13 Jan 2009

The Torturous Train of Triops Taxonomy

Dear Blue Lobster:

Can you tell me if any species of tadpole/longtail shrimp live in Taiwan? Thanks!

mj klein

Dear Gentle Sir:

The triop, also known as the horseshoe shrimp, lazarus shrimp, shield shrimp, tadpole shrimp, or, in Mandarin-speaking regions, as 盾蟹 (shield crab), are sixteen species between two genera under the Linnaean taxonomical system that comprise one of the oldest and most primitive crustacean groups still extant. They have not changed much in the last 220 million years.

The distribution of species is problematic in the historical context. One species, Triops longicaudatus, is distributed through central North America, all of South America, parts of Micronesia, and East Asia (though not Taiwan). Several theories have been put forth to explain such complex distributions, but each has serious flaws.

Paterson (1977) proposed a reclassification of extant triops species, suggesting that the distribution failed to make sense because the taxonomy was wrong. Marmur (1979) argued that the taxonomy was indeed sound but that closer study of prehistoric landmass movements was in order while Webb (1980) pushed for even more radical taxonomic reclassification.

Webb later concluded that triops were actually two species with pronounced climate adaptations causing subspeciation (1982) and made vigorous attempts at hybridization, though he sadly was committed later that year and never contributed to the topic again; Mitchell (1984) posited that the triops genera were the result of convergent evolution.

Molecular studies put an answer to the question with Capaldi (1988), who showed the lineages were quite accurate as understood. Since then other have quibbled over species and subspecies classification and discussed plate tectonics and its intersection with triops distribution.

In answer to your initial question, there are no species of triops in Taiwan, which offers a tantalizing clue to crustaceologists. Did triops never live on Taiwan, meaning the island separated from Asia before they existed? Or did triops live there for a time but, in the specialized island environment, go extinct?

Until these and other questions are answered, the multi-million year old mystery remains, tantalizing scientist and hobbyist alike. If you're desperate to see triops in Taiwan, however, try visiting some university labs, where imported specimina thrive happily. Just don't release any in the wild!