Effects of Symbiodinium Colonization on Growth and Cell Proliferation in the Giant Clam Hippopus hippopus.

Giant clams (subfamily Tridacnidae) house their obligate symbionts, Symbiodinium sp., in a specialized tubular system. Rapid uptake of Symbiodinium has been shown to increase early clam survival, suggesting that symbionts play an essential role in host growth and development. To determine whether symbionts influence development in the giant clam Hippopus hippopus, we compared growth patterns and cell proliferation in two groups of clams inoculated or not inoculated (control) with Symbiodinium sp. Symbiont uptake occurred continuously from days 8 to 26 post-fertilization, with, on average, ∼5% per day colonized. The control treatment grew even without symbionts (1.03 ± 0.41 µm per day, standard error). Inoculated individuals grew significantly faster (2.91 ± 0.37 µm per day) than control individuals (P < 0.001). However, daily shell length measurements did not significantly differ between treatments until day 22, and ∼97% of control individuals metamorphosed by day 24, suggesting a delay in growth effects. Consistent with this, at day 13, clam cell proliferation was not correlated with symbiont abundance in inoculated individuals (P = 0.13), while at day 26, it was (P < 0.01). The proliferating cell pattern also changed from being randomly distributed (P = 0.99) at day 13 to non-randomly distributed (P = 0.002), with increased likelihood of proliferation within ∼25 µm of a symbiont, at day 26. Our results indicate that H. hippopus has a longer Symbiodinium acquisition period than previously recorded, after which proliferation and development are enhanced but during which growth is unaffected by Symbiodinium.