Marine leeches (Annelida: Hirudinida) are typically fish parasites, yet little is known about species associated with invertebrates. Cochimibdella mexicana, parasitizing the two-spot octopus Octopus bimaculatus, in Baja California, México, represents a unique ecological association. This study characterizes the reproduction, development, feeding behavior, and fasting survival of the leech. Between February and April 2025, live leeches (n = 143) were collected from the mantle and arms of wild O. bimaculatus and maintained under laboratory conditions. Mating occurred within 24 hours via hypodermic insemination. Three days post-copulation, leeches deposited circular, flattened cocoons (~1.4 mm diameter) containing a single embryo. Cocoons were divided into two experimental groups (18° C and 14º C). Embryonic development lasted 36 days at 14º C and 21 days at 18° C. Cleavage, gastrulation, and organogenesis followed patterns typical of aquatic leeches, with fully formed juveniles hatching with functional sensory structures and immediate attachment behavior. In feeding trials, both hatched juveniles and adults consistently refused to feed on invertebrate prey and fed exclusively on blood from fish hosts. Hatchlings (2.1 ± 0.5 mm) initiated hematophagy within 24–72 h and survived up to 31 days unfed. Our findings provide, for the first time, key life-history traits of C. mexicana, revealing a complex ecological interaction where an invertebrate acts as a substrate and mechanical vector for a parasite that requires a vertebrate (fish) for trophic sustenance. These observations contribute to understanding the life cycle and transmission dynamics of C. mexicana within benthic–pelagic food webs.