During the juvenile stage of the western rock lobster life cycle, there is an ontogenetic shift in behaviour, that is a change in behaviour that relates to the animal’s development, from solitary to gregarious. While post-puerulus (juveniles CL < 25 mm) can sometimes be found in groups, they generally shelter alone, with one study finding 75% of post-puerulus were solitary, and what gregarious behaviour was observed was most commonly associated with larger post-pueruli (Jernakoff 1990). In the laboratory, aggressive behaviours have been observed in post-puerulus when resources are limited (e.g. food or space) (Berrill 1976). Older stages of pot-puerulus, with longer antennae, tend to dominate younger post-puerulus, with shorter antennae. They were also capable of making a rasping sound during aggressive encounters, which appears to denote an intraspecific (of the same species) threat, rather than a response to a predator.
This is in contrast to juvenile lobsters (CL > 25 mm) which have high levels of den sharing and cohabitation, with one study finding that only 3% of juveniles denned alone (Cobb 1981). Laboratory experiments have also indicated that western rock lobsters prefer to be housed in groups. Juveniles in groups of three have been shown to be more responsive to food odours compared with solitary animals (Ghisalberti et al. 2004), and those housed in groups of four to 16 had higher growth rates, as a result of reduced intermoult periods, compared to solitary animals (Chittleborough 1975). Shelter preference trials have revealed that juvenile lobsters preferentially denned in shelters containing larger groups of juveniles, compared with empty shelters or shelters with fewer lobsters (Keymer 2017). Seemingly conversely, there is also evidence of antagonistic or antisocial behaviours between lobsters. In underwater observations, Cobb (1981) noted that if a den was too crowded “there would be some movement after the new lobster had entered, and after a brief aggressive encounter, one of the lobsters would leave the den and walk directly to another”. Similarly, aggressive interactions between lobsters have been observed around pots in a laboratory setting (Konzewitsch 2009). Furthermore, studies on catch rates have revealed dominance behaviours in larger animals, with large lobsters excluding or deterring smaller animals from pots (Chittleborough 1974, Tuffley et al. 2021).