Making sense of electrical sense in crayfish.

The five sensory modalities of humans are also found in a wide range of invertebrates. Other vertebrates have evolved additional special senses, such as the magnetic sense, which are also found in some invertebrates. However, there remain a few sensory abilities that curiously appear to be found in either vertebrates or invertebrates, but not both. For example, electrosensitivity - the ability to detect electric fields in water - which should benefit vertebrates and invertebrates alike, is apparently only used by vertebrates. However, recent reports suggest that some invertebrates could have an electric sense. Here we examine that possibility further and demonstrate a behavioural threshold to low-level electrical fields in two freshwater invertebrates. The responses are not low enough for them to detect the Earth's magnetic field as some other electroreceptive species can do, but sufficiently low for them to use in navigation or prey and predator detection. This finding challenges the current view of the sensory world of aquatic invertebrates and has implications for the evolution of this ability.

Video tracking in the extreme: video analysis for nocturnal underwater animal movement.

Computer analysis of video footage is one option for recording locomotor behavior for a range of neurophysiological and behavioral studies. This technique is reasonably well established and accepted, but its use for some behavioral analyses remains a challenge. For example, filming through water can lead to reflection, and filming nocturnal activity can reduce resolution and clarity of filmed images. The aim of this study was to develop a noninvasive method for recording nocturnal activity in aquatic decapods and test the accuracy of analysis by video tracking software. We selected crayfish, Cherax destructor, because they are often active at night, they live underwater, and data on their locomotion is important for answering biological and physiological questions such as how they explore and navigate. We constructed recording arenas and filmed animals in infrared light. Wethen compared human observer data and software-acquired values. In this article, we outline important apparatus and software issues to obtain reliable computer tracking.

Exploring with damaged antennae: do crayfish compensate for injuries?

Appendages are important sources of sensory information for all animals that possess them but they are commonly damaged in nature. We describe how the tactile system of the crayfish Cherax destructor functioned when subjected to the kind of damage found in wild-caught or cultured animals. Touch information was methodically varied by the removal of antennae and chelae. The resulting behaviour was analysed in a T-maze. Crayfish with a single antenna ablated turned toward the intact appendage, however, those with only a partial ablation did not, suggesting that a tactile information threshold exists for normal behaviour. When exposed to the same environment after an antennal ablation but with no prior experience in that terrain, crayfish also turned toward the side of the intact antenna. By contrast, when animals with experience obtained in a previous trial with intact antennae were tested after ablation of one antenna, they did not turn into one arm of the maze more than the other. These two outcomes indicate that behaviour is affected by an interaction between the time at which an injury occurs and an animal's knowledge of the topography, and that an injury may affect learning. We also tested to see if other appendages could provide tactile information to compensate for antennal loss. Input from the chelae did not affect the turning behaviour of crayfish in the maze.

Corners and bubble wrap: the structure and texture of surfaces influence crayfish exploratory behaviour.

Touch is a principal sense in all animals. It is potentially important in species of freshwater crayfish that encounter murky waters or are nocturnal. Little is known about how tactile (touch) stimuli affect exploratory behaviour under these conditions. We placed animals in different tactile situations at the start of an exploration in a dark arena and tracked the position of the body and antennae to test whether subsequent search behaviour was affected. Individuals were exposed to differently textured walls, channelled out along a wall, or released in contact with no, one, or two walls. A corner arrangement of surfaces, where individuals started near two walls at right angles, produced behaviour that differed from that of other configurations; animals chose one wall and then maintained a close distance from the wall along which they were moving. The distance from a wall adopted by a crayfish walking parallel to it was affected by the texture of the wall. These results on the influence of tactile stimuli on crayfish exploratory behaviour may have implications for other taxa.

The activity of abdominal stretch receptors during non-giant swimming in the crayfish Cherax destructor and their role in hydrodynamic efficiency.

Recordings were made from the nerve innervating the stretch receptors of the abdominal muscle receptor organs and slow extensor muscles of tethered crayfish, Cherax destructor, during so-called "non-giant swimming". The stretch receptors were active during the flexor phase of swimming but the duration and pattern of activity varied from cycle to cycle. Their pattern of firing was modified by the activity of the large accessory neurons which make direct inhibitory synapses upon them. Neither the stretch receptors nor the accessory neurons were active during the extensor phase of the cycle. The timing and extent of tailfan movements during the period of stretch receptor activity were measured from video records before and after the stretch receptor nerves were cut in the second to fifth segments. The promotion of the tailfan during flexion was significantly delayed and the minimum angle to which the uropods were remoted at the end of flexion significantly larger in denervated animals. We propose that afferent information from the stretch receptors coordinates the timing and extent of tailfan movements according to variations in the positioning and movement of the abdominal segments such that the hydrodynamic efficiency of the tailfan is enhanced on a cycle by cycle basis during non-giant swimming.

Parathyroid hormone-related protein (PTHrP) production sites in elasmobranchs.

This study describes the distribution of parathyroid hormone-related protein (PTHrP) antigen and its mRNA in seven species of cartilaginous fish from six elasmobranch families. Antigen was detected using antibodies to synthetic human PTHrP and the mRNA with a riboprobe to human PTHrP gene sequence. The distribution pattern of PTHrP in the cartilaginous fish studied, reflected that observed in mammals but PTHrP further occurs in some sites unique to cartilaginous fish. Of particular note was the demonstration of PTHrP in the shark skeleton, which although considered not to contain bone, may form by a process similar to that forming the early stages of mammalian endochondral bone. The distribution of PTHrP in the elasmobranch skeleton resembled the distribution of PTHrP in the developing mammalian skeleton. Differences in the staining pattern between antisera to N-terminal PTHrP and mid-molecule PTHrP in the brain and pituitary suggested that the PTHrP molecule might be post-translationally processed in these tissues. The successful use of antibodies and a probe to human PTHrP in tissues from the early vertebrates examined in this study suggests that the PTHrP molecule is conserved from elasmobranchs to humans.

The cord stretch receptors in the abdominal nerve cord of the crayfish Cherax destructor: physiology and relationships.

The physiology and relationships of tonic cord stretch receptor neurons in the crayfish Cherax destructor were examined with intracellular and extracellular recording. Cord stretch evoked slow depolarisations leading to action potentials in tonic cord stretch receptor neurons. Intermittent post-synaptic potentials were also seen in cord stretch receptor neurons but were not the primary cause of the action potentials. Cord stretch still evoked action potentials in cord stretch receptor neurons when all synaptic activity, monitored at another known chemical synapse, was blocked using high [Mg(2+)] and low [Ca(2+)] in the bath. One source of facilitating excitatory post-synaptic potentials in the cord stretch receptor neurons was from mechanosensory hairs on the dorsal abdominal surface. Tonic cord stretch receptor neuron activity was associated with an increase in the activity of the abdominal slow extensor inhibitor motor neuron and at least one abdominal flexor excitor motor neuron in its segment, and reduced activity in the abdominal slow flexor inhibitor motor neuron. Activation of individual cord stretch receptor neurons produced a local resistance reflex. Cord stretch, activating many receptors, produced several other outcomes. One was the "extensor state" described in earlier literature. The tonic cord stretch receptor neurons of Cherax destructor appear to be stretch-sensitive interneurons that receive inputs from other elements of the abdominal control system and mediate polysynaptic reflex activity in postural motor neurons.

Muscle receptor organs do not mediate load compensation during body roll and defense response extensions in the crayfish Cherax destructor.

It has been proposed that the abdominal muscle receptor organ (MRO) of decapod crustaceans acts in a sensory feedback loop to compensate for external load. There is not yet unequivocal evidence of MRO activity during slow abdominal extension in intact animals, however. This raises the possibility that MRO involvement in load compensation is context-dependent. We recorded from MRO tonic stretch receptors (SRs) in freely behaving crayfish (Cherax destructor) during abdominal extension occurring during two different behaviors: body roll and the defense response. Abdominal extensions are similar in many respects in both behaviors, although defense response extensions are more rapid. In both situations, SR activity typically ceased when the abdominal extension commenced, even if the joint of the SR being monitored was mechanically prevented from extending by a block. Since extensor motor neuron activity increased when the abdomen was prevented from extending, we concluded that the load compensation occurring in these behaviors was not mediated by the MROs.

Laboratory and field studies on the effect of molinate, clomazone, and thiobencarb on nontarget aquatic invertebrates.

The midge Chironomus tepperi was used in laboratory experiments to assess the relative toxicity of formulated molinate, clomazone, and thiobencarb, three herbicides used in Australian rice crops. Static bioassays were initiated with first-instar larvae at herbicide concentrations between 0.0625 and 2 times the anticipated field concentrations (AFCs) expected from the registered application rates. Adult emergence success, development time, and wing length were used as indices of the effect of each herbicide. Clomazone had no effect on any parameters at concentrations up to 0.288 mg/L (p > 0.05). Molinate significantly increased development time at concentrations equivalent to the AFC (3.6 mg/L) and above (p < 0.05). Thiobencarb reduced emergence success of adult C. tepperi at 0.0625 times the AFC (0.1875 mg/L) as well as decreasing male adult size and increasing development time for males and females at 0.125 times the AFC (p < 0.05). Nontarget effects of the herbicides on aquatic invertebrate communities were assessed in shallow experimental ponds using commercial application rates. One week after treatment, only thiobencarb had a significant effect, suppressing populations of chironomids, calanoids, and cyclopoids (p < 0.05). Four weeks later, all populations had recovered, equaling or exceeding control densities.

Embryonic and postembryonic neurogenesis in the ventral nerve cord of the freshwater crayfish Cherax destructor.

Previous studies of neurogenic activity in the thoracic neuromeres of indirect developing crustaceans indicated that the temporal patterns of neurogenesis can be correlated with the appearance of the thoracic appendages during larval and metamorphic development. To test further the idea that the temporal patterns of neurogenesis in crustaceans are related to their life histories, we examined neurogenesis in the ventral nerve cord of a direct developing crustacean, the freshwater crayfish Cherax destructor, whose life history contains neither larval stages nor metamorphoses. Neurogenesis was examined using the in vivo incorporation of bromodeoxyuridine into DNA. During late embryonic development the thoracic neuromeres of the crayfish contain arrays of mitotically active neuroblasts similar to those previously described in the spider crab and lobster. The arrays in the crayfish abdomen are, however, greatly reduced compared with those of the thorax. On hatching, both the thoracic and abdominal appendages of C. destructor are capable of movement. The pleopods, however, do not beat rhythmically until the second postembryonic stage whereas the pereiopods are not used in coordinated walking movements until the third stage. An examination of the time course of neurogenesis in the ventral nerve cord revealed that neurogenic activity in each neuromere ceases during or before the moult to the developmental stage in which its segmental appendage is first used in coordinated movements. These findings indicate that the patterns of neurogenesis in crustaceans are indeed related to the maturation of the segmental appendages and, in particular, to the maturation of motor behaviours.

Insights for robotic design from studies of the control of abdominal position in crayfish.

Studies of the control of position and movement of the abdomen of crayfish illustrate a number of features of invertebrate sensory-motor systems that have implications for their use to inform robotic design. We use the abdominal slow extensor motor system to illustrate three of them here: first, the way in which a behaviorally flexible length-servo device can be achieved with very few elements; second, the importance of knowledge of the biological and behavioral context in which the elements operate; third, that design solutions resulting from natural selection have been constrained by the previous evolutionary history of the animal, which can affect the outcomes in ways that may not be immediately apparent in a design context.

Tailflick escape behavior in larval and juvenile lobsters (Homarus americanus) and crayfish (Cherax destructor).

We examined the escape behavior of larvae and postlarvae of the American lobster (Homarus americanus) and of adult immature (stage ADI) crayfish (Cherax destructor). Responses to standardized water jet stimuli delivered through a pipette were observed and analyzed. Lobster larvae did not respond to stimuli within 60 ms, indicating that they do not have functional giant fibers. The first movement by lobster larvae in response to water jet stimuli was a hyperextension of the abdomen. Larval escape responses also showed very little habituation. Postlarval lobsters and ADI crayfish showed the same range of responses as adult animals. Displacement efficiency of tailflicks exhibited by the different animals and stages was examined and related to the morphology of the animals. A separate behavior from tailflicking by larval lobsters in response to water jet stimuli was also observed. Here, the abdomen was hyperextended and the thoracic appendages were promoted. We termed this behavior a "starburst" response. The features of the tailflicking behavior suggest that it evolved to make the larvae difficult prey to handle for small, slower moving predators, and possibly to allow them to ride the bow waves of faster moving predators.

Comorbidity of hyperactivity-impulsivity-inattention and conduct problems: risk factors in social, affective, and academic domains.

Children showing a comorbid behavior pattern of hyperactivity-impulsivity-inattention and conduct problems (HIA + CP) were contrasted to children having only an internalizing and externalizing behavior pattern (I + E) and matched controls. Children displaying the HIA + CP behavior pattern were at greater risk on a number of outcome measures in social and academic domains beginning in Grade 3 and continuing into Grade 4. The most marked differences among the three groups were found on peer measures of rejection and friendship and teacher ratings of social skills. On average, almost two-thirds of the HIA + CP group were rejected by their peers compared to one-third of the I + E group and only 12.5% controls. Over 70% of the HIA + CP group had no reciprocated friendships from Grade 3 to Grade 4 compared to less than half of the I + E group and approximately one-fourth of controls. Contrary to much theorizing in the literature, the HIA + CP and I + E groups showed average levels of academic self-concept and did not differ from controls in on measures of social self-concept and academic self-concept and general self-esteem. These findings were interpreted in light of positive illusory biases and the established link between aggressive behavior and egotism. The current article offers partial support for Lynam's (1996) notion of the "fledgling psychopath."

Identification of dithiolethiones with better chemopreventive properties than oltipraz.

Oltipraz and related dithiolethiones are an important class of chemopreventive agents. Studies were undertaken to identify cancer chemopreventive dithiolethiones more active than oltipraz. Largely based upon enzyme induction activities in vitro, 17 dithiolethiones, including oltipraz, were analyzed for their ability to induce hepatic phase II enzyme activities in vivo. Of these compounds, 15 produced greater induction of NAD(P)H:quinone reductase and 11 yielded greater induction of glutathione S-transferase than oltipraz. All 17 dithiolethiones were then tested for their ability to inhibit acute hepatotoxicity by aflatoxin B1 (AFB1), which previously has been shown to be an intermediate predictor of chemopreventive activity. Rats were pretreated with dithiolethiones (0.3 mmol/kg body wt, three times a week per os) and challenged with two acutely toxic doses of AFB1 (0.5 mg/kg body wt, once daily for two successive days per os). Inhibition of hepatotoxicity was measured by changes in body weight gain during AFB1 challenge, reduction in levels of hepatic enzymes in serum and diminution of bile duct cell proliferation. Nine dithiolethiones spanning a range of responses in this toxicity screen were further tested for their ability to prevent AFB1-induced tumorigenicity, as assessed by a reduction in hepatic burden of putative preneoplastic foci. Six dithiolethiones were found to be considerably more effective than oltipraz in preventing AFB1-induced tumorigenesis. In general, dithiolethiones that were very effective in inhibition of acute hepatotoxicity were also found to be effective in prevention of hepatic tumorigenesis.

Discrepancy between definitions of learning disabilities and school practices: an empirical investigation.

Students referred by general education teachers to school study teams (SSTs) were evaluated for learning disabilities (LD) eligibility. We classified children as LD on the basis of a WISC-III Full Scale IQ of 82 or higher and a 22-point discrepancy between IQ and any WRAT-R achievement score. Research decisions were then contrasted with actual school-based decisions regarding the child as LD. Over half of the students referred to SSTs were certified by the schools as LD, yet less than half of these school-certified students with LD evidenced the aptitude-achievement discrepancy required by the state. Examination of the cases called LD by the schools revealed that children were classified as LD on the basis of low absolute achievement, regardless of whether or not a discrepancy existed. Moreover, in cases where a discrepancy was found but the school did not classify the child as LD, that child evidenced significantly higher achievement, despite exhibiting the requisite 22-point discrepancy. The school-identified students with LD constituted an extremely heterogeneous group, including students with mental retardation along with a substantial number who failed to qualify for any special education services. Findings are discussed in terms of the discrepancy between criteria specified in state regulations and what the committee members at the school site seemed to use in classifying children with LD.

Early Intervention Project: can its claims be substantiated and its effects replicated?

A comprehensive report to the National Institute of Health on the diagnosis, etiology, epidemiology, and treatment of autism indicated that early intervention has the potential of being an effective intervention (Bristol et al., 1996). In spite of this positive outlook, several research and methodological questions remain regarding time of treatment initiation, intensity of treatment and duration of treatment, random assignment, comparative treatment designs, and treatment integrity. Against this backdrop we consider the claims made by the Early Intervention Project (EIP; Lovaas, 1987, 1993; McEachin, Smith, & Lovaas, 1993). The EIP claims to produce recovery from autism in 47% of the cases and to greatly reduce its severity in an additional 42% of cases. This article evaluates the EIP against threats to internal and external validity and is found to suffer from a number of methodological problems. Based on rebuttals to criticisms of their program, the EIP authors seem unwilling to admit any methodological flaws in the sampling, design, and analysis of data of the EIP. It is recommended that parents and fair hearing officers adopt an attitude of healthy skepticism before proceeding to an unqualified endorsement of the EIP as a treatment for autism.

Learning disabilities, low achievement, and mild mental retardation: more alike than different?

Children identified with learning disabilities (LD), low achievement (LA), or mild mental retardation (MMR) were contrasted on 41 measures of ability, academic achievement, social skills, problem behavior, academic engaged time, perceptual-motor skills, and school history. Both multivariate, univariate, and meta-analytic comparisons among the three groups showed relatively large differences on measures of aptitude and achievement, with the LD group scoring higher on measures of cognitive ability than the LA and MMR groups and the LA group showing higher tested academic achievement than the LD and MMR groups. Teacher ratings of academic competence showed similar levels of functioning for the LD and LA groups. No differences among the groups were found on measures of social skills, problem behaviors, or academic engaged time, or on most indices reflecting school history. Results were interpreted in light of studies contrasting LD and LA groups. Comparisons with earlier studies were difficult in light of demographic differences in samples and the lower cognitive and academic functioning of children in the present study. The current study showed that 61% of the LD group could be differentiated from the LA group, with LD-MMR and LA-MMR differentiation levels being 68.5% and 67.5%, respectively.

The labyrinth of IDEA: school decisions on referred students with subaverage general intelligence.

A sample of 150 children referred to Student Study Teams was assessed with a psychometric battery. Behavioral and academic ratings were obtained from teachers. Forty-three children scored at or below 75 on the WISC-III. We examined schools' subsequent classification decisions to ascertain how schools dealt with low-IQ students with academic and behavioral problems. Schools reached decisions regarding 35 of the children: Only 6 were classified as having mental retardation, and 18, as having learning disabilities. Findings were discussed in terms of (a) decline in identification rates of mild mental retardation and (b) the extent to which school decisions adhere to the research criteria.