Biology Reference
In-Depth Information
As will be described in detail below, metamorphosis in lampreys involves
extensive modifi cation to the internal organs and tissues. The changes to
the internal anatomy are of such a magnitude and so widespread that the
animal is incapable of feeding until metamorphosis is fully completed; i.e.,
lamprey metamorphosis is a non-trophic phase of the life cycle. In addition,
external changes involve body colouration, a complete remodeling of the
mouth from the larval buccal funnel to the adult suctorial disc with teeth and
a tongue-like piston, completion of development of the eyes, and alterations
to the shape of the branchiopores. It is these external changes that are the
basis for the description of stages (see below) of this developmental process.
Although there are some subtle differences in the events and processes of
metamorphic change between lamprey species, the end result is basically
the same. That is, metamorphosis results in a juvenile. In non-parasitic
species, juveniles usually have a larger gonad, compared to their parasitic
member in a paired species, likely as a consequence of their more protracted
interval of larval life (Bird and Potter, 1979b). Despite the fact that they have
the same adult internal anatomy as a parasitic species, the juveniles of non-
parasitic species almost immediately commence sexual maturation and can
be capable of spawning (sexually mature adults) within 5-6 months of the
completion of metamorphosis. During this interval of sexual maturation and
after reaching sexual maturity, the adults of non-parasitic species move only
a short distance within their natal stream. In contrast, juveniles of parasitic
species are required to feed before sexual maturation commences and in
some cases, such as in anadromous species, they undertake an extensive
downstream migration to the ocean. Some parasitic, freshwater species may
undergo a short migration to fi nd a host within their natal stream. In the case
of the landlocked,
P. marinus
, and other landlocked species, the downstream
migration is to a large body of fresh water such as a lake. The length of the
parasitic period in adult life is quite variable among the species. Information is
most prevalent on the sea lamprey,
P. marinus
, where a rapid-growth, parasitic
period of around 1 year for landlocked forms (Bergstedt and Swink, 1995)
and around 2 years for the anadromous form (Beamish, 1980) are estimated.
The smaller size of parasitic forms that remain in their natal stream, suggest
a much shorter feeding period.
Feeding in adult life ceases at the time the animals commence an
upstream migration to the spawning grounds. Once again, the length of
time for this upstream migration is quite variable even among anadromous
forms. For example, although the anadromous
P. marinus
(and also the
landlocked form) migrate and spawn between March and September
(Beamish, 1980), both
Mordacia mordax
and
Geotria australis
of the southern
hemisphere and the northern hemisphere, river lamprey,
Lampetra fl uviatilis
,
have migration periods of much longer duration; i.e., up to 15 months for
G. australis
(Hardisty, 2006).
