Q12 - Phylogeny and Biogeography of Horseshoe Crabs

Theoretical B Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Horseshoe crabs are marine and only four extant species are known, while many have gone extinct. Tachypleus gigas (Tg), T. tridentatus (Tt) and Carcinoscorpius rotundicauda (Cr) are from southeast Asia, whereas Limulus polyphemus (Lp) lives on the east coast of N America. Tg and Cr overlap in their geographic range (from Andaman Sea (close to Thailand and Malaysia) to the South China Sea). Tt lives from Vietnam to Japan. Horseshoe crabs are “living fossils”.

Phylogeny of extant horseshoe crabs. The unit of scale is a million years. Black bars indicate 95% confidence interval. The two populations of Cr are from Andaman Sea and Thailand (from Obst et al. 2012).

Figure 1. Figure 1.

Figure 2. Figure 2.

Using the information and data, determine which of the statements are true or which are false.

A. From the Fig., we can conclude that horseshoe crabs must be a slowly evolving group

B. According to Fig., the Asian species constitute a monophyletic clade

C. Speciation in horseshoe crabs seems to take between 5 and 45 million years

D. The genera Tachypleus and Limulus are sister taxa

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Question reproduced from IBO 2015, Theoretical Exam B, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)