Marine Science Faculty Publications

Observations on a Trimorphic Life Cycle in Amphistegina Gibbosa Populations from the Florida Keys

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Trimorphic life cycles have been widely postulated for larger foraminifera but have only been documented for Heterostegina. Prior to 1991, extensive laboratory and field studies of Amphistegina spp. revealed evidence only for a classical dimorphic life cycle. In summer 1991, A. gibbosa populations in the Florida Keys were stricken by disease. Specimens collected from stressed populations and maintained in laboratory cultures produced lineages of up to four successive asexual generations. Gametogenesis was observed in some F3 individuals, demonstrating that trimorphic life cycles occur in Amphistegina.

To determine if biological trimorphism is manifested as physical trimorphism in protoconch diameters or test size, specimens from field collections and from reproductions in laboratory cultures were sectioned and their protoconchs measured and evaluated. Of 143 individuals randomly-selected from field samples, only five exhibited typical microspheric embryon morphologies; the rest appeared to be megalospheric. Of 32 field-collected specimens that reproduced by multiple fission in culture, three appeared to be microspheric. Of 17 specimens that underwent gametogenesis, all exhibited typical megalospheric embryon morphologies. Of the field-collected specimens suspected to be microspheric, mean protoconch diameter was 19.0 μm (SD 3.4; N 8); mean protoconch diameters of specimens with megalospheric morphologies was 39.1 μm (SD 9.4; N 184). Of the field-collected specimens that reproduced in culture, gamonts were generally larger (TD 1.23 mm, SD 0.19, N 17) than those that reproduced by multiple fission (TD 0.92 mm, SD 0.21, N 32).

This study indicates that the life cycle of Amphistegina gibbosa is trimorphic and may include both alternation of generations and successive asexual generations. The latter mode may provide an effective mechanism to rapidly increase population densities. Such a strategy may facilitate population recovery following mortality events, as well as occupation of marginal habitats and colonization of new areas.

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Journal of Foraminiferal Research, v. 28, issue 2, p. 141-147