Michael M. Walker

A Candidate Magnetic Sense Organ in the Yellowfin Tuna, Thunnus albacares

Single-domain magnetite crystals have been isolated and characterized from tissue located in a sinus within the dermethmoid bone of the skull of the yellowfin tuna, Thunnus albacares. Their chemical composition, narrow size distribution, and distinctive crystal morphology indicate that these crystals are biochemical precipitates. Experiments on the interaction between particles reveal the organization of the particles in situ and suggest a possible form for candidate magnetoreceptor organelles. The consistent localization of such particles with similar arrangement within the dermethmoids of this and other pelagic fishes suggests that the ethmoid region is a possible location for a vertebrate magnetic sense organ.

Learned magnetic field discrimination in yellowfin tuna,Thunnus albacares

Summary1.Yellowfin tuna,Thunnus albacares, were trained individually to discriminate between two Earth-strength magnetic fields by differential reinforcement of a swimming response.2.Seven subjects, of which two were trained with a double blind procedure designed to control for the possibility of cues from the experimenter, learned to discriminate between ambient and altered fields (Figs. 1–3).3.Two additional fish trained with the same double blind procedure failed to discriminate between two magnetic fields in which the gradients of intensity were equal and opposite (Fig. 4).4.The results suggest that the responses to magnetic fields by yellowfin tuna are neurally mediated and that magnetic field detection by this species can be analyzed by the same means as other sensory modalities.

Conditioned responding to magnetic fields by honeybees

SummaryIndividual honeybees were trained in two experiments to come for sucrose solution to a target set on a shelf before an open laboratory window. On some visits, the target was presented in the ambient geomagnetic field, and on other visits in a field modified in the vicinity of the target by passing a direct current through a coil under the shelf. The target contained 50% sucrose when it was in one of the two fields and 20% sucrose when it was in the other. Tested subsequently with a pair of targets, one in the ambient field, one in the modified field, and both containing tap water, the animals significantly preferred the target in the field in which they had been given the 50% sucrose during training. Four modified fields, produced with different coils and currents, were discriminated equally well from the ambient field, and performance was as good when the 50% sucrose was given in the ambient field as when it was given in the modified field. Data are provided also to illustrate the excellent discriminative performance attainable when two targets are presented on each training visit — one in a modified field, the other in the ambient field — and choice of one is rewarded with 50% sucrose while choice of the other is punished with mild electric shock. Our results show that foragers attend to magnetic stimuli at the feeding site and that discriminative training techniques are appropriate for the study of magnetoreception and its mechanism in honeybees.

Particle-Size Considerations for Magnetite-Based Magnetoreceptors

The presence of a magnetic influence upon behavior now appears to be a fairly common trait among a wide variety of organisms, as outlined and discussed elsewhere in this volume. In a broad manner, these behavioral responses can be grouped into two categories, the first of which involves the use of a relatively insensitive “compass” to obtain directional (north/south) information, and a more sensitive system involved in the “map” sense of vertebrates and the time cue of insects.

Chains of single-domain magnetite particles in chinook salmon,Oncorhynchus tshawytscha

SummaryAlthough the presence of magnetite in their tissues is correlated with the ability of different species to detect magnetic fields, proof that the magnetite is involved in magnetoreception has not yet been provided. Using the approach employed to localize and isolate magnetic particles in the yellowfin tuna, we found that single-domain magnetite occurs in chains of particles in tissue contained within the dermethmoid cartilage of adult chinook salmon,Oncorhynchus tshawytscha. The particles are present in sufficient numbers to provide the adult fish with a very sensitive magnetoreceptor system. Magnetite in the chinook can be correlated with responses to magnetic fields in a congeneric species, the sockeye salmon. Based on the presence of the chains of particles, we propose behavioral experiments that exploit the responses of sockeye salmon fry to magnetic fields to test explicit predictions of the ferromagnetic magnetoreception hypothesis.

Detection, Extraction, and Characterization of Biogenic Magnetite

Several difficulties arise when attempts are made to characterize the deposits of magnetite found in metazoans. We are usually forced to deal with very small amounts of material, dispersed in tissues, using indirect methods that are subject to contamination. Magnetite crystals in the abdomens of bees (Gould et al., 1978), and in the heads of pigeons (Walcott et al., 1979), and other vertebrates (Bauer et al., this volume; Perry et al., this volume; Walker et al., this volume) are submicroscopic (<100 nm), occupy a combined volume of 10−10 to 10−8 cm3, and have a mass of 1–100 ng. In organisms of up to 100 kg or more, detecting such quantities of magnetite from its magnetic properties depends on the crystals being highly concentrated in small, recognizable structures, and not uniformly dispersed throughout all the tissues. Extraction and recovery of the crystals likewise depend on their being sufficiently concentrated to be magnetically detectable.

Magnetoreception and Biomineralization of Magnetite Fish

Many species from different taxa respond to one or more features of the geomagnetic field (Keeton, 1971, 1972; Lindauer and Martin, 1972; Wiltschko, 1972; Walcott and Green, 1974; Martin and Lindauer, 1977; Quinn, 1980; Wiltschko et al., 1981). These responses fall into two general categories: responses to magnetic field direction and to magnetic field intensity. Magnetic compass responses include the vanishing bearings of homing pigeons (Walcott and Green, 1974) and directional preferences of migratory species in orientation arena experiments (Wiltschko, 1972; Tesch, 1974; Quinn, 1980). The postulated magnetic intensity, or “map”, response (Gould, 1980, 1982; Moore, 1980; Walcott, 1980) refers to the apparent ability of homing pigeons to determine their position to within a kilometer or two using some feature related to geomagnetic field intensity. This response has been inferred from the vanishing bearings and homing speeds of birds released at geomagnetic field anomalies and during magnetic storms (Keeton, 1969, 1971, 1972; Gould, 1980, 1982; Walcott, 1980). Gould (Chapter 12, this volume) provides a full discussion of this research.

Geomagnetic field detection by yellowfin tuna

Yellowfin tuna (Thunnus albacares) learned to discriminate between two Earth-strength magnetic fields. In addition, magnetite was localized within the ethmoid bone complex of the skull. The easily-conditioned response to magnetic field stimuli indicate that the tunas possess a geomagnetic sense, while the presence of magnetite suggests a mechanism for sensing the geomagnetic field.

Magnetic Sensitivity and its Possible Physical Basis in the Yellowfin Tuna, Thunnus Albacares

Many animals are known to orient to magnetic fields. However, two central problems in the study of magnetic sensitivity have been the almost complete failure of magnetic field conditioning experiments and the lack of evidence for a feasible transduction mechanism. In the studies reported here yellowfin tuna learned to discriminate between two Earth-strength magnetic fields in a discrete-trials/fixed-interval conditioning procedure. Magnetometry experiments, diffraction spectra and electron microscope analyses demonstrated single-domain crystals of the ferromagnetic mineral magnetite in the head of this species. The crystals are concentrated in tissue contained within a sinus formed by the ethmoid bones of the skull. Theoretical analyses show that the crystals would be suitable for use in magnetoreception if linked to the nervous system. The physical properties of the crystals would determine the operation of magnetoreceptor organelles and constrain the capacities of the magnetic sense. Tests of these constraints in appropriately designed conditioning experiments will provide powerful tests of the ferromagnetic magnetoreception hypothesis.

An automatic feeder for liquids and wet or dry solids

An inexpensive and simple, remote-control conveyor-belt feeder is described. The feeder handles liquids and wet or dry solids.