Life History & Behaviour
Feeding
Tropical spiny rock lobsters are typically known for being omnivorous scavengers feeding on a diverse assemblange of benthic species including: molluscs, crustaceans, echinoderms, seagrass and algae (Pitcher et al, 1992c). Food is processed and passed on by the mouthparts of the lobster called maxillipeds. The food is then crushed and ingested by its mandibles before being swallowed (Trendall & Bell, 2006).
Reproduction
The life history of P. ornatus has some key differences than that of the typical spiny rock lobster (Pitcher etal. 1995). The ornate rock lobster grows significantly faster than other Panulirus species, having the shortest larval development phase and weighing around 1kg within 2 years of hatching (Johnston, 2006).
The breeding behaviour among Palinurus species is very similar and begins when a male lobster, possibly attracted by sex pheremones, mating with an intermoult female that has well developed ovaries (Pitcher et al. 1992c). Fertilisation is external and involves the males depositing their sperm via paired penile projections onto the sternal plates of the females (Pitcher et al, 1992c). Eggs are then extruded from the female into a chamber enclosed by the sternum, whereby the eggs are then fertilised (Pitcher etal. 1992c). The eggs are then brooded under the female’s tail for about a month before they hatch as larvae (Pitcher etal. 1994).
Development
Palinurus species generally exhibit five major phases within their life cycle: adult, egg, larval stage (phyllosoma), post-larval stage (pueruli) and juvenile. Following hatching the phyllosoma larvae are dispersed via currents over large distances and majority of larval development and maturation occurs 100s-1000s km's away from hatching site (Pitcher et al. 1992c). After the larval stage, strong currents return the crayfish to the Torres Strait for settlement (Mohamed & George, 1968). The post-larval stage is the transition phase between pelagic larvae and benthic juveniles. After settlement the post-larvae lobsters are very cryptic and juvenile lobsters are not observed until 6-12 months of growth. The developmental growth rate is influenced by factors such as food, temperature and density (Pitcher et al. 1992c).
Moulting
Crustaceans such as tropical rock lobster have a hard outer shell, or cuticle, which serves as a exoskeleton of organic armour (Schwab, 2006). Lobsters can only grow during moulting periods whereas the growth of internal tissues are continuous (Pitcher etal, 1992c). Interestingly, the process of shedding the shell is controlled by a gland in the eyestalk (Schwab, 2006). The epidermal cells secrete the entire shell and water is pumped inside to force the shell away from its newly formed soft shell.
Image: (Above) The molted shell of Panulirus Ornatus
Migration
As spiny rock lobsters mature and develop they typically migrate from the shallow water of nursery grounds to breeding grounds in deeper water. P. ornatus undergoes is characterised by one of the longest breeding migrations known to the Palinurus genus. The juvenile lobsters depart Torres Strait in the spring and arrive on the coastal reefs of Papua New Guinea (over 500km away) in summer. This mass annual summer migration can occur in single-file ques, with up to 64 individuals in each line (Pitcher et al, 1992c).
Behavioural Study
Background
The study was conducted on the coral reef of Heron Island, Queensland, Australia. The surrounding reef provides an ideal habitat for lobster dens and shelters a high volume of spiny rock lobsters. In this case, the most prevalent species Panulirus ornatus was caught in a coral bommie den in the surrounding reef-flat of the island.
The eyestalks on the tropical rock lobsters are an important sensory appendage used for orientation. Eyestalk ablation has been shown in many crustaceans, including lobsters to accelerate sexual development and moult cycles (Schwab, 2006). It is a process that has been used to advantage by commercial crustacean farmers. However, it has been shown to result in a number of different physilogical and behavioural side effects on lobsters including: loss of responsiveness to chemical stimulation, disturbed feeding behaviour through an increased discoordination of feeding movements and decreased navigational ability (Dennis et al. 2006).
The lobster eyestalks are compound eyes and are used in conjunction with the statocyst for for vision, orientation and movement. Panulirus ornatus are nocturnal organisms and spend their daylight time sheltering in their dens and foraging nocturnally (Pitcher et al. 1992c). There have been studies suggesting Panulirus species are capable possess a magnetic compass sensory ability demonstrated by their remarkable ability to migrate consistent geographical courses to breeding sites during periods of complete darkness. The degree of magnetic sensitivity in lobsters is unknown; however, these findings suggest there is great sensory ability in these lobsters (Pitcher et al. 1992c).
True navigators are animals that after displacement to an unfamiliar location can assess the environment and effectively determine their position relative to a goal destination without relying on cues or familiar surroundings (Mohamed & George, 1968). The findings of this study will further contribute to whether or not Panulirus species capable of true navigation when all visual sensory cues are unavailable.
Aims
This experiment will further investigate the behavioural preference for light when eye stalks are covered or uncovered during the day. It will also investigate whether or not covered eyestalks have an effect on lobster orientation and whether or not there are enough sensory cues without vision to re-orientate.
Methods
Eye stalk coverings were fitted to the one adult lobster (Panulirus ornatus; carapace length 90mm) and were released into plastic aquatic tanks where half of the arena was exposed to the bright sunlight and the other half was covered by thick shade cloth, effectively blocking all light.
The lobsters were placed into the arena, facing a random direction and timed till they reached the shaded area. All trials were conducted during daylight, ensuring the exposed half of the arena was in direct sunlight. After each trial, the lobster specimen was removed for a 2 minute interval before the next trial began.
 
Images: (Above) Panulirus ornatus eye stalks being completey covered with alfoil and tape.
 
Images: (Above) Ornate rock lobster with covered eye stalks seeks shelter under shade cloth and the aquatic arena setup.
For the orientation trials the lobster was placed firmly on its back with covered and uncovered eye stalks in separate trials and its orientation righting behaviour was observed. The behavioural movements of antennae, legs and tailfans were observed throughout both uncovered and covered trials.
Results
The lobsters undergoing the covered treatments took on average 21.58 seconds longer than uncovered trials (Table 1). It was also observed that the movement courses taken by covered trials were not always consistent and change of direction occurred frequently in the trials.
Table 1. The trials and times associated with P. ornatus reaching the shade cloth area.
Trials
|
Uncovered
|
Covered |
1
|
34.6 |
61.3
|
| 2 |
32.3 |
76.1 |
| 3 |
27.5 |
43.5 |
| 4 |
24.8 |
80.2 |
| 5 |
68.1 |
41.4 |
| 6 |
24.9 |
53.2 |
| 7 |
43.6 |
62.1 |
| 8 |
24.2 |
33.6 |
| 9 |
52.7 |
63.7 |
| 10 |
22.5 |
55.9 |
Conclusions
The results of this trial suggest that covered eye stalks definitely inhibit sensory cues to reach an ideal destination. The covered lobsters, managed to seek out the dark, cooler, shaded area. This suggests that the eyestalks are not the primary receptors for sensing changes in light intensity. This may be due to statocyst receptors on the lobster that can detect changes in temperature and/or light.
The uncovered lobsters were observed to be very sensitive to handling during the experiment with tail-flipping escape mechanism occurring frequently resulting in not being able to record accurate data for orientation trials. The study wasn't able to determine orientation sensory cues available to P.ornatus with covered eye stalks.
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