Question

The evolutionary Imprtance of ammocoetes larva for 5 marks

Answer 1

Answer:

Explanation:

Systematics

Chordata P, Metameria, Vertebrata sP, Agnatha C, Petromyzontida O, Petromyzontidae F

Lamprey eels belong to Agnatha, the oldest known taxon of living vertebrates. Of a handful of living lampreys, the parasitic marine lamprey, Petromyzon marinus , is the most common and most readily available for laboratory use, although it is not the best example of its taxon. Agnaths are vertebrates with no paired appendages, no jaws, no cranium and no vertebrae. The skeleton is cartilaginous and the notochord persists through life. The larva is the ammocoetes, which closely resembles cephalochordates such as Branchiostoma (amphio­xus). The ammocoetes (AM oh sete ease) is the vertebrate most like the most vertebrate-like invertebrate. As a vertebrate with striking similarities to a non-vertebrate it is an important evolutionary link supporting the hypothesized close relationship between protochor­dates and vertebrates.

Laboratory Specimens

Ammocoetes larvae of marine lampreys or brook lampreys are available from biological supply companies as wholemount slides of small specimens and cross section slides. Preserved larger larvae are also available from some suppliers. This laboratory exercise utilizes preserved specimens, wholemount and pharyngeal cross section slides and can be used with either marine or brook lamprey larvae. The larvae on wholemount slides are necessarily very small, usually less than 2 cm. Preserved specimens for this exercise should be much larger, about 100-120 mm.

The name “ammocoetes" (or ammocoete), is an old generic name retained from earlier days before we realized they were larval agnaths and were thought to be a genus of cephalochordates. The larvae live in freshwater streams, burrow in sediments, and are filter feeders utilizing phytoplankton. The larvae live two to six years before metamorphosing into parasitic adults. See Hyman (1942), Weichert (1961), or Lehman (1983) for additional information. The biology student’s first exposure to the lamprey and its ammocoetes is usually in a course in comparative vertebrate anatomy.