality
Natural mortality rates appear to be highest in small/young animals, decreasing with age and size (Figure 3) (Chittleborough 1970, Phillips et al. 2003). The mortality during the first year after settlement (from ages 1–2 years) has been estimated at between 80 and 97% (Phillips et al. 2003). Modelling has revealed that only very small numbers, 3 – 20%, of the settling puerulus live to recruit into the fishery some 4-5 years later (Phillips et al. 2003).
Several studies have found that mortality in juveniles is density-dependent; with high rates of morality found in areas of higher density (Phillips et al. 2003; Chittleborough and Phillips 1975; Phillips 1990). One such study removed juveniles from one reef to lower the density and then compared the growth and mortality rates of juveniles with a control reef (an unaltered reef) (Ford et al. 1988). While there were no differences in the growth rates of juveniles at the two reefs, lobsters on the control reef had significantly higher mortality rates than those on the reef with reduced density. Density-dependent mortality may be a result of competition for resources (e.g. shelters for protection and food) and as well as higher rates of predation.
Fine-scale spatial modelling has revealed variation in juvenile density-dependent mortality throughout the spatial distribution of the species (de Lestang et al. 2009). The estimates produced by modelling found density-dependent mortality to vary greatly, with the highest levels in the Abrolhos Islands and lowest at Lancelin. This was in line with previous research which found the highest levels of density-dependent mortality at the Abrolhos Islands, and the lowest along the south coast (Caputi et al. 1995). The Abrolhos Islands is an area of higher density compared to the coastal areas, especially in the southern region. This variation highlights the importance of habitat and resource availability in determining density-dependent mortality. In areas with limited resources, greater puerulus settlement will bring more lobsters to the reef than the resources can support, and therefore natural mortality will increase.
Less is known about natural mortality in adult lobsters. It is generally assumed that mortality rates are lower in adult lobsters compared with juveniles; a reduction in predation rates alone would be expected to result in lower mortality rates. Additionally, density-dependent morality is probably less prevalent among adults, as mortality during the juvenile years will have already thinned the density to a suitable limit, and if resources are limited adults and larger juveniles are more capable of moving to less dense reefs. It is thought that migrating white lobsters, due to the fact they are more exposed to predators as they walk over bare sandy areas, have a higher rate of natural mortality than do similar-sized red lobsters, who remain on and around reef systems.
Because of their ability to moult and other anti-aging mechanisms, lobsters rarely succumb to disease and age-related diseases, including cancer, are virtually unknown (Vogt 2012). One male lobster lived in aquaria until he was at least 24 years old (Gray 1992).
Figure 3: Depletion of pueruli and post-pueruli from settlement (age 1 year) to age 8 years assuming density-dependent mortality and no fishing effort (Phillips et al. 2003).
References
Gray H (1992) The Western Rock Lobster, Panulirus Cygnus: A natural history. Book 1. Westralian Books.