Potential Fecundity of Landlocked Sea Lamprey Larvae, Petromyzon marinus, with Typical and Atypical Gonads

Abstract

Lampreys produce a fixed number of oocytes early in larval life, which represents their total reproductive potential or potential fecundity (Beamish and Thomas, 1983). In the landlocked sea lamprey of the Great Lakes, considerable oocyte atresia is thought to occur in postlarval stages (Hardisty, 1965b, 1969) such that actual fecundity may be appreciably less than potential fecundity (Applegate, 1950; Vladykov, 1951; Hardisty, 1965b). In the anadromous sea lamprey, the actual fecundity of adults and potential fecundity of larvae are similar (Hardisty, 1964). The much lower actual fecundity of the landlocked sea lamprey is thought to be directly related to the smaller adult body size (Hardisty, 1964; Beamish and Thomas, 1983). Sex differentiation in landlocked sea lamprey is typically complete when larvae reach total lengths of approximately 100 mm (Hardisty, 1965a, 1965b; Docker, 1992). Females are characterized by a large horseshoe-shaped ovary with fingerlike lobes, each containing two rows of oocytes greater than 60 xum in diameter (Hardisty, 1965b; Docker, 1992). Stromal tissue is scarce between the germinal elements, and there are few undifferentiated germ cells (Docker, 1992). Undifferentiated, or primordial, germ cells develop into oocytes through two growth phases. During the first growth phase, occurring in larvae 80-100 mm, cells increase synchronously in size (12-40 ,xm diameter) and are called first-stage oocytes (Hardisty, 1971). A second growth phase follows, with oocytes synchronously increasing in diameter to 80 ,Im. These second-stage oocytes are typically found in larvae greater than 110 mm total length (Hardisty, 1971). Males have an angular-shaped testis without lobes (Docker, 1992). Oocytes may be present but are few in number and usually less than 18 uxm in diameter (Docker, 1992). Stromal tissue is abundant, and clusters of germ cells are present (Hardisty, 1965b). Our recent observations (unpubl.) on landlocked sea lamprey larvae from the Great Lakes basin have revealed highly atypical patterns of gonadal development. Gonads of larvae with atypical gonads exhibit varying degrees of male and female characteristics. For example, gonad morphology may be similar to a female; however, primordial germ cells may be abundant. Oocytes may be asynchronous in their development, such that both firstand second-stage oocytes are present, or a high proportion of atretic oocytes may be present. The present study was undertaken to compare potential fecundity of typical and atypical larvae collected from streams tributary to the Great Lakes. The present study also compared current estimates of potential fecundity with those made several decades earlier (Hardisty, 1964) before larval abundance was regularly reduced in streams by chemical treatment.