Phyllosoma
The average length of the planktonic life stage is between 9 and 11 months (Phillips et al. 1979). During this time phyllosoma moult approximately 15 times, through 9 developmental stages (Braine et al. 1979), though the temporal length of each phyllosoma stage is unknown. The nine developmental stages were first described by Chittleborough and Thomas (1969) and further improved by Braine et al. (1979). Below is an illustration of the final phyllosoma stage, stage IX. The phyllosoma have some swimming ability, with their mobility limited to diurnal (night and day) vertical movements.
Phyllosoma growth is significantly impacted by temperature. Liddy et al. (2004) found that increases in temperature (19, 22, and 25 °C) significantly reduced the intermoult period (time between moults), thereby increasing the growth rate. However, the same study also found that for some stages, the reduced intermoult period, induced by warmer waters, resulted in smaller post-moult sizes. There is therefore a trade-off between the intermoult period and post-moult size. The study concluded an optimal temperature of ~22 °C. This is supported by Caputi et al. (2001), who found that high puerulus settlement years were associated with warmer water temperatures, 22 to 22.5 °C, while low settlement years were associated with temperatures below 21 °C, in February-April in the area occupied by the phyllosoma off the Western Australian coast.
Puerulus
“The most profound transformation at a single moult known in the Decapoda”
(Gurney, 1942)
Gurney’s statement was in reference to the transformation that occurs from the final stage phyllosoma as it metamorphoses into the free-swimming puerulus (Figure 2). The puerulus looks like a miniature, transparent lobster, of around 35 mm in total length (with the antenna stretched out).
The metamorphic moult occurs primarily at the continental slope region adjoining the shelf-break off Western Australia (McWilliam and Phillips 1997; Phillips and Pearce 1997). After moulting, the puerulus must make the 40-60 km journey to the coast where they settle on the inshore reefs that will become their juvenile lobster habitat. As pueruli are a non-feeding stage, it is important that this moult occurs close enough to the coast, so that the puerulus can survive this journey sustained only by their current energy stores.
What triggers this penultimate moult to occur at the shelf break, and not further offshore, is unknown. Previous research and literature reviews have found no evidence that this final metamorphosis is triggered by any direct environmental cue (Phillips and Pearce 1997). Because the puerulus are non-feeding, it has been suggested that metamorphosis results from the culmination of sustained nutrition and reserve energy levels through the later phyllosoma stages (McWilliam and Phillips 1997). Satellite data show chlorophyll concentrations (likely related to phytoplankton and zooplankton abundance) are highest along the edge of the continental shelf during late winter-spring. However, widely variable stores of energy have been recorded in offshore puerulus, making it highly unlikely a particular energy reserve level triggers this moult (Phillips et al. 2006).
Based on basal (resting) metabolic rate and observed changes in energy reserves, the swimming puerulus stage has been estimated to last a maximum of ~21.6 days (Lemmens 1994). When the estimated cost of actively swimming across the continental shelf was taken into account, this estimate was reduced to around one week. However, this is probably an underestimate as more recent research has indicated that the energy consumption by puerulus during migration onshore is significantly lower than had been previously estimated for this, and other rock lobster species (Phillips et al. 2006). It seems reasonable, therefore, to conclude that the puerulus stage lasts somewhere between seven and 21 days.
Figure 2. Clear puerulus (left image). After the clear puerulus settle they begin to pigment (gain colour) within a few days (left image, first three lobsters on the right). They then moult into the first stage juvenile within around 13 days post-settlement (right image, last two lobsters on right). (Images: DPIRD)
Post-Puerulus
When the swimming puerulus reach the shallow inshore reefs, they settle into their new juvenile habitat. Clear puerulus begin to pigment (gain colour) within a few days of settling, having fully pigmented by around seven to nine days post-settlement (Lemmens 1994). They then moult into the first stage juvenile between eight and 13 days post-settlement (Lemmens 1994). Both these timeframes have been shown to be temperature-dependent, with warmer waters associated with faster development (Lemmens 1994). The term post-puerulus has generally been used to describe any lobsters less than 25 mm in carapace length, and therefore, probably includes both the settled puerulus and early juvenile stages. More information on post-puerulus habitat and behaviour can be found in Juveniles.
Development of feeding structures
There is a substantial change in the feeding structures of the western rock lobster in its transition from phyllosoma, to puerulus, to juvenile. The latter stage phyllosoma are well equipped for capture and mastication (chewing) of food, with well-developed external feeding structures, i.e. mouth parts (Lemmens and Knott 1994) and hooks on their legs to capture prey. Upon metamorphosising into the puerulus, the mouth parts are poorly equipped for food mastication and the gut lacks the structures required for internal food digestion. However, upon the first stage juvenile moult, the animals are again equipped with well-developed external, as well as internal feeding structures (Lemmens and Knott 1994). Part of this transformation involves the calcification of the mouth region in juveniles. Since internal calcium reserves are small in the puerulus, calcium may initially be taken up directly from the seawater following the first stage juvenile moult (Lemmens 1995).