Navanax, Navanax inermis

Navanax, Navanax inermis. Underwater photograph taken in the greater San Diego area, November 2019. Photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Navanax, Navanax inermis, Juvenile and Adult. Photographs  taken in the greater San Diego area,  February 2020. Photographs courtesy of Ben Cantrell, San Diego, California.

Phylogeny:  The Navanax, Navanax inermis (J.G. Cooper, 1863), is a member of the Aglajidae Family of Aglajid Slugs. The genus Navanax is one of sixteen genera in that family, and it contains four species. It is also as the California Aglajid and in Mexico also as Navanax.

Morphology:  The Navanax has a rectangular profile and a slightly flattened head. The parapodial flaps are wide and overlap the sides of the animal, nearly meeting along the midline of the back. Navanax may be brown, black, dark purple, or tan in color. The margins of the parapodium are orange or yellow, bordered by electric blue dots. There are  usually numerous white or yellow lines that run the length of the body, though the number of lines is variable. The internal shell is calcified and is 2.0 cm (0.75 inches) in diameter in adult slugs. Navanax are the largest Sea Slug in this Family, reaching a maximum of 25 cm (9.8 inches) in length.

Habitat and Distribution:  Navanax are found on mudflats, muddy sand in shallow bays, and on mud, rock and sand offshore. They reside from the intertidal zone to depths up to 30 m (98 feet). The Navanax is a resident of all Mexican waters of the Pacific Ocean, including the entire Gulf of California.

Diet:  Navanax are predatory carnivores, eating other mollusks such as bubble shells of the Bulla and Haminoea genera. They also eat nudibranchs, small fish, and even other smaller Navanax. Navanax track their prey by following the slime trail and then swallow the prey whole.

Predators:  Information related to the predators of Navanax has not been documented. There is information on how they chemically modify their slime trail to deter predators, but no mention of what those predators are. This gap in information seems unusual for such a common species.

Reproduction:  Navanax are simultaneous hermaphrodites. They track potential mates by following their slime trail. Mating starts with a form of courtship in which the role of male and female is decided. The role of female seems to be the preferred role, though individuals play the male role as often as they do the female role. Fertilization is internal. The eggs are laid, as a gelatinous mass, on marine vegetation. There can be up to 800,000 eggs in the mass. The eggs hatch into planktonic larva after seven to nineteen days.

Ecosystem Interactions:  Navanax is known to host the ectoparasitic copepod Anthessius navanacis.

Human Interactions: Navanax have no direct impact on human activities. From a conservation perspective they have not been formally evaluated however they are  common with a wide distribution and should be consider to be of Least Concern.

Synonyms:  Aglaja bakeri, Doridium purpureum, Navarchus inermis, Posterobranchia maculata, and Strategus inermis.