larval period

the time from the beginning of exogenous nutrition (feeding externally) until the formation or ossification of the axial skeleton and until the undifferentiated median fin fold is differentiated or no longer apparent. Some embryonic organs persist and special larval organs develop which are later replaced by different organs of the same function or disappear when their function is no longer needed. This period is absent in some fishes, e.g. Salmonidae and extremely long in others, e.g. 1-3 years in Anguilliformes. This period has two phases:- protopterygial from the transition to exogenous feeding and the beginning of differentiation of the embryonic median fin fold (appearance of lepidotrichia buds and dorsal and anal fin outlines), and pterygiolarval from the beginning of the differentiation of the median fin fold until it is entirely differentiated or no longer apparent. An alevin phase is found in species that have no larval period and lasts until complete scalation or ossified spines, e.g. in Salmonidae, equivalent to the elver phase in Anguillidae, the metamorphic phase in Elopidae, the tholichthys phase in Chaetodontidae, the acronurus phase in Acanthuridae and the exterilium phase in Brotulidae