Feeding of Boeckella gracilipes (Copepoda, Calanoida) on ciliates and phytoflagellates in an ultraoligotrophic Andean lake

The calanoid copepod, Boeckella gracilipes, is the dominant crustacean zooplankton in South Andean deep ultra-oligotrophic lakes. Combining field and experimental data we explored the feeding of the copepod and its access to the mixotrophic ciliate, Ophrydium naumanni, in Lake Moreno Oeste (Patagonia, Argentina). Phytoplankton was dominated by nanoflagellates throughout the water column. Ophrydium naumanni, which accumulates much of the chlorophyll a, as do copepodites and adults of B. gracilipes, has a deep distribution during the day, with maximal abundances around 30 m depth. Mouth-part morphology analysis of B. gracilipes indicated that the copepod has an omnivorous diet. Laboratory experiments showed that B. gracilipes could access O. naumanni only when it is offered as a single food item. However, when natural phytoplankton and ciliate assemblages (including O.naumanni) are offered, B. gracilipes did not eat Ophrydium and preyed on the oligotrich, Strombidium viride, and phytoflagellates like Chrysochromulina parva. The range of ingested sizes was broad (3.9–33 μm of equivalent spherical diameter) but all selected particles were motile ones with distinctive movements, which would enhance the copepod particle detection.