Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 33

The reproductive system is highly variable and complex. Fish usually show separate sexes (producing sperm or ova), but hermaphroditism and parthenogenesis occur (Sloman 2011). Some bony fish show sexual dimorphism (Table A1.3). Some of the skin changes can be mistaken for pathology. Hermaphroditism is particularly common within Perciformes, namely parrotfish (Scaridae), wrasses (Labridae), damsels (Pomacentridae), and gobies (Gobiidae) (Sloman 2011). Hermaphroditism may be sequential or simultaneous. Sequential hermaphroditism can be divided into protandry (males become females, e.g. gilthead seabream [Sparus aurata] and clownfish [Amphiprioninae]), and protogyny (females become males, e.g. Indo‐Pacific cleaner wrasse [Labroides dimidiatus]). Simultaneous hermaphroditism is defined by capacity to release viable eggs or sperm during the same spawning, e.g. hamlets (Serranidae). Parthenogenesis is less common, but examples are found, e.g. Amazon molly (Poecilia formosa) where all individuals are females.

Typical teleost testes are elongated, paired, and either lobular (most common) or tubular (Roberts and Ellis 2012). They are supported in the coelom by a mesorchium and surrounded by a tunica albuginea. They can be hard to differentiate grossly from ovaries in young animals, but wet mounts of the tissue can be diagnostic. The ducts can serve for both sperm transport and storage (Schulz and Nobrega 2011).

Teleost ovaries vary from clusters of follicles to a more organized organ that may be paired, fused, or coiled. They are supported within a thin mesovarium. Mature ovaries can take up a substantial amount of the coelom (up to 70%). Ovaries in bony fish are typically paired, but they are fused in some species, e.g. lampreys and hagfish (Agnatha), mollies and guppies (Poecilia spp.), and medaka (Oryzias sp.). There are three types of ovary (Table A1.4). In gars (Lepisosteidae) and most teleosts, the ovary is continuous with the oviduct, while in trout and salmon (Salmonidae), the oviduct is diminished or even absent and ova enter the coelomic cavity before exiting the genital pore (Helfman et al. 2009; Roberts and Ellis 2012). In live‐bearing species, embryos develop in either the oviduct or the uterus (Turner 1947).

Fertilization occurs externally or internally (Table A1.5). Under human care, normal reproduction may be inhibited or altered which may result in health problems such as egg retention and oophoritis.

The reproductive systems of fish are enormously varied and important additional details can be found elsewhere (Stoskopf 1993; Farrell 2011; Wootton and Smith 2014).