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Marine Vertebrate Zoology Laboratory
Larval Fish Identification
Introduction
The study of larval fish is an undertaking not for the faint of heart. Studying microscopic
fish, which in many cases bear no resemblance to the adult, can be tedious. With this
said, it should also be noted that without an understanding of larval fish identification,
development and growth, a crucial piece of knowledge is missing for any given species.
To have a clear picture of a species life history (ontogeny) we must know the whole
story, including its early life history. Before we discuss the larval stages of fish we must
address how they are created; through the reproduction of adults.
Reproduction
Spawning behavior has fascinated biologists for centuries. The reproductive behavior of
a given fish can be tied directly to the ecology of the species and its adaptations. Most
fishes can be separated into one of three groups:
1. Nonguarders
2. Guarders
3. Bearers
Nonguarders – these fishes do not protect their embryos and young once
reproduction is completed.
Pelagic spawners – this type of nonguarder is the most prevalent
reproductive method used by marine fishes. Many schooling species and several
anadromous species employ this strategy. The true advantage to this method is
dispersal via currents. For dispersal to occur, individuals must be positively
buoyant and this is usually achieved by the presence of an oil globule. This
reproductive strategy experiences staggering mortality due to dispersal to
unfavorable areas, or predation. To compensate this, females are usually
extremely fecund and have extended spawning periods.
Benthic spawners – use vegetation or other structures on the bottom to
attach its eggs. Adhesive on the eggs is used to hold them in place until the larvae
hatch, whereupon they usually adopt a pelagic existence.
Guarders – usually tend to both the embryo and larval stages. These fishes
remain in a single location and are territorial. Embryos are usually guarded by the
males (except in Cichlids), which afford protection, maintain high O2 levels, (by
fanning currents) and clear debris. Guarders will spend a few days to several
months protecting its young.
Bearers - fish that carry their embryos (or young) around with them, either
externally or internally. This group is easily identifiable by the Syngnathids
(Seahorses) and the Elasmobrachs, which we have discussed in class.
Early Life History
After spawning and before recruitment into the adult population, fishes undergo
drastic morphological and physiological changes. When we examine the early
life history of marine fishes it is useful to categorize the distinct developmental
stages and the factors that impact them (Figure 1.)
Egg Stage – this period of development is where the individual is
dependent on yolk from the egg for nutrients. Rapid cell division and the
formation of organ systems occur, resulting in the eventual hatching of a larval
fish.
Yolk sac – in this stage of development the yolk sac is still the principal
source of nutrition. The individual has now left the confines of the egg and is
able to orientate itself in the water column. At the end of this stage the yolk sac is
absorbed and the pigmentation of the larvae becomes evident.
Larval Stage – this is the most dynamic stage of a fishes life. Several
morphological and physiological changes are occurring. Individuals are now able
to feed (or starve!) and adopt many characteristics beneficial for a planktonic
existence. The duration of this stage varies by species and the conditions needed
for metamorphose into the juvenile stage. It is during the end of the larval stage
when flexion occurs. This is the upturn movement of the last vertebral elements
forming the caudal fin.
Juvenile Stage – the individual now resembles a miniature adult (in
most cases). All characteristics adaptive for a planktonic existence have been lost
and the structure and habitat of the adult is adopted.
Larval Fish Identification
Worksheet
Sample # ____________________
Date________________
Gear Type____________________
Sampling Technicians__________________________
Time of sample_________________ Duration (min)__________
Numbers
Scientific Name Total Subsmpl
Record individual lengths on a separate sheet.
Lengths (mm.)
Total Max Mean
Min