Invertebrates
Clam Shrimp
Spinicaudata · also called clam shrimps
A clam shrimp looks exactly like a tiny clam, its entire soft body tucked inside a hinged, two-part shell, but crack open the resemblance and you find a crustacean instead, only distantly related to any actual mollusk. Many species have adapted to a genuinely demanding lifestyle, racing from egg to reproductive adult within the days or weeks a temporary desert pool holds water before it dries up completely.
Overview
Clam shrimp, order Spinicaudata, are small filter-feeding crustaceans found in temporary and permanent freshwater pools worldwide. A hinged, bivalve-like shell encloses the entire body, and leg-like appendages inside sweep algae and organic particles from the water.
Built to outlast the drought
Desiccation-resistant eggs can survive being completely dried out for months or years, hatching only once rain refills a suitable pool — a critical adaptation for the many species specialised to breed in short-lived desert puddles. Some clam shrimp lineages have changed remarkably little in tens of millions of years.
Status
Least Concern — common and resilient across both temporary and permanent freshwater habitats worldwide, thanks largely to its remarkably tough, long-lasting eggs.
Clam Shrimp: common questions
Is a clam shrimp actually related to a clam?
No, despite the strong visual resemblance created by its hinged, two-part shell, a clam shrimp is a crustacean, more closely related to shrimp, crabs, and other members of that group, while a true clam is a mollusk belonging to an entirely different and much more distantly related branch of the animal kingdom, making the similarity between the two a case of superficial appearance rather than any close biological relationship.
How can a clam shrimp complete its whole life cycle in such a short time?
Many clam shrimp species are specifically adapted to breed in temporary desert pools that may only hold water for days to a few weeks after rainfall, so they have evolved to hatch, grow, and reach reproductive maturity extremely quickly under favourable conditions, ensuring the next generation of desiccation-resistant eggs is produced and left behind in the soil before the pool dries up completely.
How do clam shrimp eggs survive without water for so long?
Clam shrimp eggs have a tough, specialised outer coating and can enter a dormant state that allows them to withstand complete drying out, extreme temperatures, and other harsh conditions for months or even years buried in dry soil, only resuming development and hatching once sufficient rainfall refills a suitable temporary pool, an adaptation shared with several other temporary pool specialists such as fairy shrimp and triops.
Do clam shrimp look the same as they did millions of years ago?
Certain clam shrimp lineages show a strikingly similar overall body plan to fossil relatives dating back tens of millions of years, suggesting relatively little evolutionary change over that extremely long period, which has made some clam shrimp species genuinely useful subjects for researchers studying long-term evolutionary stability and the fossil record of freshwater crustaceans.