Abstract:
Righting behavior was examined in the asteroids Henricia
leviuscula and Leptasterias aequalis by repeating a standard laboratory
righting trial with a large number of individuals of each species,
and analyzing the observations made by statistical methods. Detailed
observations on the righting method, including leading pairs of
arms and righting time were made. The relationship between body
size and righting time, and the effect of mechanical stimulation on
leading arms was analyzed.
Both species used somersaulting in righting. The folding over
method, and a combination of folding over followed by somersaulting
were only used by L. aequalis. Somersaulting appears to be characteristic
of the entire class Asteroidea.
Both species are active during the pre-righting period, suggesting
that there is no quiescent period. The righting response was present in H. leviuscula at the beginning
of the righting reaction; in L. aequalis, it appeared later.
Suppression of activity of tube feet of non-leading arms plays an important
part in the first movements of the righting reaction.
There was a leading pair of arms in both H. leviuscula and L.
aequalis. In both species, the preference for certain arm pairs to
lead in the righting reaction is a species rather than an individual
characteristic. Leading arms are not adjacent to the madreporite.
The preference for a specific pair of arms is not due to the length of
the arms or greater number of tube feet, since the leading arms
were the same length as the non-leading arms.
The time required for righting in both species was very variable.
The average time taken for righting was 230 seconds in H.
leviuscula and 181 seconds for L. aequalis. Differences in righting
time between species appears to be an expression of the differences
in morphology rather than environmental variables or behavioral
modes.
In both species, there was a significant relationship between
the body size and the righting time, with smaller individuals righting
faster than larger. H. leviuscula took longer times to right in successive
trials, even with six and 12 hour rest intervals between
trials. This increase in righting time may be due to a process
similar to habituation. The same phenomena was not observed in L. aequalis.
Mechanical stimulation affected the arms used in righting in
L. aequalis. The arm used to turn the sea star over was used less
frequently as a leading arm. No such effects were observed in H.
leviuscula.
A righting reaction is of obvious survival advantage because
both species live on intertidal rocky shore areas which are exposed
to heavy wave action. Since there was no difference in the righting
time between the somersaulting and folding over method in L.
aequalis, neither method would appear to be more advantageous for
survival in nature.
