7723 - Laserfiche WebLink

Home Browse Search

Some keys require myomere counts (lateral muscle segments; Fig. 3-4). For accurate counts, the first myomere must be identified and included. It is partial (epaxial, dorsal only), sometimes deltoid or triangular in shape, and located above and often slightly anterior to the cleithrum or base of the pectoral fin (Fig. 4). To observe myomeres, experiment with both reflected (from above) and transmitted (from beneath) light. For small, somewhat translucent specimens, bright transmitted light will pass through the larva and make myomeres more visible. In many cases, the addition of a pair of polarizing fllters will actually highlight individual myomeres, including the first. One polarizing lens is placed between the light source and specimen (under the dish) and the other is positioned or held between the specimen and the objective lens of the microscope. The highlighting effect is achieved by rotating the upper lens until the background is black or as dark as possible. Only light waves that are turned 900 as they pass through the larva are observed. The position of the specimen rela- tive to the plane of light passed by the lower filter is important as well. Once background light is minimized, rotate the specimen itself, or the dish, for optimal effect. For larger or more opaque specimens, reflected light at a low to near-horizontal angle casts shadows that help defIDe myomeres. Fin ray counts also are required for some keys. Developing fID rays must not be con- fused with folds or creases in the finfold or membrane, or with the skeletal structures that precede and support median fin rays, i.e., the hypural elements of the caudal fin and pterygiophores of the dorsal and anal fIDs (Fig. 3). The hypural elements eventually fuse and form the hypural plates. Externally, the upper or articulating ends of pterygio- phores appear as small nodules at the base of the developing dorsal and anal fIDs. Since pterygiophores appear before the rays they support and since there is a one to one correspondence between pterygiophores and principal fin rays, pterygiophore counts can substitute for principal dorsal and anal fID counts near the end of the mesolarval phase. Also use them to determine presence of all principal dorsal or anal rays (a requirement for transition to the metalarval phase). For median fIDs, principal rays also must be distinguished from rudimentary rays (Figs. 3 and 4). Principal dorsal and anal fID rays include all branched rays and the long unbranched ray preceding them. Principal caudal rays include all branched rays and the long unbranched rays immediately above and below them. Terminal branching for some principal rays of the dorsal and anal fIDs might not be evident until late in the meta- larval phase, but the pterygiophores asso- ciated with them are obvious on close exa- mination. The most posterior principal ray for dorsal and anal fIDs consists of two elements that articulate with the last ptery- giophore. By convention, these elements are said to branch at the base and are counted together as one principal ray. In early meta- larvae, the most posterior element of the last ray is often difficult to observe. Again, rely on the association between principal dorsal and anal fID rays and their pterygiophores to confirm counts. All fin rays are included in pectoral and pelvic fin ray counts. Do not mistake the spine-like pelvic splint at the leading edge of the pelvic fin for a fill ray. Transmitted light and polarizing fllters are useful in observing pterygiophores and fID rays. Keys are seldom perfect. When a speci- men is near the boundary between develop- mental phases, use the adjacent key to help confirm results of the proper key. Similarly, within keys, if a character state is nearly borderline or if the user is unsure which of the criteria the specimen meets, try both branches of the key. In either of these situ- ations, if the conclusions are different, and remain so after review, either the identity via the more appropriate key or branch should be considered tentative (add a question mark, "?", to the name and footnote other possibili- ties) or the identity of the specimen should be left in question (label to family or genus only). When character states are so similar between species that positive diagnosis is not possible (based on characters studied), the keys conclude with two or more possible species. As previously noted, some of these specimens might be hybrids. Use both the comparative summary and species accounts to help confirm conclusions reached through these keys. Non-morphological data, such as collection date and location, can be used sometimes to delimit possible species (e.g., neither Utah, white, nor razorback sucker have been reported in the White River in 38