The ecology and functional morphology of Trigonothracia jinxingae (Bivalvia: Anomalodesmata: Thracioidea) from Xiamen, China



All anomalodesmatans are ‘rare’ but Trigonothrucia jinxingae is relatively common in Xiamen Harbour, Fujian Province, China. This is because the species has a life span of approximately one year and is a simultaneous hermaphrodite, probably with either a short or absent planktonic larval stage. That is, success results from rapid maturation, self-fertilization, direct development and within-habitat recruitment over an extended period in early summer.

Trigonothracia jinxingae is interesting in another way, however. The Thraciidae is the Mesozoic stem group of the Thracioidea which also contains the more modern (Caenozoic) Laternulidae and Periplomatidae. Features of the anatomy of T. jinxingae, such as the method of hydraulically moving the foot by the pumping of blood into a capacious pallial haemocoel, and the structure of the stomach, are reminiscent of the earliest (Palaeozoic) anomalodesmat́ans, i.e. the Pholadomyoidea, represented today by Pholadomya candida. The thraciid Asthenothaerus sp. (Pelseneer, 1911) even has, like P. cundida, an opisthopodium on its visceral mass. P. candida, however, fed on sub-surface deposits using its foot. T. jinxingae is also a deposit feeder, but on surface deposits using the inhalant siphon.

Modern periplomatids resemble thraciids in their separate siphons, but both representatives of this family and the Laternulidae are suspension feeders with extensive sorting areas on the wall of the stomach to process such material. The Thraciidae thus form a link between the oldest, pedal feeding, pholadomyoidean anomalodesmatan and the most advanced, suspension feeding, laternulids and periplomatids.