Latitudinal effects on breeding in Semibalanus balanoides
The arcto-boreal intertidal
barnacle Semibalanus balanoides
releases its nauplii in Spring, when they are able to take advantage of the
phytoplankton bloom. Semibalanus produces a single brood each
year, and the eggs and nauplii develop over the autumn and winter, during a
prolonged period of anecdysis. There is a large literature on the breeding
system of the species (see Anderson, 1994 for review). Maturation and
fertilization of the egg masses takes place in autumn, and it is easy to
distinguish hard, fertilized egg masses from soft, developing masses; this is
an all-or-nothing endpoint and requires minimal skill to monitor.
The environmental control of
maturation and fertilization is still unresolved, though Tige-Ford (1967) long
ago showed that constant light inhibited the process. Davenport & Crisp
(1973: unpublished data) found that shortening daylength appeared to be a
controlling factor, with very low continuous light intensities (rather more
than starlight!) inhibiting breeding. However, it is not known whether there is
any thermal modulation of the process. It is a general feature of ectothermic
animals that development takes longer at lower temperatures. If Semibalanus breeding onset is solely
controlled by autumn day length, then this would suggest an automatically
longer developmental period the higher the latitude (reinforcing the thermal
effect). The work proposed would help to elucidate environmental control, and
determine whether time of egg mass hardening may be a useful tool for
monitoring climate change (comparable perhaps to the shifts in breeding times
shown for some passerine birds [Sanz, 2002]).
Three approaches will be
adopted. First, the proportion of hard egg masses will be monitored in the
autumn of two years at sites readily accessible to the project participants at
as wide a range of latitudes as possible, and at which the Semibalanus are distributed densely enough to permit cross
fertilization. During the first year, at least 50 barnacles taken from the
middle shore will be assessed at weekly intervals from August until all egg
masses are hard. The time (year-day) of 50% hardening will be assessed by
Probit Analysis. When these data are available, the sampling programme for the
second year will be designed in collaboration with all participants.
Second, at at least two
sites, the effect of shore height on time of egg mass hardening will be
assessed. If hardening is controlled by a combination of day length and light
intensity, then it might be predicted that hardening would start earlier the
lower the shore position.
Third, laboratory
experiments will be conducted on Semibalanus
to determine whether egg mass hardening time is influenced by environmental
temperature. Animals will be held in natural and artificial light regimes at
two different temperatures, while egg mass hardening is monitored.
Refs
Anderson,
D.T. (1994). Barnacles. Structure,
function, development and evolution. Chapman & Hall,
London.
Sanz,
J.J. (2002). Climate change and breeding parameters of great and blue tits
throughout the western Palaearctic. Global
Change Biology 8: 409-422.
Tige-Ford (1967). Possible
mechanism for the endocrine control of breeding in a cirripede. Nature, London 216: 920-921.
John Davenport Wednesday, June 12, 2002
