Fish larvae
Laboratory techniques and terminology

Two techniques are normally used to identify eggs and larvae of marine fishes: (1) rearing eggs collected in the field, or eggs artificially fertilized in a tank, and following the developmental stages; and (2) following the developmental series of field-collected larvae from juvenile stage to yolk-sac larva.

Accurate identification of eggs and larvae is difficult. Identification is based on a variety of morphological characters: shape, number and position of melanophores, meristic characters, relative position of fins, shape and size of fin rays, myomere/vertebra counts, head spination, and so on. For each taxon, a combination of different characters is used for identification. A recently hatched larva is about 3 mm long, and reaches the metamorphosis stage at about 10-30 mm. During this short larval development period, morphological features change dynamically. Therefore, it is normally difficult to construct keys for larval identification.

Along with superficial morphological characters, clearing and staining techniques are used to study fin ray formation, bones supporting fin rays, size at ossification of different components of bony structure, and formation of vertebrae and accessories. To differentiate cartilaginous from ossified bones, a double staining technique using alcian blue and alizarin red is commonly used (Dingerkus and Uhler, 1977; Potthoff, 1984).

The early life of fish is normally divided into five stages: egg, yolk-sac larva, larva, transformation (or metamorphosis), and juvenile (Kendall et al., 1984). Most marine fish eggs in plankton samples are spherical, transparent, and ca. 1 mm in diameter. Overall size ranges from ca. 0.5 to 5.5 mm, with ca. 70% of the species ranging between 0.7 and 1.6 mm (Ahlstrom and Moser, 1980). The eggs are enclosed in a thin shell or chorion that appears smooth under the dissecting microscope (Moser, 1996d). In the taxonomy section we mention some eggs which have specific structures, such as filaments, spines, or a hexagonal network on surface. The larval stage is further divided into three sub-stages according to the degree of curvature of the distal end of notochord (Ahlstrom et al., 1976a). This is convenient because caudal fin formation is directly related to larval swimming capacity. The pre-flexion larva normally has only a membranous median fin-fold along the dorsal and ventral margins, a straight terminal notochord, and thus limited active swimming capability. During the flexion stage the fins and basic organ system differentiate, resulting in enhanced swimming ability. The post-flexion larva normally has most fins formed, and is an active swimmer.

The most widely used measurements of fish larvae (as defined by Olivar and Fortuño, 1991), are:
notochord length (NL): the distance along the mid-line of the body from the tip of the snout to the posterior end of the notochord in pre-flexion and flexion larvae. Standard length (SL) is the same but to the posterior end of the hypural bones in post-flexion, transformation, and juvenile stages.
Pre-anal length (PA): the distance along the mid-line of the body from the tip of the snout to the anus. The PA is important for determining the phylogenetic level of taxa; thus, larvae of the more primitive orders Albuliformes, Elopiformes, Anguilliformes, Clupeiformes, Gonorynchiformes, Salmoniformes and Stomiiformes have a relatively larger PA (as a function of total length) than the other, more evolved, orders.

Definition of head spination and general terminology of larvae are shown in schem. descript. of fish larva, following Lagler et al. (1962), and Moser (1996d).