Life History & Behaviour
LOCOMOTION
Sea stars are very slow movers, but do so in a coordinated pattern. Tube feet on the underside of each arm are used for locomotion. A stepping motion is conducted by all tube feet and the suckers that are located on the ends of these feet. The feet swing forward, grip the surface they are moving to, and then move backwards. Not all arms move in unison, but some take the lead in the direction of movement, and the others follow. The suckers are adhesive and enable the sea star to move into vertical positions and stay comfortably on the undersides of rocks (Ruppert et al. 2004, p. 882). If the sea star were to lose a tube foot, it would not affect them as the muscles attached have a valve that can be effectively switched on and off to block processes between the feet and the rest of the body.
FEEDING
Sea stars are both scavengers and carnivores that seek out benthic prey (those species that don’t enter the water column in the adult stage). Their prey consists of a variety of bivalves, snails, other echinoderms and crustaceans. On the central disc, the mouth is located on the underside. Their prey must be releasing substances for the sea star to have any chance of detecting and locating it. Unique to the Order: Valvatida, is a stomach that extends outside of the central disc to be directly applied to the prey (Ruppert et al. 2004, p. 885). This means that larger prey items can be consumed as digestion occurs outside of the body. An example is that a sea star can open up a bivalve, insert its stomach inside and consume the animal there and then.
RESPIRATION
Sea stars have very specialized gills located on the dorsal surface of the arms (the opposite side to the tube feet) and are used for respiration. The gills are very sensitive to the availability of oxygen and are ventilated with sea water externally. The gills are thin, hollow and are parts of the body wall that have turned inside out to end up on the surface of the arms. The bumps caused by this reversed action are called papulae (Ruppert et al. 2004, p. 883).
REPRODUCTION
Linckia multifora reproduces both sexually and asexually. The occurrence of each reproductive method varies greatly with location and environmental variables (Crawford, 2007). Although from previous studies, it is believed that L. multifora use asexual reproduction more than sexual reproduction (McAlary, 1993).
Sexual:
This sea star is gonochoric, which means that it is either a male or a female (Ruppert et al. 2004, p. 887). There is one breeding season per year for each L. multifora population. The timing of this varies with location and could occur at any time between November and May (Ruppert et al. 2004, p. 887; Crawford, 2007). Fertilization takes place in the water column once the eggs and sperm spawn. Later developmental stages of this sea star occur mostly in the plankton (Ruppert et al. 2004, p. 887). In the few studies that have observed sexual reproduction in L. multifora, low fecundity was the result (McAlary, 1993; Crawford, 2007). Sexual reproduction does result in a greater gene flow between different locations compared with asexual reproduction. 
Asexual:
Extremely unique methods of asexual reproduction are used within the genus, Linckia. Autotomy is the voluntary detachment of a whole arm that regenerates into a completely new individual sea star (Edmondson, 1935). Autotomy occurs all year round and involves both males and females equally, so it does not matter whether there are mature gonads present or not (McAlary, 1993). Only the largest arm of the parent sea star is lost to autotomy (Crawford, 2007). There are seven different phases of asexual reproduction, where the comet phase is usually the most abundant (although this can vary with location) (Rideout, 1978). The comet phase is when the new regenerated arm begins sprouting four small arms from the end. These comets can regenerate from a range of sizes (Crawford, 2007). The presence of these comets shows that a population has successfully reproduced through autotomy (Crawford, 2007). Autotomy only has a short regeneration period of less than 40 days where the arms develop into young comets. This time period is variable, because previous studies in Hawaii have observed the comets taking 49 days for the mouth to form, and 27 days in Guam (Edmondson, 1935; Rideout, 1978). The sea star does not have to be a parent for autotomy to occur. Each arm has the ability to contribute a minimum of 5 asexual arms to the population before they are deemed a disc-parent (Rideout, 1978).
PHOTORECEPTORS
Sea stars have photoreceptor cells that have the ability to detect light. These cells are located at the ends of each arm in a location called the optic cushion (Ullrich-Luter et al. 2011). This is present from a very early juvenile stage in the life cycle. Sea stars have many tube feet on the underside of each arm to allow them to move. Phototaxis is a locomotory movement by an animal in response to light. Sea stars require these tube feet, nerves and the optic cushion for phototaxis (Ullrich-Luter et al. 2011). The need for tube feet to be able to sense light is a phenomenon for animals under the phylum, Echinodermata. Little research has been completed for sea stars, but many studies have focused on sea urchins to study phototaxis. It has been suggested that the tube feet cells, and their connection to other nerves, could be acting as the central nervous system for the organisms. This is emphasized by the fact that genes most commonly found inhuman eyes were also found in adult sea urchin tube feet (Ullrich-Luter et al. 2011).
Most research on Echinoderm photoreceptor cells is focused on how they detect the light and the responses at the cellular level. There is no research on the behavioural responses of sea stars to light and dark changes. Using Linckia multifora, a small experiment was conducted on Heron Island, Great Barrier Reef, Australia, to try and determine a basis to behavioural responses. It was hypothesized that the sea stars would move towards the light after being exposed to pitch darkness.
Methods:
A plastic container, with the measurements 425 x 335 x 215 mm, was used as the environment for the sea stars. The container and its lid were completely covered with black material to eliminate surrounding light sources. A hole was placed in the side for a small light to fit through. A total of 9 sea stars were used in this experiment. Coral rubble, from the natural environment at Heron Island, was used in the container to act as a protective cave for the sea stars. Prior to each control and treatment experiment, the sea stars were arranged so that 4 were on the coral rubble, and 5 were surrounding it.
A control experiment was conducted where the sea stars were left incomplete darkness for 30 mins without a light source. For the light treatment, the sea stars were allowed 20 mins to settle in complete darkness before the light was turned on. These 20 mins occurred prior to each repeat of the experiment. A light was then turned on (without lifting the lid) and left for 30 mins. At the end of the 30 mins, observations were recorded and photographed. This experiment was repeated 5 times.
The experiment was then modified. There was still a 20 minute period where the sea stars were allowed to settle. The modification was to leave the sea stars for a longer period of time with the light on. The new time frame was 1 hour after the light was turned on, with observations and photographs taken every 20 minutes. This longer experiment was completed twice. At the conclusion of both experiments, the sea stars were measured from tip to tip of their longest pair of arms, and the diameter of their disc. These were recorded before the sea stars were released.
Table 1: The measurements of all sea stars from tip to tip and the disc diameter.
|
Sea Star
|
Tip to Tip (mm)
|
Disc diameter (mm)
|
|
1
|
95
|
9
|
|
2
|
53
|
7
|
|
3
|
74
|
8
|
|
4
|
79
|
8
|
|
5
|
63
|
10
|
|
6
|
60
|
6
|
|
7
|
72
|
7
|
|
8
|
63
|
6
|
|
9
|
49
|
4
|
Table 2: The first hourly experiment showing the number of sea stars directly in the light and near the light.
|
Each 20 minute interval
|
Number of sea stars directly in the light
|
Number of sea stars near the light, but not directly in it
|
|
1
|
1
|
4
|
|
2
|
1
|
1
|
|
3
|
0
|
3
|
Table 3: The second hourly experiment showing the number of sea stars directly in the light and near the light.
|
Each 20 minute interval
|
Number of sea stars directly in the light
|
Number of sea stars near the light, but not directly in it
|
|
1
|
2
|
3
|
|
2
|
1
|
3
|
|
3
|
1
|
1
|
Results and Discussions:
It was noted prior to the start of the experiment that 2 sea stars were of much lighter colouring than the others. For the purpose of this experiment, they were assumed to be juveniles. Although, there was found to be no correlation between the lengths of the tip to tip measurement of the arms and the disc diameter measurement of each sea star (see table 1).
From the results of this experiment, the hypothesis cannot be either accepted or rejected. There was definitely movement in the tank once the light was switched on, but it was unclear whether the sea stars were moving towards the light or away from the light. The results from the first experiment when the sea stars were only exposed to the light for 30 minutes are not displayed. These results were so variable that no conclusions can be made about the movement of the sea stars, except that there was movement. The negligible results are therefore not displayed above.
The results from the hourly experiment are shown in tables 2 and 3. From these results it is clear that the sea stars moved towards the light at some point, but then some moved away within the hour. As the container had to be covered over to simulate darkness and then to eliminate other light sources, constant observations were not conducted. This has resulted in no confirm results as to whether the sea stars are attracted to the light. Further experimentation in the behaviour of sea stars in the presence of different light sources needs to be conducted to gain a better understanding of their responses.
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