California Aglaja (Navanax inermis)

California Aglaja (Navanax inermis)

California Aglaja (Navanax inermis)

The California Aglaja (Navanax inermis) is a large predatory sea slug known to tropical fish keeping enthusiasts in Mexico as Navanax, is found in the eastern Pacific Ocean from from Tomales Bay, California, in the north; to Laguna Manuela and to Jalisco in Mexico, in the south; including the Gulf of California.

The California Aglaja resembles a large nudibranch but is actually a sea slug that belongs to a more ancient line of opisthobranchs called the cephalaspideans, or headshield slugs and snails.

Navanax inermis are typically found on sandy, muddy, and rocky substrates associated with the rocky, intertidal regions and sub tidal mudflats of their range where they feed on a varietry of sea slugs and bubble snails of the order Cephalaspidea, and nudibranchs such as Hermissenda crassicornis, Polycera atra, and Dirona picta.

They are usually collected from tide pools, rocky beaches, mud flats, the muddy sand in shallow bays, and from the mud, rock, and sand offshore from the intertidal zone to depths of 98 feet or more.

The California Aglaja does not possess a radula or any organs associated with vision.  Despite being blind, they find their prey and locate potential mates by tracking the slime trails they leave behind using their chemoreceptors. When alarmed, they use chemical pheromones that can be sensed by other members of its species to communicate danger.

California Aglaja (Navanax inermis)

California Aglaja (Navanax inermis)

The California Aglaja has a rectangular profile with a slightly flattened head and varies in length from 2.5″ to 8.7″.   They have two large protruding lateral flaps on either side that run the length of the body and almost meet at the middle of the back, and a small internal calcified shell, somewhat like that of squid. Navanax have a dark body color that varies from black, brown, dark purple, to tan.  The margins of the parapodium are orange or yellow, edged with small electric blue dots.   Numerous white or yellow lines run the length of the body.

Because of their size and specialized feeding habits, the California Aglaja has a relatively short life expectancy in an aquarium environment. Although they can be kept as a single specimens in an established reef tank of at least 100 gallon capacity with a crushed coral or sandy substrate, some mature live rock, and various shelled species of Bulla and Haminoea for them to prey on; they are not considered suitable for aquarium keeping.

Like all marine invertebrates, Navanax inermis are sensitive to rapid changes in water quality and need stable water conditions. A good filtration system, protein skimmer, and powerhead for some moderate water movement will keep them healthy and happy.

Like all other Opisthobranchs, California Aglaja are simultaneous hermaphrodites; that possess both female and male reproductive organs. From January into the summer months, gelatinous egg masses can be found on the eelgrass of their range. Copulation frequently occurs in small groups, or “chains” of up to four individuals that can produce upwards of 800,000 eggs in a single spawn.  The eggs hatch out into embryos after 7 to 20 days and live as plankton in the upper water column.  In the southern portion of its range, Navanax inermis spawn year round.

The California Aglaja is a highly predatory sea slug that feeds on bubble snails, mudibranchs, small fish, and a variety of other sea slugs. In an aquarium environment, they require the same diet, which makes keeping them almost impossible to maintain.

The California Aglaja (Navanax inermis) are not normally available to tropical fish keeping enthusiasts.   Aquarium specimens are occasionally collected and kept for short periods.   Their feeding requirements make them unsuitable for most aquarists.

California Aglaja (Navanax inermis)

California Aglaja (Navanax inermis)








Minimum Tank Size: 100 gallons
Aquarium Type: Established Reef
Care Level: Extensive
Temperament: Aggressive
Aquarium Hardiness: Relatively hardy
Water Conditions: 72-78°F, dKH 8 to 12, pH 8.1-8.4, sg 1.023-1.025
Max. Size:  8.7″
Color Form: Yellow, White, Brown, Black, Purple
Diet: Specialty Carnivore
Compatibility: Reef Compatible
Origin: eastern Pacific Ocean
Family: Aglajidae
Lifespan: Unkonwn
Aquarist Experience Level: Expert Only


One Response to “California Aglaja (Navanax inermis)”


  1. […] toxic nature of their flesh repels most predators except lobsters, starfish, and the ophistobranch Navanax inermis which consumes juvenile Sea […]

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