Western rock lobsters are an omnivorous species that consumes an extremely wide range of benthic food items and are therefore classified as generalist consumers (Joll & Phillips 1984, Edgar 1990, MacArthur et al. 2011).
The majority of published studies on diet have focused on juveniles in shallow coastal ecosystems (< 5 m depth). These studies have found lobsters to feed on mobile invertebrates, such as crabs and gastropods, sessile filter feeders (e.g mussels), non-coralline and coralline algae, and seagrass (Edgar 1990, MacArthur et al. 2011, Dumas et al. 2013). One study found that post-puerulus consume a variety of plant and animal material, with a dietary spectrum similar to that of larger juveniles from the same locations (Jernakoff & Phillips 1993).
There appears to be a general preference for animal prey over plant material in juvenile western rock lobsters, and indeed, a higher proportion of animal matter is associated with greater growth rates (Chittleborough 1976a, Joll & Phillips 1984, Edgar 1990). Feeding trials have further revealed prey preferences in juveniles, with strong preferences for abalone and crabs, some preference for mussels, octopus, and teleost fish, and no preference for urchins (Chittleborough 1975, Dumas et al. 2013). Bait may also be an important source of nutrition for lobsters in areas where fishing occurs, with studies finding that bait can contribute up to ~30% of lobster nutrition in typical juvenile shallow-water habitats (< 20 m) (MacArthur et al. 2011), and 30-80% of lobster nutrition in deepwater habitats (35 – 60 m) (Waddington et al. 2008).
The western rock lobster’s diet appears to be highly variable, with several studies demonstrating that the juvenile diet varies temporally (through time) and spatially (in different areas), and is related to the availability of prey items (Edgar 1990, MacArthur et al. 2011). MacArthur et al. (2011) found that juveniles in macro-algal and sand dominated habitats consumed significantly more sessile filter feeders (sponges and colonial ascidians) than those in the adjacent seagrass-dominated sites, where gastropods were consumed in much greater volumes. The authors concluded that this was due to differences in the volume of prey items in each habitat type. Edgar (1990) also found that the juvenile western rock lobster diet is affected by prey availability, with the proportion of molluscs in the diet related to spatial and temporal variations in mollusc settlement and distribution. When slow moving benthic prey, especially molluscs, were in low abundance, rock lobsters incorporated large quantities of plant material into their diet (Edgar 1990).
Lobsters in deep-water habitats, irrespective of size or sex, appear to be primarily carnivorous, with diets consisting predominately of bait (from the fishery) and small crustaceans – crabs and amphipods/isopod (Waddington et al. 2008). In contrast to studies in juvenile habitats, where coralline algae and mussels are highly important, these items were only minor dietary components in these deepwater habitats, despite overlap in the size structure of lobsters studied. Evidently, habitat, and the associated benthic assemblage, is the main driver of variations in lobster diet, likely through the availability of prey items. Moreover, it appears that neither lobster size nor sex has a significant effect on diet (Waddington et al. 2008, MacArthur et al. 2011). However, Dumas et al (2013) found small differences between large adults, and medium adults and juveniles, with the former preferring crabs and mussels over gastropods (kelp shells) and urchins, while the latter preferred only crabs over mussels, gastropods, and urchins (Dumas et al. 2013).
Experiments limiting food supply in juveniles have demonstrated that food shortages reduce growth rates (Chittleborough 1975 1976b), and when food is completely withheld, death occurs within 24 and 43 weeks (Chittleborough 1975). Food consumption rates have been correlated with numerous factors in western rock lobsters, including temperature, density, and moult cycle.
Because increased water temperature can stimulate growth, food consumption generally increases with water temperature. One study demonstrated that two-year-old juveniles held at 26°C consume significantly more food than those held at 23°C (Chittleborough 1975). Aquaria studies have also found that solitary animals and, to a lesser extent, animals housed in pairs, were more sedentary and consumed significantly less food, than those at a density of three or four animals per tank (Ghisalberti et al. 2004, Waddington et al. 2005). Western rock lobsters are also known to eat less when in a pre-moult condition, with consumption rates dramatically increasing again several days after moulting (Chittleborough 1975, Joll & Phillips 1984, Jernakoff & Phillips 1993). According to one study, food intake rises rapidly to a peak on the fourth or fifth-day post moult and follows a downward trend thereafter (Chittleborough 1975).
Moult stage appears to affect the ratio of food types in the fore-gut of juveniles, namely coralline algae and exuvia (moulted exoskeleton), with the same pattern likely in adults. Animals in a post-moult condition have a higher proportion of coralline algae in the gut than those in an intermoult or pre-moult stage (Jernakoff & Phillips 1993). It is hypothesised that coralline algae may be consumed by post moult animals in an effort to build up calcium carbonate levels in the blood and thus help strengthen the new exoskeleton, as research has demonstrated that juveniles are able to transfer calcium from algae to the exoskeleton (Joll & Crossland 1983). However, coralline algae has also been shown to provide between ~20 and ~80% of the western rock lobster’s energy requirements and may aid in the uptake of nitrogen when ingested as a supplement to animal prey (Joll & Crossland 1983, MacArthur et al. 2011). Western rock lobsters are known to consume both their own and other lobsters’ exoskeletons after moulting. This behaviour has been observed in both adults and juveniles in the laboratory (per. obs. Emma-Jade Tuffley), and in juveniles in the field, where fragments of lobster shell have been found in the gut (Joll & Phillips 1984). The condition of the shell fragments and the parts found (antennae, antennules, and pereiopods) indicate that these fragments were the result of consuming exuvia and not cannibalism, and indeed the proportion of exoskeleton in the foregut was found to increase at times of high moulting activity (Joll and Phillips 1984). Although the shell fragments found in the guts of wild-caught lobsters have been commonly considered the result of consuming exuvia, cannibalism is known to occur, especially in a laboratory setting. Cannibalism in aquaria has been documented in juveniles and appears to be most common in recently moulted and smaller animals (especially post puerulie), and increases at higher densities or when food is supply insufficient or of poor quality (Johnston et al. 2006, Moyle et al. 2009). Additionally, observations of cannibalism of recently moulted adults in the laboratory confirm its occurrence in larger animals (per. obs. Emma-Jade Tuffley). It is unknown how much, if any, cannibalism occurs in the wild, however, cannibalism in wild populations has been observed in other spiny lobster species, and it is likely that at least some of the density-dependent mortality of western rock lobster, especially in post puerulus, is the result of cannibalism.