PART I. ANATOMY OF BABY FISH IN LAKES AND PONDS

Since larval fishes are the babies of adult fish, they are organized into the same families, genera and species. However, baby fishes do not look like adult fishes such as described by McAllister and Coad (1974) and elsewhere. What do baby fishes look like, and how are they different? Baby fish start out life as swimming embryos; their bodies and internal organs are transparent but coloured with species-specific melanin spots (maculae). They begin without bones or scales, and range in length from about 5 to 15 mm (about the size of a mosquito).

One big difference from adults is that they do not possess fins, at least early in life, so other methods must be used to identify and classify them. The bottom line says that newly hatched fishes do not look or act like their adults. Because of this, ichthyologists should include descriptions of larval fishes in regional fish guides, as some Japanese authors already have done. In fact, baby fish possess very different morphological, physiological, ecological and behavioral features from their adults. Certain of these features appear and disappear during several days or weeks. Since the basic morphological features of baby fish do not always lend themselves to the same groupings as adult fishes, the systematics or taxonomy of baby fish will not correspond exactly with their adults (Ahlstrom and Moser, 1976).

The life of any fish - from the moment of fertilization until death in old age - can be subdivided into five artificial but definable and recognizable periods: A) Embryonic Period, B) Larval Period, C) Juvenile Period, D) Adult Period, and E) Senescent Period (Balon, 1975). Most fishes in our region develop and change gradually (in very small steps or stages) like true bugs (hemipteran insects) rather than with spectacular metamorphic changes as with butterflies (larva, pupa and adult). Despite the almost anomalous terms "larva", "larval fish", "fry", "eleutheroembryo" and others, these words are well established in the scientific literature and in the everyday vocabulary of fishery biologists. Au contraire, staff working at fish hatcheries avoid these "scientific terms" and use their own "hatchery jargon", such as "yolk sac fry", "alevins", "fingerlings" and so forth. To add to the confusion to specialists and non-specialists alike, numerous other terms are used in the scientific literature, depending upon author and continent (Ahlstrom and Moser, 1981).

As has been mentioned, larval fish or babies, with their distinctive morphological features, look very different from adult fishes. Certain baby structures or features that are transient or even ephemeral characterize their bodies. Gill slits (pharyngeal pouches) in the early embryonic life of birds and mammals are an analogous but classical textbook example. Because of their importance but unfamiliarity, certain of these transient anatomical structures will be discussed first. Only easily recognized structures will be mentioned.

Transient Structures

The finfold (figs. I-1-A and I-1-B) is a transparent membrane which lies medially around the caudal part of the baby. It extends along the midline of the back from near the head, around the tail and then forward to the anus. A small, separate section of finfold extends forward from the anus to a position below the paired pectoral fins (fig. I-1-D). The unpaired dorsal, caudal and anal fins are eventually formed from portions of this membranous tissue.

Yolk materials (fig. I-1-A) consist of yolk (protein and so forth) per se and sometimes a sphere of oil (lipids) or a group of oil globules (not shown). Yolk materials are food reserves which diminish as physiological growth proceeds. Carotenoids normally colour yolk materials some shade of yellow or orange. The blood is colourless owing to colourless erythrocytes; haemoglobin ln the blood increases with growth and becomes red some time later.

The notochord (fig. I-1B), an endoskeletal structure, extends mid-dorsally from the brain to the tip of the caudal fin (fig. I-1D). It is pliant, rod-like and formed of special vesicular con connective tissue which serves as a foundation about which the backbone is eventually built. A small canal in the middle of the centra (vertebra) of adult fishes represents the location of the remnant of the notochord. Myomeres or myotomes (figs. I-1A and I-1B) serve as the muscular system. Baby fish show strong segmentation or metamerism as do all vertebrate embryos.

The pigmentation (figs. I-1C, I-1D and I-1E) is different on each side of preserved baby fish and consists of various melanophore maculae (black pigment spots) (figs. I-1C, I-1D and I-1E) organized in species-specific arrangements. While alive, some babies possess guanophores (white pigment cells, not shown) and/or xanthophores (yellow pigment cells, not shown). Pigment patterns and colours change gradually but completely as growth proceeds. Other anatomical and physiological features, which we will not discuss here, are also transient.


External Anatomy

Most baby fish that can be captured and seen in the Ottawa region are similar to golden shiner babies. The external anatomical features shown here are specific for the golden shiner (Notemigonus crysoleucas) but are similar to those of many other freshwater fish babies. Figures I-1A to I-1E show three lengths or "stages" during the development of golden shiners. Most internal features are not shown here because these figures were drawn from formalin-preserved specimens in which the tissues have already turned from transparent to opaque. Internal structures are described in the next section.

The body (fig. I-1A) of the golden shiner is elongate and tapers gradually from head to tail. Mouth (fig. I-1B) is terminal; a pair of nostrils (fig. I-1C) lies between the mouth and eyes. Eyes (figs. I-1C and I-1D) comprise opaque spherical lenses and black donut-shaped optic cups (figs. I-1D and I-1E). At the back of the head is the operculum or gill cover (figs. I-1D and I-1E) covering the gill filaments (fig. I-1E). The finfold (figs. I-1A, I-1B and I-1D) persists from hatching until the beginning of the juvenile period. Fins which are identifiable include the paired pectoral fins (figs. I-1C and I-1D) and the medial caudal fin (fig. I-1D). Dorsal fin, anal fin and paired pelvic fins are not yet formed.

The intestine (fig. I-1D) is tubular and ends at the anus or vent (figs. I-1D and I-1E). The tip of the notochord (fig. I-1D) is visible at the tail. Fin rays (fig. I-1D) are beginning to form in the ventral part of the caudal fin. The air bladder or swim bladder (fig. I-1D) is ovoid-shaped and filled with gas; it has an ovoid melanophore maculae over it. The pattern of pigmentation consists of various melanophores which occur in various places and on various parts of the body. Sometimes in response to light, temperature or neurohumors, these black pigment cells change from small and round to large and dendritic, or vice versa, while larvae are still living (See: melanophores).

Figure I-1. Transient Structures and External Anatomy of Baby Fish.

A. Lateral view of recently hatched larva of golden shiner, 2.7 mm. (From Snyder et al., 1977.)
B. Lateral view of preserved golden shiner larva, 4.6 mm, which is about five days old. (From Snyder et al., 1977.)
C. Dorsal view of preserved golden shiner larva (8 mm) which is about two weeks old. (Illustration by Sally Gadd.)
D. Lateral view of golden shiner larva (8 mm). (Illustration by Sally Gadd.)
E. Ventral view of golden shiner larva (8 mm). (Illustration by Sally Gadd.)

Internal Anatomy

The internal structures of preserved baby fish are normally difficult to see because they are hidden by muscle tissues which have become opaque through preservation. Cross-sections of these three-dimensional, preserved, baby fish allow certain anatomical structures to be seen. Most baby fishes easily captured and seen in the Ottawa region are similar to white sucker babies. The internal anatomical features presented here are specific to the white sucker (Catostomus commersoni) but are similar to those of other fish babies. Figures I-2A, I-2B and I-2C show cross-sections of a white sucker babies at three locations. Figure I-2D shows these locations where cross-sections were taken. The anatomy of baby fish is somewhat less complicated than that of their adults.

The central nervous system is relatively simple at this size. A neural tube (figs. I-2B and I-2C) extends from the head into the caudal part of the body; a thickened portion occurs as the brain (fig. I-2A). The tubular brain surrounds a brain ventricle (fig. I-2A). Auditory vesicles (fig. I-2A) hold calcified otoliths (fig. I-2A) which aid in swimming and balancing. Cross-sections of eyes show spherical lenses (fig. I-2A) and surrounding optic cups (fig. I-2A). Olfactory placodes (fig. I-2A) develop as thickenings of the skin and eventually locate within the nostrils. Yolk materials (fig. I-2B) are composed of numerous constituents including proteins, fats, lipids, sterols, pigments, vitamins and salts. Myomeres (figs. I-2B and I-2C) dominate the bodies of larval fishes. In the tail region (figs. I-2B and I-2C) the neural tube is small and the notochord is large. Other circular structures shown include urinary ducts (urinary vessels, figs. I-2B and I-2C), the aorta (blood vessel, figs. I-2B and I-2C), intestine (food vessel, fig. I-2B) and anus (fig. I-2C).

Figure I-2. Internal Anatomy of Baby Fish

A. Cross-section through head region of newly hatched baby white sucker. Fig. I-2D shows position of cross-section.
B. Cross-section through body region of newly hatched baby white sucker. Fig. I-2D shows position of cross-section.
C. Cross-section through tail region of newly hatched baby white sucker. Fig. I-2D shows position of cross-section.
D. Lateral view of baby white sucker showing position of figures I-2A, I-2B and I-2C. Illustrations redrawn from Stewart (1926).

Literature Cited

Ahlstrom, E.L. and H.G. Moser. 1976. Eggs and larvae of fishes and their role in systematic investigations and in fisheries. Revues Travaux d'Institut de Peches Maritimes, Nantes 40(3-4): 379-398.

Ahlstrom, E.L. and H.G. Moser. 1981. Systematics and development of early life history stages of marine fishes: achievements during the past century, present status and suggestions for the future. Rapports et Proces-verbaux des Reunions 178(III-IV): 541-546.

Balon, E.K. 1975. Terminology of intervals in fish development. Journal of the Fisheries Research Board of Canada 32: 1663-1670.

McAllister, D.E. and B.W. Coad. 1974. Fishes of Canada's National Capital Region/Poissons de la capitale du Canada. Fisheries Research Board of Canada, Misc. Spec. Publ. 24. 200 pp.

Snyder, D.E., M.B.M. Snyder and S.C. Douglas. 1977. Identification of golden shiner, Notemigonus crysoleucas; spotfin shiner, Notropis spilopterus; and fathead minnow, Pimephales promelas, larvae. Journal of the Fisheries Research Board of Canada 34(9): 1397-1409.

Stewart, N.H. 1926. Development, growth and food habits of the white sucker, Catostomus commersonii Le Sueur. Bulletin of the Bureau of Fisheries 42: 147-184.

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