Elthusa californica

The body of the Elthusa californica is oblong/oval shaped, typical of the vaulted body shape expected from the Elthusa genus.[6][7] On average, the body length is around 16 mm long while the body width is around 7 mm wide. In general, the body length is slightly more than twice its body width. Due to the slightly shorter body length on the right side of the E. californica, the body is slightly twisted.[7]

a) Adult female E. californica. b) Lateral view of thoran[7]

The head of the E. californica is approximately 2 mm wide.[7] It is triangular and has a strong and blunt outline on the anterior end.[4] The cephalon posterior margin is not trilobed, once again typical characteristic of organisms found under the Elthusa genus.[6] Two large, oval eyes sit on the post-lateral angles on the head. The eyes are approximately apart by the width of one-eye, making them rather close-set.[7][4]

On the head, two pairs of antennae may be found. Each pair of antennae is made up of eight or nine articles. The antennule is the shorter pair compared to the longer antenna and is shorter than all the pleopod lamellae.[7][6] The antennule is slender and weak and extends to the seventh article on the other larger pair.[7]

E. californica has maxillipeds composed of two articles while the mandible has palps made up by three articles. The thorax is that of a typical Elthusa genus organism. It is made up of eight segments where the first segment with the head embedded into it, is the largest to an approximate size of 2mm in length. Both second and third segments are equal in length of approximately 1.5 mm. The fourth, fifth, and sixth segments are all 1 mm in length. The seventh segment is then the shortest one with a length of 0.5 mm. Narrow plates called epimera are separated along the last segments on the thorax. In the last segments, the epimera is capable of fully extend across the segment. The terminal thorax is at the posterior and it is about 4mm wide and 3.5 mm long.[7]

a) Maxilliped. b) Second maxilla. c) First maxilla. d) Palp of mandible.[7]

E. californica has legs that are capable of grasping objects. The legs also have long and slender dactyli.[7]

In adult females, the pleotelson is as wide as its length, which fits the typical description of the Elthusa genus.[4]

In juveniles, the color pigment granules are concentrated in the melanophores, thus, giving its lack of distinct pattern appearance.[4]

Elthusa californica may be typically found on the shores of California, near San Francisco, hence its species name. They are also distributed along the Pacific coast from Canada to Peru.[4][8]

E. californica is an ectoparasite and has been found on many reported host species. These include the actinopterygian fishes Cymatogaster aggregate and Atherinops affinis, Artedius lateralis, Clinocottus analis, Oligocottus maculosus, S. marmoratus, and Fundulus parvipinnis.[8][9] Once they find a suitable host, they attach inside the opercular cavity of the gill chamber, making a protruding shape.[8][9]

Seventh leg of the E. californica [7]

Inside the gill chamber, E. californica feeds on the blood and tissue of the host fish. To ensure secure attachment to the host, the isopod uses seven pairs of sharply hooked legs and its specialized mouthpart.[8][9] Due to the loss of blood for the host fish, the parasitic behavior of E. californica is hypothesized to influence the host growth process.[9] Studies have shown that the presence of E. californica in host fish may subject them to lower levels of vitality when encountering a decrease in the seawater salinity. However, the general understanding is that E. californica is relatively harmless for the host fish as its activity is not fatal.[9]

They have an ecological preference as they like to select hosts that live in the benthic or that belong to a demersal school. Due to its smaller body size and less complex body structure compared to endoparasites, they are regarded as good indicators for environmental change such as those caused by pollution or climate.[8]

Lastly, studies have indicated that E. californica is capable of cross-fertilizing and hybridizing with other species of the same genus such as the Elthusa vulgaris.[8]