Biology Reference
In-Depth Information
including the shape of the branchiopores. It is noteworthy that modifi cation
to the larval buccal funnel includes a complex set of changes including
the loss of larval cirrhi, the development of teeth and a tongue-like piston,
and special laminae, and the appearance of a ridge of fi mbriae on the outer
surface of the developing oral disc. The sequential development of these
features in the oral region are all important for stage identifi cation.
Detailed descriptions of all events in the alteration of the fi ve main
features is beyond the scope of this review, but several are worthy of a
summary. For instance, the eye which is beneath the skin in larva, fi rst
becomes larger and almost oval-shaped patch (stage 1) and darker and more
rounded by stage 2. The circular shape persists as the eye further enlarges
and starts to show separation of the pupil and iris (stage 3); these features
develop further to such an extent that the eye bulges from the body surface
(stage 4). By stage 5 eyes with distinct pupil and pale-grey iris protrude
further from the surface. Subsequent stages show further eye enlargement
and a much more silver iris. The branchiopores show a progressive change
from triangular shape in larva and early stages to more oval by stage 4.
Body colouration retains the larval brown ventral and dorsal surfaces until
stage 4 when a blue-grey sheen is observed. Subsequent stages can be
distinguished on the basis of the degree to which the dorsal surface becomes
blue-black and the ventral surface a silver sheen. Stage 4 is identifi ed when
the transverse and lateral lips of the larval buccal funnel fuse to produce the
oral disc, the larval cirrhi are reduced to a few papillae-like projections, and
the undeveloped piston is fi rst prominently visible. Teeth are fi rst visible as
small points at stage 5 when the piston shows its laminae and the infraoral
lamina is discernible. The changes in these and other features also manifest
themselves in changing behaviour that can be species specifi c. For instance,
the changes are so advanced by stage 6 in
P. marinus
that the animals may
leave their burrows and may be capable of adhering with their suctorial
disc (Youson and Potter, 1979).
Since the animal length does not change appreciably in most lamprey
species, in addition to the fi ve main visible external changes, measurements
of body proportions (% of total length) are also useful to follow during
metamorphosis. These types of measurements show reductions in branchial
and trunk lengths and an increase in prebranchial length. Although
tail length remains relatively constant, the length of the disc increases,
particularly in species such as
P. marinus
(Potter et al., 1978b). A comparison
of three species showed a common increase in eye diameter and height of
both the fi rst and second dorsal fi n (Potter et al., 1982).
Following the reports cited in the previous paragraph, the seven
stages of lamprey metamorphosis (1, earliest to 7 fi nal stage) have been
universally adopted. Staging is now available for the southern brook
lamprey,
Ichthyomyzon gagei
(Beamish and Thomas, 1984), the far eastern
