Assessment of interspecific interactions between the invasive red-claw crayfish (Cherax quadricarinatus) and the Mozambique tilapia (Oreochromis mossambicus)

The Australian red-claw crayfish, Cherax quadricarinatus, has been introduced for aquaculture purposes worldwide and consequently colonized natural environments, where it might cause ecosystem services losses or adversely affect native species and the local environment. This species was first found in Pequenos Libombos Reservoir in Maputo Province, Southern Mozambique in the late 2009 and is linked to reduction in tilapia fisheries. This study, conducted in 2015 under controlled conditions, aimed to assess the interspecific relationships between the Mozambique tilapia (Oreochromis mossambicus) and the alien crayfish. For both species, no significant differences in growth and survival rates were found between animals reared in the presence versus the absence of heterospecifics, indicating no direct deleterious interspecific effects. Behavioural observations revealed that fish and crayfish competed for shelter and food. Both species reduced the foraging in the presence of heterospecifics during feeding period, in the daytime. Crayfish seemed to have an advantage in competition for shelter, suggesting that they may interfere with tilapia sheltering activity and make tilapia vulnerable to predators in natural habitats.

Retention of acoustic conditioning in St Peter's fish Sarotherodon galilaeus

Groups of St Peter's fish Sarotherodon galilaeus were trained during 28 days of classical conditioning to respond to an acoustic signal by approaching a feeder. Their ability to retain the acoustically conditioned behaviour was tested after eight different intervals ranging from 12 to 188 days from termination of training. Retention tests comprised 7 consecutive days of retraining and the fish response was compared with that of naive fish. Fish response drastically diminished with time. After 55 days, there was no apparent expression of memory on the first test day but fish response was restored on subsequent retraining days, indicating the existence of latent memory. Six months from the end of training, there was no indication of behavioural retention and the response completely vanished. The study was conducted in the context of sea ranching, with classical conditioning used to gain control over fish location. In light of the present results, it is suggested that periodic reinforcement of the acoustically conditioned behaviour should be used.

Intersex Red Claw Crayfish, Cherax quadricarinatus (von Martens): Functional Males with Pre-vitellogenic Ovaries

Intersex individuals, possessing both male and female genital openings, were assessed in two groups-7 and 19 months old-of Australian red claw crayfish (Cherax quadricarinatus). All intersex individuals investigated were functional males, as suggested by their malelike morphology and the presence of testes, sperm ducts, androgenic glands, and viable spermatozoa. When an ovary was present in an intersex individual from either group, the gonadosomatic index, the diameter of the oocytes, and the ovarian cytosolic polypeptide profile were similar to those of immature, pre-vitellogenic females. We conclude that intersexuality in C. quadricarinatus does not indicate a case of protandric sequential hermaphroditism, as previously suggested. The case of intersexuality described here presents a unique model for the study of the role of the androgenic gland in the regulation of sex differentiation in crustaceans.

The Agonistic Behaviour of the Three Male Morphotypes of the Freshwater Prawn Macrobrachium Rosenbergii (Crustacea, Palaemonidae)

Abstract

The agonistic behaviour of the freshwater prawn, Macrobrachium rosenbergii, was studied in the laboratory. One hour long contests were conducted between prawns matched to size in each of the three sexually mature male morphotypes of this species; Small Males, Orange Clawed males and Blue Clawed males. An agonistic ethogram was established, consisting of 18 different acts. These acts were classified into indicators of dominance or subordinance following the application of cluster analysis. Prawns contests consisted usually of 3 phases: pre-escalated fighting phase, escalated fighting phase and post-escalated fighting phase. Contest phases differed both in the frequency of several acts and in the distribution of acts between the eventual winner and loser. In addition, the relative orientation and elevation of the opponents changed across phases. The inter and intra-individual sequences of acts of the winner and loser during the escalated fight were studied. Despite the similarity found in the behaviour of the two opponents during the escalated fighting, the eventual winner could be predicted by the relative number and duration of nips or by the relative number of times in which Complete and Incomplete-Lifting (two major displays of fight) were performed simultaneously with the snapping of the claws. Differences found among morphotypes pointed out a tendency of shifting from interactions with physical contact towards ritualized interactions without physical contact. This was correlated with the morphotypic developmental pathway and the concomitant increase of claw size.