Biomedical Engineering Reference
In-Depth Information
motion parallel with the LGF and possess chains of magnetite
crystals oriented linearly with respect to their cylindrical outer
membranes (Kalmijn, 1981; Frankel, 1986a, 1986b). Sharks when
swimming across the LGF sense the potential difference thus
generated (Kalmijn, 1981). Rays have been shown to use the
LGF to determine the direction of their circular path around a
tank aligned symmetrically with the LGF. During migration, whales
exhibit magnetosensitivity, and temporal changes in the LGF due to
solaractivityandaperiodicchangesintheearth'smantlemaycause
the well-knownbeach strandings of schools (Walker et al., 1986).
Befitting the conductivity of the marine environment, electrore-
ception is also observed, the local electric field (LEF) being used
for sensing purposes (Kalmijn, 1974, 1981; Adey, 1981). Marine
vertebrates such as sharks and rays exhibit predatory feeding
behaviourthatdependsuponsomeLEFemittedbytheprey.Flatfish
may attempt to escape by burying themselves in the sand on the
seabed, but this does not su
ciently camouflage their bioelectric
fields to avoid well-aimed attacks by both sharks and rays (Adey,
1981). Indeed, the wriggling motion of flatfish requires the firing of
electric signals in its muscles, which create the fields that are then
detected by the predator.
Turning to biogenic methods of field amplification, there is
evidence for LGF amplification amongst winged species (Kirschvink
and Walker, 1986). Presti and Pettigrew located ferromagnetic
deposits in the neck muscles of pigeons and white-crowned
sparrows (1980) and found that biogenic magnetic particles were
spreaduniformlythroughouttheskull.Theyalsoexamined40other
species for inducible remanence, with widely varying levels being
found;appreciablelevelswereassociatedonlywithmigratorybirds.
To provide the observed remanence, they estimated that about
6
10
6
aligned, single-domain cubic particles of size 0.07 mm
were needed, provided the domains did not interact. They further
proposedthetorqueduetotheseparticlescouldstimulateaspindle
muscle receptor.
In the marine environment, a similar magnification of the LGF
seems to occur. Frankel (1986a, 1986b) suggested that certain
bacteria and algae could navigate along the LGF due to the torque
produced by the LGF and chain-like deposits of magnetite forming
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