[2] It is a small, Clupeoid fish with a large mouth and a long, laterally compressed body, which strongly resembles the European Anchovy (Engraulis encrasicolus) with only slight differences in girth and fin position.

Californian anchovies exhibit the basic anchovy body form like a large sub-terminal mouth that extends past the eye, a centrally located single dorsal fin on a compressed & elongated body, a forked caudal fin, the absence of a visible lateral line, and weak, shiny scales colored by crystalline guanine that complete its countershaded camouflage.

[11][4][3] Trawling surveys in San Francisco Bay starting from the 1920s by famous ichthyologist Carl Leavitt Hubbs up to the present by scientists from UC Davis seem to support the existence of a separate "bay" form of the Californian anchovy that has fewer vertebrae than the oceanic "green-back" form, a proportionally larger head, a tan to transparent dorsum due to the near absence of guanine crystals in their chromatophores, and a residential lifestyle with no migration to coastal waters.

Although these subpopulations somewhat differ in certain characteristics like operculum width, body size, transferrin pattern types, and spawning times, they are not genetically distinct enough to warrant reclassification into separate subspecies.

The larvae live in the photic zone of the waters near their birth as ichthyoplankton, where they float around at the mercy of tidal and oceanic currents and are thus regularly consumed by planktivores like pacific herring and other anchovies until the survivors mature into schooling sub-adults and begin moving into more saline waters, which physiologically corresponds with the increased guanine crystals (and therefore bluer & greener dorsum coloration) in their chromatophores; eventually making it into the open ocean where they become adults.

As climate change accelerates, the coastal upwelling that pelagic species in East Pacific waters rely on may weaken due to increasingly invariable winds, which can be detrimental to the spawning and rearing of forage fish.

If left unchecked, anchovy fisheries may experience a similar collapse to the sardines during the mid 20th century, which will put the countless species that rely on their continued abundance at serious risk.