When they must travel, South African leeches (Marsupiobdella africana) prefer to catch a crab. Unlike many leech species, which attach their eggs to hosts, like frogs or fish, to safeguard them from predators, M. africana has a one-of-a-kind internal pouch where offspring are raised for weeks. Yet these kangaroo leeches still latch on to crabs as adults, though they don’t feed on them. That means this phoretic relationship—where one animal uses another merely for transportation—must offer other benefits to the worm. While surveying Cape river crabs near Potchefstroom, South Africa, researchers noticed this ferrying behavior varies by season and by the sex of the crab . Kangaroo leeches began blanketing male crabs just before breeding season. More males were caught by bait traps, suggesting that males are foraging more during this period, as they move into the best positions for courtship. Female crabs witnessed a similar, but smaller, postnuptial bump in the number of leech riders, potentially when extra foraging is needed to satisfy pregnancy demands. Individual crabs averaged only a few dozen leeches during the rest of the year, according to a study published in the International Journal for Parasitology: Parasites and Wildlife. By hopping on crabs when they’re covering the most ground and clinging to inaccessible spots, which are harder to groom, kangaroo leeches may use the clawed crawlers as safe harbor from predators, while expanding their freshwater territory.

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