How?
Little is known about how the puerulus actually settle onto their new benthic habitat. Presumably, when they contact a suitable substrate, they cease beating their pleopods, and hold on with their legs; thus becoming benthic. Whether any preferential swimming towards suitable habitat, i.e. swimming down towards the reef, occurs is unknown, but likely, as puerulus have well-developed structures for both chemosensory (sensing chemical cues) and sensing vibrations (Phillips & Macmillan 1987). Puerulus may sense their desirable habitat, either through chemical cues related to the benthos or by sensing vibrations related to reef activity, and intentionally swim onto it. Experiments on post-puerulus (less than 30 days post-settlement, and 10 – 15 mm in CL) revealed preferential habitat selection based purely on chemosensory (Brooker et al. 2022), and it is possible that settling puerulus exhibit the same behaviour.
Monitoring Settlement
Puerulus are frequently observed clinging to pot ropes and floating rafts of seagrass. Whether these animals have mistaken these objects for their final destination, or are simply resting or catching a lift, is unknown. Since the late 1960’s this behaviour has been exploited by DPIRDs puerulus collectors; floating devices that mimic these seagrass rafts to capture puerulus, thus providing a method for monitoring settlement. Interestingly puerulus have been observed actively avoiding clumps of this artificial seagrass in the field (Phillips and Olsen 1975), which may indicate that capture by these collectors is passive, with puerulus unintentionally being caught.
The settlement monitoring program started in 1968 at Seven Mile Beach north of Dongara, and since then other sites have been progressively added so that now eight locations between Kalbarri and Cape Mentelle are sampled every month. These sites are sampled within 5 days of the full moon each month. Watch the video below or head here for more information on this program.
When?
Puerulus start settling on inshore reefs in May, and continuing until around April the following year, with peak settlement occurring in late winter or early spring (August-September) (de Lestang et al. 2012). There is a correlation between the strength of the Leeuwin Current (the warmer water south-flowing current that runs along the WA coast and is strongest in the winter months) and the timing of puerulus settlement (Caputi et al. 2003). In years of stronger Leeuwin Current, water temperatures off the WA coast are warmer, and as increased water temperatures are associated with faster growth in phyllosoma, settlement tends to happen earlier in years with strong Leeuwin Current.
Where?
Substantial puerulus settlement has only been recorded in shallow reef and seagrass areas in depths of less than 5 m. Oceanographic modelling has shown that during the offshore phase ocean currents completely mix the larvae from all spawning regions (Griffin et al. 2001, Feng et al. 2011). This means that settlement in an area is not related to the hatching location, and the adult population forms a meta-population (i.e. one genetic stock). The distribution of settled puerulus has also been correlated with the strength of the Leeuwin Current (Caputi et al. 2001, 2003). In years of strong Leeuwin Current, the peak settlement of puerulus occurs further south by up to 2o of latitude, compared with years in which the Leeuwin Current was weaker (Caputi 2008).
Click here for up to date information on settlement by location, for past and current settlement seasons.
Settlement success and abundance
The factors affecting the successful settlement of puerulus along the coast have been the subject of much research, especially since the recruitment failure of 2008/09 (de Lestang et al. 2015). The proportion of puerulus returning to the coast after 9-11 months at sea depend on natural mortality, as well as favourable oceanographic conditions returning them to the coast.
Natural mortality
An indication of natural mortality rate was calculated by Feng et al. (2011) from the catches of phyllosoma in the surveys conducted by Rimmer and Phillips (1979), which indicated that numbers declined by around 85 – 90 % from stage I to stage IX. Predation by plankton-feeding fish species is likely a major source of mortality and is supported by observations that few phyllosoma, beyond the freshly hatched stage I, are found on the shelf (Rimmer 1980), where fish abundance is highest. Additionally, food availability and quality will impact phyllosoma survival, especially considering the substantial energy reserves required for the non-feeding puerulus’ journey to the shore (McWilliam & Phillips 1997).
Nutritional condition
Studies have found significant variability in the nutritional condition of phyllosoma from different eddies (circular current systems) (O’Rorke et al. 2015, Wang et al. 2015). Phyllosoma from warmer-water cyclonic eddies were in better nutritional condition than those found in cooler-water anticyclonic eddies, despite feeding on similar prey items. Potentially cooler water directly impact the phyllomosa by reducing growth or impacting their ability to accumulate nutritional reserves (O’Rorke et al. 2015, Wang et al. 2015). Alternatively, the cooler water eddies may indirectly impact the phyllosoma by impoverishing the nutritional value of the phyllosomas’ foodweb (O’Rorke et al. 2015).
Oceanographic conditions
Warmer water temperatures have been associated with greater settlement success by increasing growth rates, therefore decreasing the temporal length of the larval phase, minimizing natural mortality. Warmer waters at the beginning of the larval phase as a result of a stronger Leeuwin Current in February – April, have been demonstrated to increase settlement abundance the following season (Caputi et al. 2001, Caputi 2008, de Lestang et al. 2015). Similarly, phyllosoma hatching in the north of the species’ distribution has an increased likelihood of settlement because they are exposed to warmer waters (Feng et al. 2011).
Correspondingly, the timing of hatching has been associated with survival as it affects the water temperatures the larvae are exposed to. Oceanographic modelling indicates that larvae that had hatched early in the season, from mid-October to December, grew faster because of longer exposure to warm summer temperatures, and therefore had reduced mortality and greater settlement success (Griffin et al. 2001, Feng et al. 2011). Additionally, earlier hatching was associated with preferable conditions in winter for the return of late-stage larvae and puerulus, back to the shore. These results were supported by the relationship between the annual level of puerulus settlement and the average time that settlement occurs, with higher settlement associated with an early peak in settlement (Caputi et al. 2001).
However, more recently, modelling and statistical analyses has indicated that very early spawning may in fact be associated with lower puerulus settlement (de Lestang et al. 2015, Caputi et al. 2018). Westerly winds associated with winter storms have been identified as important for the onshore transport of later-stage phyllosoma and puerulus (Caputi & Brown 1993, Feng et al. 2011). Earlier spawning since the mid-2000s, as a result of warmer winter temperatures, may have caused a timing mismatch with these winter storms, as well as other important environmental factors, such as peak food productivity, required for metamorphosis (de Lestang et al. 2015, Caputi et al. 2018) resulting in a shift in the timing and magnitude of settlement.
The impact of the Leeuwin Current on the puerulus settlement has also been recently found to be more complex than previously thought. While the strength of the Leeuwin Current has generally been correlated with high settlement (Pearce & Phillips 1988, Caputi & Brown 1993, Griffin et al. 2001, Caputi et al. 2003, 2010), recent work has demonstrated that a seasonal variation may be important. While a strong Leeuwin Current in winter assists onshore transport and therefore promotes settlement, a stronger Leeuwin Current in summer, and its associated weakened Cape Current, negatively affects puerulus settlement by transporting early-stage phyllosoma south (Kolbusz et al. 2022). This work also identified complex relationships between settlement and the El Niño–Southern Oscillation (ENSO), with the aforementioned “recruitment failure” associated with a nine-year period in which there was neither a moderate La Niña nor El Niño event (Kolbusz et al. 2022). It appears that this broad-scale oceanographic system plays a vital role in puerulus settlement on a relatively large temporal scale.