Scarpa, J.
Abstract:
Many species of marine fish larvae initially subsist on endogenous
food (i.e., yolk), but soon need live food to continue development.
This first-feeding stage is where fish culturists have often failed due
to the lack of an acceptable or correctly sized live or artificial feed.
Bivalve trochophore larvae, particularly oyster trochophores, are of
an appropriate size (40-60/ m) and nutritional quality for first-feeding
marine fish larvae. However, this swimming unshelled stage is
short-lived developing into the prodissoconch I stage at which time
the shell starts to form. The short time period for the trochophore
stage also means that a daily supply must be produced or stored.
Cryopreserved oyster trochophores are already available on a
commercial basis (trademark Trochofeed). However, storage of the
trochophores requires a source of liquid nitrogen that may not be
available to culturists. Therefore, the development of a simple and
inexpensive method for producing developmentally arrested bivalve
trochophores would be useful for many culturists.
A general method for arresting development of bivalves at or prior to
the trochophore stage is by altering the sperm DNA with ultraviolet
irradiation (UV, 254 nm). A sperm suspension from a bivalve is
obtained and placed in an open container with low sides, such as a
petri dish, so that the suspension thickness is </= 1 mm thick. The
suspension is then exposed to UV (e.g., from a germicidal bulb in a
hood) for a brief time period (15-180s) depending on sperm density
and UV power. A visual examination is made with the aid of a
microscope to determine the approximate proportion of sperm
moving. Sperm movement is used as an indicator of sperm viability
and fertilization capacity. The irradiated sperm suspension is then
used to inseminate the bivalve eggs. After the eggs have undergone
two cell divisions the egg suspension may be placed in a temperature
controlled unit set at a lower temperature to delay development or
placed at an elevated temperature to hasten development.
Ultraviolet irradiation of bivalve sperm used for insemination results
in developmental arrest of the embryo at or prior to the trochophore
stage. This has been found for a number of different bivalve species
in research on producing gynogenetic haploid offspring. Only when
irradiation exposure has been for too short a time period or with too
low an output has normal development ensued in a portion of the
larval population. Fertilization is proportional to the number of
mobile sperm and yield is proportional to the number of eggs
obtained and fertilized. At HBOI, oyster (C. virginica) and clam (M.
mercenaria) trochophore larvae have been produced consistently for
use in fish culture. The general utility of the method presented is that
it is easy and inexpensive, it can be used on any bivalve species in
which sperm and eggs are retrievable and can be kept viable in vitro,
and it virtually eliminates any chance of introducing a non-native
species into a new habitat.
(Harbor Branch Oceanographic Institution, Inc., Aquaculture Division
5600 U.S. I North, Ft. Pierce, FL 34946, USA)