Role of chemical substances from fish hosts in hatching and host-finding in monogeneans.

Hatching responses to chemical stimuli appear to have evolved independently in different kinds of monogenean skin and gill parasites of fishes, particularly in those parasites associated with bottom-dwelling hosts. Some monogeneans, such asEntobdella soleae, have two hatching strategies, responding readily to host skin mucus but hatching spontaneously in small numbers in the absence of the host. Other monogeneans, such asAcanthocotyle lobianchi, have abandoned spontaneous hatching and rely entirely on a "sit- and-wait" strategy, but improvements in the speed of hatching provide opportunities to take advantage of brief periods of contact between the eggs and the host. This has led to the loss of ciliated epidermal cells and to the inability to swim. Comparison of the eggs and hatching responses of two unrelated monogeneans,Leptocotyle minor andHexabothrium appendiculatum, which share the same dogfish host, reveals evidence of convergence. Small, stable molecules such as urea, excreted by the host, have been implicated as hatching stimulants in monogeneans. There is evidence that host recognition inE. soleae is by chemoperception but, in contrast with the lack of specificity of the chemical hatching stimuli, this appears to be of a specific nature.