| dc.description.abstract | Isopods are members of the order Isopoda that is contained in the subphylum Crustacea. Isopods have seven pairs of similar walking legs and the name isopod is derived from the Greek words “Iso” meaning same and “pod” meaning foot or leg. So far, more than 10,600 isopod species have been identified. The first isopod to evolve was Hesslerella shermani which is thought to be the ancestor of all modern isopods. About 225 million years ago, some isopods made evolutionary transitions from the marine habitat to freshwater and about 110 million years ago, some isopods made evolutionary transitions from aquatic habitats to land. During the evolutionary transition from water to land, a major challenge for these isopods would have been avoiding desiccation. Terrestrial isopods often huddle together in close proximity and this aggregative behaviour decreases evaporative surface area and reduces desiccation. In 2011, aggregative behaviour of the terrestrial isopod, Porcellio scaber was studied by Devigne and his colleagues. The experimental set-up in this study consisted of a cylindrical arena containing two transparent dark red shelters, placed at either side of the arena that provided coverage for the animals and reduced the exposure to light, as terrestrial isopods display negative phototaxis. The majority of the group of P. scaber individuals chose one of the two red shelters at random and aggregated under it thereby showing social aggregation. Prior to 2013, it was not known whether social aggregative behaviour evolved in isopods after their evolutionary transition to land to help reduce desiccation or whether marine isopods also display social aggregative behaviour. It was decided to test whether the marine isopod, Cirolana harfordi, which is anatomically similar to P. scaber, displays social aggregative behaviour. C. harfordi populations are found in the intertidal rocky sea beaches of the Pacific coast of North America, Australia and Japan. As C. harfordi lives in the intertidal zones of coastal areas and estuaries, it may often experience fluctuating salinities. The salinity of estuarine water can decrease from 35 ppt to 10 ppt after rainfall events. Prior to this study, it was not known whether the physiology and behaviour of C. harfordi are affected by hyposaline water. In the marine environment, isopods use their antennules for sensing food odours. Isopods have one pair of antennae and one pair of shorter antennules on the cephalon (head). The antennae and antennules of crustaceans are often decorated with numerous articulated cuticular projections known as setae. These setae can have chemosensory and / or mechanosensory functions. During the evolutionary transition from water to land, terrestrial isopods evolved shorter antennules with fewer setae as compared to their marine ancestors. This might have occurred because terrestrial isopods do not rely as much on the antennules for food searching out of water. Prior to 2013, it was not known, however, whether C. harfordi relies on antennules for food searching behaviour in water. To investigate whether C. harfordi shows social aggregative behaviour, specimens were placed in a 20 cm diameter acrylic cylinder arena that contained two 5 cm diameter circular acrylic shelters that were tinted red (to reduce light transmittance), one at either end of the arena. To investigate whether C. harfordi individuals require light reduction to display social aggregative behaviour under a shelter, experiments were performed in a similar arena mentioned above with the exception that both of the shelters were clear. To characterize C. harfordi’s ability to withstand short term (two days or less) exposure to hyposaline water, specimens of C. harfordi were transferred from 100% artificial seawater to four different artificial seawater dilutions, 100%, 50%, 25% and 0% artificial seawater and their survival rate, weight gain, oxygen consumption and righting behaviour were examined for 48 hours. To investigate whether C. harfordi requires antennae or antennules for food searching behaviour, a combination of antennae and antennules ablation experiments were performed. There were seven treatment groups that had the following structures ablated; left antennae, right antennae, both antennae, left antennules, right antennules, both antennules, both antennae and antennules. Animals were anaesthetized by placing on ice before performing ablation experiments. Food searching experiments were performed in a 20 cm diameter circular arena by placing an individual animal in a small holding bay at one side of the arena and two eppendorf tubes (one of these tubes contained canned tuna and the other was kept empty) on the other side of the arena. The holding bay was removed from the arena and movements of animals to the food were video recorded. The data obtained from the recorded video were used for measurement of searching duration, travel distance and speed to locate the food. Moreover, light and scanning electron microscopy were performed to investigate the structure of the setae on the antennae and antennules. When specimens of C. harfordi were placed in the arena with two red shelters, the majority of the group of animals chose one of the two identical shelters at random, and congregated under this winning shelter. These data indicate that the aggregative behavior displayed by this marine isopod species is driven by social conspecific attraction. When C. harfordi specimens were placed in the arena with two clear shelters, the animals again picked one of the two clear shelters at random and significantly more animals aggregated under this shelter as compared to the other. These data indicate that C. harfordi specimens do not require negative phototaxis to display social aggregation. When specimens of C. harfordi were transferred from 100% artificial seawater to 100% 50%, 25% or 0% artificial seawater (and the entire experiment repeated a total of three times), none of the animals died in the 100% and 50% dilutions up to 48 hours. 97% of the animals survived in 25% artificial seawater for 48 hours, which was not significantly different to control animals, but no animals survived after 24th hours in 0% artificial seawater. The mean percent weight gain of the animals exposed to 50% artificial seawater was significantly increased at 4, 24 and 48 hours compared to control animals whereas a significant difference in the mean percent weight gain was found at 2, 3, 4, 24 and 48 hours in the animals exposed to 25% artificial seawater as compared to controls. The mean percent weight gain of the animals exposed to 0% artificial seawater was significantly higher at 2, 3 and 4 hours compared to controls. When animals were exposed to 25% and 0% artificial seawater for 90 minutes, their righting reflex time was significantly increased compared to controls. When animals were exposed to 50% artificial seawater for 6 hours, their mean oxygen consumption significantly increased compared to control animals. When animals were exposed to 25% and 0% artificial seawater for 6 hours, their mean oxygen consumption was significantly decreased compared to control animals held in 100% artificial seawater. These data indicate that whether C. harfordi can tolerate an episode of hyposaline water depends on the degree of hyposalinity and the length of exposure. C. harfordi individuals that had both antennules ablated required a significantly longer time to locate food as compared to control animals and this indicates that specimens of C. harfordi require antennules for efficiently locating food sources. Light and scanning electron microscopy data showed that C. harfordi individuals have a variety of setae types on the antennae and antennules. In conclusion, C. harfordi is a social animal, it depends on its antennules for locating food and is able to survive in quarter strength seawater for days which suggests that C. harfordi is adapted to survive in estuaries. | en_AU |
