As it turned out, this was only partly true. All four species showed genetic patterns resulting from a recent population expansion following the last glacial maximum, when sea levels rose ~ 120 meters. However, their patterns of genetic differentiation across the Coral Triangle were not all that concordant, with both seastars showing more genetic differentiation than the ectosymbionts. Of the two ectosymbionts, the specialist haemolymph-sucking gastropod Thyca crystallina had a spatial pattern that faintly echoed that of its host, Linckia laevigata, while the generalist shrimp Periclimenes soror showed hardly any genetic differentiation across the whole Coral Triangle!
These results taught me two things about phylogeographic patterns in the sea. One is that habitat matters. The specialist symbiont had a pattern that mirrored its host, while the generalist, which can live in any habitat that supports any species of seastar, has much less structure. These habitat requirements can affect the overall size of the population and how it reacts to environmental changes such as sea-level fluctuations. The second thing is that population size matters. Species with large population sizes can have a lot of variance in the spatial pattern of structure, just due to the random effects of genetic drift.
Crandall, E. D., Jones, M. E., Muñoz, M. M., Akinronbi, B., Erdmann, M. V., & Barber, P. H. (2008). Comparative phylogeography of two seastars and their ectosymbionts within the Coral Triangle Molecular Ecology, 17(24), 5276–5290. doi:10.1111/j.1365-294X.2008.03995.x