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<br />mouth structures are diagnostic for some cyprini-
<br />forms, especially later in the larval period, The
<br />timing of mouth migration from a terminal to an
<br />inferior position is particularly useful during a
<br />portion of the metalarval period in catostomids.
<br />The length, timing of the occurrence of the first
<br />loop, and eventual degree and form of the loops
<br />or coils of the gut can be important diagnostic
<br />characters for many fish. They are among the more
<br />obvious characters used to distinguish the late
<br />mesolarvae, metalarvae and early juveniles of the
<br />bluehead and flannel mouth suckers (Catostomus
<br />disaobo7..us and C. 7..atipinnis respectively).
<br />
<br />Pigmentation: The basic patterns of chromato-
<br />phore distribution, and changes in these patterns
<br />as fish grow and advance developmentally, are
<br />characteristic at the species level (for some fishes
<br />at the subspecies level). Used with caution,
<br />preferably in combination with other characters if
<br />feasible, and with an awareness of both intra- and
<br />inter-regional variation, the chromatophore distri-
<br />bution and patterns of many fishes are among the most
<br />useful characters available for identification at the
<br />species level. However, in some instances, differ-
<br />ences are so subtle that use of pigmentation is
<br />impractical and may be misleading.
<br />
<br />Pigmental variation, for a specific develop-
<br />mental stage within a species, exists largely in
<br />the number of chromatophores exhibiting pigment,
<br />either in general or in specific areas, rather than
<br />differences in the basic pattern. Complete loss
<br />of pigment in an area, of course, eliminates that
<br />portion of the overall pattern. In addition, the
<br />pigment in chromatophores can be variously displayed
<br />from tight, contracted spots, giving a relatively
<br />light appearance, to widely expanded, reticular
<br />networks which gives a dark or more brilliant
<br />appearance to the area affected. Differences in
<br />environmental conditions and food can significantly
<br />affect the appearance of pigmentation. Cultured
<br />specimens accordi ngly, can appear quite different
<br />from field-collected material.
<br />
<br />In cypriniform fishes, as well as most other
<br />fishes, chromatophores other than melanophores have
<br />not been sufficiently studied for identification
<br />purposes, in part because they are typically
<br />neither as numerous nor as obvious,and because of
<br />the difficulty in preserving these pigments over
<br />a period of time. Melanin, the amino acid breakdown
<br />product responsible for the dark, typically black,
<br />appearance of melanophores (Lagler et al. 1977),
<br />remains relatively stable in preserved specimens.
<br />Melanophores are, however, subject to fading and loss
<br />of pigment if specimens are stored or studied exten-
<br />sively in bright liqht or if subjected to changing
<br />concentrations in the fluids in ~Ihich they are stored
<br />or studied. To ~inimize the latter effects, as well
<br />as shrinkage ana deformation, dilute formalin
<br />solutions (3-5%, preferably buffered) are strongly
<br />recommended over alcohol solutions as storage media.
<br />Most of the following discussion refers to chromato-
<br />phores in general, but in this guide, as well as
<br />previous guides to freshwater species in North
<br />America, pigmentation typically refers to melano-
<br />phores only.
<br />
<br />According to Orton (1953), pigment cells
<br />originate in the neural crest region (dorsal portion
<br />of body and tail) and migrate in amoeboid fashion
<br />in waves to their eventual position. The first
<br />wave of chromatophores occurs late in the embryonic
<br />period or early in the larval period and establishes
<br />a relatively fixed basic or primary pattern of
<br />
<br />FISH EGGS 13
<br />
<br />chromatophore distribution. In a few (mostly
<br />marine) species, the cells become pigmented prior
<br />to migration and the actual migration can be observed
<br />and documented. But in cypriniform fishes, as in
<br />most other freshwater species, pigment is not
<br />present (or appears not to be present) in the
<br />chromatophores until some time after the cells have
<br />reached their ultimate destinations.
<br />
<br />Pigmentation often changes considerably as
<br />fish grow and develop. Most of the change is due
<br />to the increased numbers and spread of chroma to-
<br />phores. Observable pigmentation may also be lost
<br />from certain areas, usually through either a loss
<br />of the pigment or chromatophores themselves, or,
<br />in the case of subsurface or internal chromatophores,
<br />by the thickening and increasing opacity of covering
<br />tissues. Internal melanophore pigmentation can be
<br />observed more readily by careful clearing of the
<br />larva.
<br />
<br />COMMENTS ON THE IDENTIFICATION OF FISH EGGS
<br />
<br />Identification of fish eggs or embryos has
<br />received very little attention in the published
<br />literature, especially for freshwater species. Due
<br />to lack of distinctive features for most freshwater
<br />forms, all but the latest stages of the late-embryo
<br />(tail-free) phase of most fishes are very difficult,
<br />if not nearly impossible, to identify to species.
<br />The latest embryonic stages can sometimes be
<br />identified to species or designated as belonging to
<br />one of several related species by use of diagnostic
<br />characters for the recently-hatched larvae.
<br />
<br />Certain egg and embryo characteristics are
<br />suffi ciently di sti ncti ve to a 11 ow most specimens
<br />to be identified as belonging to one or more
<br />specific families or subfamilies. Characteristics
<br />useful in this respect are egg diameter and shape;
<br />nature of the chorion (e.g., smooth or patterned);
<br />projections or invaginations; attachment threads,
<br />filaments, or stalks; presence and form of an
<br />obvious micropyle; number and thickness of
<br />chorionic membranes; gelatinous or adhesive coatings;
<br />homogeneous or segmented (granular) yolk;
<br />type of cleavage; and at specific stages the number,
<br />position and size of oil globules in the yolk,
<br />and the size of the perivitelline space.
<br />
<br />Size of the egg and perivitelline space and
<br />the presence and nature of oil globules (coupled
<br />with time of year, location, and apparent nature of
<br />egg deposition) are characteristics particularly
<br />useful in identifying the eggs of freshwater
<br />species. Most freshwater fish eggs, including those
<br />of the cypriniforms, are round, relatively smooth,
<br />and without distinctive surface features, stalks,
<br />filaments or coatings; cleavage is typically
<br />meroblastic. Exceptions in North America include
<br />Lepisosteus species, Notropis girardi, Ieta7..urus
<br />punatatus and Perea f7..avesaens with special coatings
<br />or outer envelopes; Osmerus mordax with an attachment
<br />stalk; Labidesthes ,siaau7..us and Menidia audens with
<br />chorionic filaments or threads; and Acipenseridae,
<br />Polyodontidae, Lepisosteidae and Amiidae with semi-
<br />holoblastic cleavage. Cypriniform eggs are typically
<br />demersal with moderate to little perivitelline space
<br />and no oil globules, though most are readily trans-
<br />ported if dislodged in moderate to strong currents.
<br />Exceptions include Notropis atherinoides and N.
<br />amoenis which have pelagic or semipelagic eggs with
<br />expanded chorions enlarging the egg diameter to
<br />about 3 mm and providing for a large perivitelline
<br />
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