Offshore transport and distribution
After hatching most early-stage phyllosoma are transported offshore, with very few larvae remaining on or near the continental shelf (Phillips et al. 1979; Rimmer 1980). The relative abundance of stage II and III phyllosoma increases with the distance from shore, indicating that the offshore transport occurs soon after hatching (Rimmer 1980).
Planktonic trawl surveys found phyllosoma over an extensive area of the south-eastern Indian Ocean (Phillips et al. 1979) (Figure 1). These surveys did not determine the maximum westward extent of the phyllosoma distribution as significant numbers of phyllosoma were still found as far as 1, 500 km offshore, at the furthest offshore station sampled. The area of greatest abundance was due west of the approximate centre of the adult distribution on the coast, approximately in line with Geraldton.
Offshore movement of the first stage phyllosoma has been estimated to occur at a minimum rate of about 5 km per day (Rimmer 1980). Phyllosoma hatch during the summer months (mainly October – February), which are characterised by strong southerly winds along the West Australian shoreline. Oceanographic modelling has demonstrated that these winds, balanced by geostrophic currents (an oceanic current in which the pressure gradient force is balanced by the Coriolis effect), aid the offshore movement of early-stage larvae, from their mid-shelf hatching sites, into the open ocean (Feng et al. 2011; Griffin et al. 2001). The diurnal vertical migrations of early-stage phyllosoma also aid in this offshore transport, and in maintaining the phyllosoma offshore.
Diurnal vertical migrations
Phyllosoma typically undergo diurnal (day-night) vertical migrations in response to light intensity; ascending to the surface at night and descending to deeper depths during daylight hours (Rimmer and Phillips 1979). Rates of ascent and descent generally increased with development stage (Rimmer and Phillips 1979). Early-stage phyllosoma were found to ascend at a mean rate of 13.7 m/h, and descend at 13.0 m/h, while mean rates of ascent and descent in mid-stages were 16.0 and 16.6 m/h respectively, and 19.4 and 20.1 m/h in late-stages (Rimmer and Phillips 1979).
Early-stage phyllosoma (stages I and II) have been shown to occur at the surface at night, regardless of moon intensity, and return to depths of 30 – 60 m during the day (Rimmer and Phillips 1979). This places them at the surface at night when offshore vectors of wind-driven ocean-surface transport dominate, and below the surface during the day when offshore vectors are small or onshore. Late-stage phyllosoma, on the other hand, only concentrate at the surface on nights with less than 5% full moonlight, and descend to deeper depths, 50 – 120 m, during the day (Rimmer and Phillips 1979). Because of their deeper day-time distribution and absence from the surface on moonlit nights, these stages are predominately subject to ocean circulation and underlying currents rather than surface advection as with the early stages. This likely aids their return to the continental shelf by exposing them to favourable water transport features.
Onshore transport and movement
During winter, eastward water movement and currents that feed the enhanced Leeuwin Current (a southward flowing current of warm water, that runs along the WA coast, with peak flow in winter) facilitate the onshore movement of late-stage larvae towards nearshore habitats (Feng et al. 2011). As mentioned above, this process is facilitated by the diurnal vertical migration of the late-stage phyllosoma, which places them at greater depths, and therefore more exposed to current-driven water transport, than surface wind-driven advection.
Upon reaching the continental shelf region, the stage IX phyllosoma metamorphoses into the free-swimming puerulus and begin the final 40 – 60 km journey to the inshore reefs. Because pueruli are non-feeding (or have very limited feeding capacity) they must survive on the energy reserves accumulated during their final phyllosoma stage for the duration of this journey. Studies on energy use and reserves indicate that the swimming puerulus have approximately 21 days to reach these reefs and settle (Lemmens 1994). As pueruli are capable of swimming speeds of up to 45 cm/s with the current (Phillips and Olsen 1975), they are potentially capable of covering this distance within a few days. Pueruli are considered to follow, more or less, a direct route shoreward from the continental shelf to the inshore reefs. A straight trajectory is supported by energy reserves at settlement that are generally inversely related to continental shelf width (Limbourn et al. 2009). However, the high level of variability in these reserves indicates that pueruli may travel more complex trajectories to the coast, affected by the strength and direction of on-shelf currents (Limbourn et al. 2009).
Energy consumption per km travelled is lower in the western rock lobster compared with other spiny lobster species, indicating that western rock lobster puerulus likely use physical oceanographic processes, ocean currents and/or wind, as well as active swimming for onshore transport (Phillips et al. 2006). As the strength of the Leeuwin Current and winter-spring westerly (onshore) winds have been shown to positively affect the level of puerulus settlement along the coast (Caputi and Brown 1993; Pearce and Phillips 1994; Caputi et al. 1995), it is likely that pueruli use these processes to aid in their onshore transport.
How the puerulus orient themselves towards the coast is yet unknown. It has been suggested that they may detect ocean swell noise, and other vibrational cues associated with the coast, and orient themselves towards this (Phillips and Macmillan 1987; Phillips and Penrose 1985). Indeed, investigations of the sensory structures on the antenna and antennules of the puerulus indicate a complex receptor system that appears well equipped to detect vibrations (Phillips and Macmillan 1987). Additionally, the structure of puerulus’ eye indicates that they are very effective in the perception of polarized light (Meyer-Rochow 1975), which may help pueruli set a consistent swimming direction.
Upon reaching the inshore shallow reefs the puerulus settle onto their new juvenile habitat, where they become benthic (bottom-dwelling), thus ending their pelagic and swimming phases.
Puerulus swimming behaviour
Whether puerulus exhibit similar photosensitive vertical migrations, as phyllosoma do, is unknown. In plankton trawls ranging in depth from the surface to 50 m, very few puerulus were captured despite observing stage IX phyllosoma moults (Ritz 1972). They concluded that the puerulus must make their shoreward journey at some subsurface level (> 50 m). However, subsequent observations of swimming behaviours revealed that puerulus can detect and avoid objects (Phillips and Olsen 1975), which may account for the low numbers in the Ritz study. Studies investigating the impact of the lunar cycle on puerulus settling on artificial collectors found that almost all settlement occurred during the new moon period, with the majority occurring during the period of no moon and ceasing when moonlight intensity became greater than 10% (Phillips 1975). As artificial collectors are placed at the surface, this lunar-sensitive settlement pattern suggests that puerulus do exhibit photosensitive vertical migrations similar to phyllosoma, rising to the surface only on dark nights. Additionally, observations of swimming puerulus confirmed that puerulus are capable of detecting light (Phillips and Olsen 1975). However, in these observations, the puerulus were attracted to the underwater light, which seems counterintuitive if puerulus descend on moonlit nights.
As mentioned above, pueruli are estimated to be capable of swimming at speeds of 45 m/s in a horizontal direction (Phillips and Olsen 1975). This estimate was made from observations of maximum against current speeds of 33 cm/s, and average against current speeds of 15 m/s, over a distance of 1 m. These impressive speeds are achieved by rapidly beating the large pleopods to provide propulsion and swimming in a streamlined profile; holding the antenna together in front of the body, extending the abdomen, streamlining the tail-fan, and retracting the legs (Phillips and Olsen 1975). Other observations regarding the swimming behaviour of puerulus made in the same study include:
- Puerulus swam alone, with no contact between individuals.
- Puerulus were capable of detecting objects in the water and displayed avoidance reactions by a rapid flexing of the abdomen.
- When the swimming puerulus was disturbed by contact with an object, it spreads its antennae to an angle of approximately 60° and the legs, abdomen and tail fan are also extended, while the animal either remains motionless or sinks slowly downwards.
- Puerulus were capable of rapid backward movement by rapid flexing of the abdomen, the same characteristic motion used by juveniles and adults.
- Some animals swimming very close to the surface were disturbed by wave action and would react by this flexing of the abdomen which would carry them down approximately 5 cm from the surface.
- Individuals appeared disorientated by the light.
References
Caputi N, Brown RS (1993) The effect of environment on puerulus settlement of the western rock lobster (Panulirus cygnus) in Western Australia. Fish Oceanogr 2:1–10.