Tropical Mole Crab

Emerita rathbunae exhibits an extreme form of sexual dimorphism, with tiny neotenous males attaching themselves to the female's appendages, "thus carrying the tendency for small males in this genus almost to the verge of parasitism". Sexually mature females are typically 33–44 mm (1.3–1.7 in) in carapace length, while males are only 2.5–3.0 mm (0.10–0.12 in).

Emerita rathbunae is found on the shores of the eastern Pacific Ocean, from the southern end of the Gulf of California in Mexico to Iquique in northern Chile, including the Galápagos Islands. In the south of its range, E. rathbunae co-occurs with the southern population of E. analoga, which occurs as far north as mainland Ecuador.

Despite their sympatry, the closest relative of E. rathbunae is not E. analoga; rather, E. rathbunae is part of a clade that also includes species from the western Atlantic Ocean, including E. portoricensis and E. benedictii. The last common ancestor of the genus is thought to have lived in the Pacific Ocean, and to have colonised the Gulf of Mexico when the Isthmus of Panama was submerged, and E. rathbunae is thought to have similarly recolonised the Pacific from ancestors that lived on the Atlantic side of the Isthmus of Panama.

Emerita rathbunae lives in the intertidal zone, where it uses the swash for transportation and filter-feeding. In Ecuador, it is one of the most abundant animals to surf the swash, together with the sea snail Olivella semistriata. The two avoid competition since E. rathbunae prefers steeper beaches with coarser sediments and rougher swash than O. semistriata, and positions itself lower in the swash than O. semistriata. Based on comparisons with other Emerita species, E. rathbunae is thought to be a very capable swimmer, and to burrow rapidly into the sand.