Biology Reference
In-Depth Information
within about 17 days post-fertilization (PF). At this time the prolarvae are
dependent on their yolk sacs for nutrition but by about 33 days PF the
young larvae, 9-10 mm long, begin to feed on detritus, algae, desmids and
diatoms. Although the larvae (ammocoetes) likely move within their natal
stream their primary habitat is a burrow within the soft sediment. Since
they cannot tolerate even dilute seawater, they remain far upstream from
any tidal effects. On the other hand, should their natal stream empty into a
larger body of freshwater, such as in the Great Lakes or in Lake Champlain
of North America, they can be found at the mouth of the stream or even in
the substrate of a lake.
The length of the larval period of the lamprey life cycle varies with the
species, but it is usually never less than 2 years and seldom higher than 7
years. According to Hardisty (1979), there has been a trend to lengthen larval
life and eliminate feeding during the evolution of lampreys. Hence, if one
accepts the view that non-parasitic lampreys are the more derived adult
life history type, then larvae of non-parasitic species are older than those
of parasitic species at the time of the onset of metamorphosis. However,
there are documented cases in experimental situations in the wild where
larvae of parasitic,
Petromyzon marinus
, have been recorded at ages greater
than 10 years (Potter, 1980). Historically, the age of larval lampreys has
been determined through modes that appear within length-frequency data
from animals collected in a given stream. More recently, annuli on statoliths
from the otic capsules have been applied, but data from this new approach
is often correlated with animal length or length-frequency data. Also, one
has to sacrifi ce the animal to obtain the statolith data. Although the early
year classes are quite clear as modes in the length-frequency data, likely
due to rapid growth spurts, it is the older age groups where age distinction
is less clear because growth slows as the larva approaches the upper ages.
The rates of growth also seem to vary between different populations (i.e.,
different streams) of the same species due to differences in both habitat
and water temperature conditions. The slowing or arresting of growth in
the oldest larvae is refl ected in an overlap in lengths of animals which are
either immediately pre-metamorphic and metamorphosing and those that
will wait another year before they reach either of these intervals. Thus, in
some species of lampreys it is diffi cult to be certain which larvae are about
to enter metamorphosis. The two most important criteria to consider in
selecting immediately pre-metamorphic animals are knowledge of the time
of the year when they generally undergo metamorphosis and the length
of metamorphosing animals in wild populations. There are also cases
where lipogenesis and fat storage is refl ected in increased body weight of
immediately pre-metamorphic animals so that animal length and weight
(collectively referred to as size) can be used as indicators as the season for
metamorphosis approaches.
