In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each other, further blurring any distinctions. Terms that are sometimes used synonymously but have more precise meanings are cryptic species for two or more species hidden under one species name, sibling species for two species that are each other's closest relative, and species flock for a group of closely related species that live in the same habitat. As informal taxonomic ranks, species group, species aggregate, macrospecies, and superspecies are also in use.
At least six treefrog species make up the Hypsiboas calcaratus–fasciatus species complex.
The fly agaric comprises several cryptic species, as is shown by genetic data.
The forest elephant (shown) is the bush elephant's sibling species.
Mbuna cichlids form a species flock in Lake Malawi.
Morphology is a branch of biology dealing with the study of the form and structure of organisms and their specific structural features.
Morphology of a male skeleton shrimp, Caprella mutica