Density of antennal sensilla influences efficacy of communication in a social insect.

Effective communication requires reliable signals and competent receptors. Theoretical and empirical accounts of animal signaling focus overwhelmingly on the capacity of the signaler to convey the message. Nevertheless, the intended receiver's ability to detect a signal depends on the condition of its receptor organs, as documented for humans. The impact of receptor organ condition on signal reception and its consequences for functional behavior are poorly understood. Social insects use antennae to detect chemical odors that distinguish between nestmates and enemies, reacting aggressively to the latter. We investigated the impact of antennal condition, determined by the density of sensilla, on the behavior of the weaver ant Oecophylla smaragdina. Worker aggression depended upon the condition of their antennae: workers with fewer sensilla on their antennae reacted less aggressively to nonnestmate enemies. These novel data highlight the largely unappreciated significance of receptor organ condition for animal communication and may have implications for coevolutionary processes in animal communication.

Crayfish recognize the faces of fight opponents.

The capacity to associate stimuli underlies many cognitive abilities, including recognition, in humans and other animals. Vertebrates process different categories of information separately and then reassemble the distilled information for unique identification, storage and recall. Invertebrates have fewer neural networks and fewer neural processing options so study of their behavior may reveal underlying mechanisms still not fully understood for any animal. Some invertebrates form complex social colonies and are capable of visual memory-bees and wasps, for example. This ability would not be predicted in species that interact in random pairs without strong social cohesion; for example, crayfish. They have chemical memory but the extent to which they remember visual features is unknown. Here we demonstrate that the crayfish Cherax destructor is capable of visual recognition of individuals. The simplicity of their interactions allowed us to examine the behavior and some characteristics of the visual features involved. We showed that facial features are learned during face-to-face fights, that highly variable cues are used, that the type of variability is important, and that the learning is context-dependent. We also tested whether it is possible to engineer false identifications and for animals to distinguish between twin opponents.

Crustacean social behavioral changes in response to isolation.

Periods of isolation during which animals have no social contact are common in the design of behavioral experiments. They are used, for example, to test memory and recognition responses, or to ensure a baseline condition before experimental manipulations commence. We investigated the effect of isolation periods on the aggressive behavior of matched pairs of the crayfish Cherax destructor in two contexts. The first experiment tested the effects of a period of isolation between two encounters. The second experiment tested the effects of isolation before an encounter by pairing one crayfish from a communal living environment with another crayfish from an isolated one. Fight outcome and aggression levels were analyzed, resulting in three conclusions about the social biology of C. destructor. First, encounters between familiar opponents are influenced by the outcome of the familiarization fight for about 2 weeks. Second, the level of aggression and the outcome of an encounter are affected over different time frames. Third, individuals that are isolated before an encounter can be disadvantaged. These data suggest that isolation, or events that occur during periods of isolation, affect multiple elements of social behavior in C. destructor. This suggestion has implications for the interpretation of previous results and future studies in crustaceans and other taxa.

Parathyroid hormone-related protein production in the lamprey Geotria australis: developmental and evolutionary perspectives.

This study explored the distribution of parathyroid hormone-related protein (PTHrP) and its mRNA in tissues of the lamprey Geotria australis, a representative of one of the two surviving groups of an early and jawless stage in vertebrate evolution. For this purpose, antibodies to N-terminal and mid-molecule human PTHrP were used to determine the locations of the antigen. Sites of mRNA production were demonstrated by in situ hybridisation with a digoxigenin-labelled riboprobe to exon VI of the human PTHrP gene. The results revealed that antigen and its mRNA were widely distributed among similar sites of tissue localisation to those described for mammalian and avian species. However, some novel sites of localisation, such as in the gill and notochord, were also found. Some differences in PTHrP localisation were noted among individuals at different intervals of the life cycle, indicating that the distributions of PTHrP, and possibly its roles, change with the stage of development in this species. The widespread tissue distribution in G. australis implies diverse physiological roles for this protein. The presence of PTHrP in the lamprey, a representative of a group of vertebrates, which apparently evolved over 540 million years ago, strongly suggests that it is a protein of ancient origin. In addition, the successful use of antibodies and probes based on the human sequence in the lamprey also provides evidence that the PTHrP molecule may have been conserved from lampreys through to humans.

Exploration in a T-maze by the crayfish Cherax destructor suggests bilateral comparison of antennal tactile information.

Many crayfish species inhabit murky waters or have a crepuscular lifestyle, which forces them to rely on chemical and mechanical information rather than visual input. Information on how they use one form of mechanical information-tactile cues-to explore their local environment is limited. We observed the exploratory behavior of the crayfish Cherax destructor in a T-maze under red light. Animals moved forward along the long arm of the maze and then moved equally in one of two available directions. The arm chosen by one crayfish did not affect that selected by a second crayfish tested immediately after in an unwashed maze. Previous experience in the maze also did not affect the choice. We found, however, that crayfish with one antenna denervated or splinted back to the carapace turned more often toward the unaltered side. Our data support the hypothesis that crayfish bilaterally compare information from their antennae.

The relationship between body size and the field potentials generated by swimming crayfish.

Aquatic animals generate electrical field potentials which may be monitored by predators or conspecifics. Many crustaceans use rapid, forceful contractions of the flexor and extensor muscles to curl and extend their abdomens during swimming in escape and locomotion. When crayfish swim they generate electrical field potentials that can be recorded by electrodes nearby in the water. In general, it is reasonable to assume that larger bodied crayfish will generate signals of greater amplitude because they have larger muscles. It is not known, however, how activity in particular muscles and nerves combines to produce the compound electrical waveform recorded during swimming. We therefore investigated the relationship between abdominal muscle, body size and the amplitude of nearby tailflip potentials in the freshwater crayfish (Cherax destructor). We found that amplitude was correlated positively with abdominal muscle mass. The mean amplitude recorded from the five smallest and five largest individuals differed by 440 microV, a difference sufficiently large to be of significance to predators and co-inhabitants in the wild.

Morphology and connections of the abdominal accessory neurons of the crayfish Cherax destructor.

Associated with the abdominal muscle receptor organs of crayfish are accessory neurons that inhibit the activity of the stretch receptors. Cobalt infusion into their cut axons reveals four accessory somata associated with each hemiganglion in the abdomen of the crayfish Cherax destructor. These conform to the pattern described previously for these neurons: The cell bodies are in the ganglion posterior to the one from which they exit. We recorded intracellularly from the largest accessory neurons, Acc-1 and Acc-2, and stained them with intracellular dye to establish unambiguously the characteristics defining their identity and structure. We describe their branching patterns in the ganglion of origin and the ganglion of exit. This morphological information permitted us to distinguish all four accessory neurons in preparations with dye infused through their cut axons, and we propose a revised, unambiguous nomenclature for the two smaller ones. Our intracelluar recordings allowed us to reexamine the physiological relationships of Acc-1 and Acc-2, the only accessory neurons for which there are data in the literature. In general, the connections and inputs described in previous studies were substantiated, although there has clearly been confusion between the two, and they differ in a number of significant ways. We found that they are seldom active together, have different firing patterns, and may operate with different clusters of extensor and flexor motorneurons. The results illustrate the level at which the accessory neurons operate within the abdominal control system but do not distinguish between competing hypotheses concerning their role in behavior. The data are consistent with the view that accessory neurons assist in timing between adjacent segments.