Biology Reference
In-Depth Information
(ammocoetes) and adults. Furthermore, the larva are restricted to freshwater
habitats (Morris, 1980) and some adult lampreys may occupy both fresh-
and salt-water and others are compelled to live in either fresh water or in
a marine environment for at least part of their adult life. To accommodate
all of the physical and physiological differences between larval and adult
lampreys requires a major interval of change between the two periods of the
life cycle, i.e., a phase of metamorphosis. Since there is a metamorphosis in
the life cycle of lampreys, the post-hatched development of larvae to adults
is indirect (Youson, 1988).
2.2 Adult Life History Types
There are two adult life history types among the 38, known, species of
lampreys, but this number will undoubtedly change in the coming years
(for example, see Renaud and Economidis, 2010). Thirty-four of these
species reside in the Northern Hemisphere (holoarctic) and there are four
species of southern hemisphere lampreys. Twenty of these species, referred
to as non-parasitic, are restricted to fresh water during their adult life and
they never feed again once metamorphosis commences. The remaining 18
species, parasitic species, feed either on the blood or tissues of host fi shes
as juveniles in either fresh water or salt water. In some cases a non-parasitic
and a parasitic species share a common evolutionary history, likely arising
from a common parasitic ancestor. These related species are referred to as
“paired” or “satellite” species and there are situations where they cohabit
a stream. There are some isolated reports of unusually large-sized, sexually
mature non-parasitic individuals that likely must have fed as adults
(Manion and Purvis, 1971). Also, there is at least one population of non-
parasitic western brook lamprey,
Lampetra richardsoni
, that produces two
immediately postmetamorphic morphotypes (R.J. Beamish, 1987). One of
these delays sexual maturity for a year and is capable of feeding and the
other follows the normal pattern of non-parasitic species and starts sexual
maturation shortly after the completion of metamorphosis (Youson and
Beamish, 1991). These cases, and other evidence, has led to the belief that
there is a certain amount of plasticity in lamprey metamorphosis (Youson,
1999; Youson, 2004). It is commonly accepted that, despite the apparent
plasticity towards the timing of sexual maturation, neoteny does not exist
among lampreys (Hardisty, 2006).
2.3 Life Cycle
The lamprey life cycle begins with external fertilization of eggs by sperm
in nest of stones in a freshwater stream. The embryos develop relatively
quickly with the prolarvae ready to embed themselves in soft silt and sand
