Читать книгу Parasitology - Alan Gunn - Страница 23

Although most parasitic relationships involve two different species, intra‐specific parasitism also occurs. Brood parasitism is a common example of intra‐specific parasitism among many birds (Tomás et al. 2017) and some social wasps (Oliveira et al. 2016). It involves a female laying her eggs in the nest of a conspecific (member of the same species) – this means that the costs of rearing, the young will be borne by another individual. Intra‐specific parasitism sometimes occurs during sexual reproduction when the male attaches to the female and becomes dependent upon her for the provision of nutrients. For example, in certain deep‐sea angler fish belonging to the suborder Ceratioidea, the larval fish develop in the upper 30 m of sea water and then gradually descend to deeper regions as they metamorphose into adults. The adolescent males have a very different morphology to the females: they are much smaller; they have larger eyes, and, in some species, they develop a large nasal organ that presumably helps in their search for females. Furthermore, the males cease feeding and rely upon reserves laid down in their liver during the larval period to fuel their swimming. Upon finding a suitable female, the male grasps onto her using special tooth‐like bones that develop at the tips of his jaws (his actual teeth degenerate during metamorphosis). After attaching, the male grows (although he remains much smaller than his consort), and his testes mature. His skin and blood vessels fuse with hers at the site of attachment, and he remains attached for the rest of his life and draws all his nourishment from her. Some authors suggest that the male must find a virgin female. However, although most females carry only a single male, there are records of females with three or more males attached to them. This is presumably an adaptation to life in the deep‐sea regions in which the opportunity to locate mates is limited. Nevertheless, this raises questions about how sexual selection occurs because it is unusual in nature for a female to mate with just one male for life, especially if that male is the first one to turn up. This type of relationship is not found in all ceratioid anglerfish; in some species, the males are facultative parasites rather than obligate ones as described in the above scenario, whilst in other species the males are free‐living, capable of capturing their own food, and form only temporary attachments to females. Molecular evidence suggests that the development of the parasitic males is a relatively plastic phenomenon among anglerfish and has evolved and subsequently become lost on several occasions (Pietsch 2005).