The behavioral mechanism of competition for food between tilapia (Oreochromis hybrid) and crayfish (Cherax quadricarinatus)

Studies on polyculture have usually investigated the consequences of this widespread culture method for growth and survival of the cultured species. However, research into the behavioral mechanism underlying competition between co-cultured species is lacking. For the first time in co-cultured fish and crustaceans, this study explored experimentally the behavioral interaction between red tilapia and red-claw crayfish in the context of food competion. The effects of the presence of heterospecifics (absent or present), the size of fish relative to crayfish (larger or smaller) and the number of food patches available for the fish (one-versus two-patch conditions) on the foraging decisions and aggression of fish and crayfish were tested. Time spent by fish in the bottom food patch (accessible for fish and crayfish) was shorter in the presence of crayfish, for small but not large fish and when there was an alternative patch for the fish (accessible only for fish). Time spent by crayfish in the bottom food patch was reduced in the presence of large, but not small, fish and when there was no alternative food patch for the fish. Fish were most aggressive towards conspecifics whereas interspecific aggression was exhibited only by crayfish. At the individual level, the dominant fish and crayfish spent more time in the bottom patch and performed more aggressive actions than the other conspecifics. In fish, this was not altered in the presence of crayfish. In crayfish, the overall reduction in foraging duration and aggression was due to an effect of large fish on the dominant individual. The implication of these findings for fish-crayfish communal culture is discussed

Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft

Crabs of the genus Lybia have the remarkable habit of holding a sea anemone in each of their claws. This partnership appears to be obligate, at least on the part of the crab. The present study focuses on Lybia leptochelis from the Red Sea holding anemones of the genus Alicia (family Aliciidae). These anemones have not been found free living, only in association with L. leptochelis. In an attempt to understand how the crabs acquire them, we conducted a series of behavioral experiments and molecular analyses. Laboratory observations showed that the removal of one anemone from a crab induces a “splitting” behavior, whereby the crab tears the remaining anemone into two similar parts, resulting in a complete anemone in each claw after regeneration. Furthermore, when two crabs, one holding anemones and one lacking them, are confronted, the crabs fight, almost always leading to the “theft” of a complete anemone or anemone fragment by the crab without them. Following this, crabs “split” their lone anemone into two. Individuals of Alicia sp. removed from freshly collected L. leptochelis were used for DNA analysis. By employing AFLP (Fluorescence Amplified Fragments Length Polymorphism) it was shown that each pair of anemones from a given crab is genetically identical. Furthermore, there is genetic identity between most pairs of anemone held by different crabs, with the others showing slight genetic differences. This is a unique case in which one animal induces asexual reproduction of another, consequently also affecting its genetic diversity

Culture Of The Australian Red-Claw Crayfish (Cherax Quadricarinatus) In Israel Crayfish Incorporation Into Intensive Tilapia Production Units

This study tested the suitability of the Australian red-claw crayfish Cherax quadricarinatus for rear- ing in an intensive culture system as a supplement to Oreochromis niloticus. Fish were grown in twelve 5.5 m3 tanks at high density (33/m3) for 133 days, alone or with crayfish at two stocking den- sities (10/m2 and 20/m2) with added shelters or with crayfish at the lower density (10/m2) without shelters. Tilapia survival ranged 90.3-95.0% with no significant differences among treatments. The growth rate of the tilapia raised with crayfish (2.05 g/day) was significantly higher than that of tilapia grown alone (1.88 g/day) probably because the fish were feeding on part of the crayfish pellets. Among treatments, there were no significant differences in fish yield. Crayfish survival was extremely low in the ‘no shelter’ treatment (2.9±2.7%) but reasonable (approximately 60%) when raised with shelters. The growth rate of the crayfish raised with shelters was significantly higher at the lower density (0.21 g/day) than at the higher density (0.18 g/day). Further research is needed on rearing tilapia and crayfish to market size in intensive systems, to establish the economic prof- itability of this culture strategy