The Feeding Tactics of Balanus glandula
Barnacles feed during two of their three
life stages. They feed
as member of the planton during their nauplier stage. The cyprid stage subsists
on reserves sequestered during the final of six naupliar instars. As juveniles
and adults, B. glandula suspension feeds with elongated cirri.
Naupliar stage feeding
As nauplius larvae, which undergo six instars
in the span of approximately a couple of weeks, B. glandula is a feeding
member of the plankton. The duration of the first three instars is much shorter
than the final three and the first instar does not feed! Feeding is important
to the nauplius larvae since the reserves the larvae store will sustain them
as cyprid searching for a place to settle and call home for the remainder of
their lives. Experiments, done by Hentschel and Emlet (2002), which manipulate
the amount of food available to nauplius larvae at various times during the
six instars, show connections to how long the nauplius stage lasts and the health
of the resulting cyprid larvae. Increased amounts of food during the latter
half (instars 3-6) of the naupliar period, may cause a shortening of the naupliar
period. If instead the food supply is reduced, the naupliar period might be
extended. However this is only true up until a certain point. The later an increase
or reduction in food occurs in the naupliar period, more likely that the length
will be unaffected by this variation in food abundance. The timing of metamorphosis
from naupliar stage to cyprid stage seems to be fixed after about 60% of the
naupliar stage is completed which corresponds to the beginning of the sixth
and final instar. After the sixth instar begins changes in food abundance do
not effect the time spent in naupliar stage sequestering reserves however such
changes continue to effect the size of the resulting cyprid larvae. Thus,
patchiness of plankton in both time and space will effect the timing of barnacle
metamorphosis and the size of the resulting cyprid larvae. This flexibility
in age and size at metamorphosis from feeding nauplii to non-feeding cyprid
larva barnacles shows adaptation to what is probably a common sort of environmental
irregularity and contrasts the conditions of the next metamorphosis into juveniles
which is cued by strict substrate cues.
Juvenile and adult feeding
After a cyprid cements itself to a substrate,
it undergoes metamorphosis into a juvenile. From this point on, B. glandula
suspension feeds. While under water and undisturbed, the movable scuta and
terga open and the barnacle extends through the aperture. The animal unrolls
three pairs of long, biramous cirri like a filtering spoon. This “spoon net”
sweeps out and then closes down toward the scuta side of the aperture. During
this “closing” stroke, fine setae on each cirri trap food and usher it closer
to the mouth. The lower cirri pairs are used to remove particle from the three
pairs which were extended cirri and transfer the food to the mouthparts for
handling and ingestion. What we see is a beautiful sweeping motion. While some
barnacle species leave their “spoon net” out for a period of time, B. glandula
rhythmically extends, unrolls, sweeps and closes.
Back to B. glandula index page
Leah Behrends, 2002.