Feeding rates and prey : predator size ratios of the nauplii and adult females of the marine cyclopoid copepod Oithona davisae

We studied the feeding behavior of the nauplii and adult females of the marine cyclopoid copepod Oithona davisae in the laboratory. Functional response experiments showed that O. davisae can feed on a broad size range of prey but that high clearance rates only occur in a narrow prey size range. Neither the nauplii nor the females were able to feed on Nannochloropsis oculata (2.5 mm), but . 4 mm prey were ingested. The highest clearance rates occurred when the nauplii and females were offered the dinoflagellate Oxyrrhis marina and the ciliate Strombidium sulcatum, respectively. O. davisae females preyed on Acartia grani nauplii but not on nauplii of their own species. Optimal prey : predator size ratios were similar for the nauplii and the females (, 10% of the predator’s body length) and were higher than those reported for suspension-feeding calanoid copepods (2–6%). The finding that the nauplii and females of O. davisae feed on relatively larger prey appears to be a consequence of their strict ambush-feeding behavior, which constrains feeding activity to prey large enough to create hydromechanical signals above the detection threshold. Very high weight-specific ingestion rates (. 150% d21) were obtained when O. davisae fed on relatively large prey. Such high daily rations are much higher than those that can be calculated indirectly from egg production. Size measurements of the mouth of O. davisae females indicate that those prey resulting in extreme feeding rates were too large to be swallowed completely and suggest the presence of sloppy feeding in Oithona. Despite the numerical dominance of the genus of marine copepods Oithona in most of the oceans (Gallienne and Robins 2001) and the expected important role of Oithona in the pelagic system (Paffenhöfer 1993; Nielsen and Sabatini 1996), many basic aspects of the biology and ecology of the genus are still poorly known. This lack of information limits our capacity to understand the function of Oithona in marine food webs. We know that their strict ambush-feeding behavior (Svensen and Kiørboe 2000; Paffenhöfer and Mazzocchi 2002), which reduces the encounter rate with potential prey, is associated with feeding and metabolic rates comparatively lower than those of calanoid copepods (Castellani et al. 2005b; Saiz and Calbet 2007). Moreover, the few available data obtained in the laboratory under controlled conditions also suggest that the adult females of Oithona may satiate at low food concentrations (50–100 mg C L21; Lampitt and Gamble 1982; Saiz et al. 2003; Zamora-Terol and Saiz 2013). Knowledge of the feeding behavior of Oithona is still deficient in comparison with that of calanoid copepods, and this limited information has resulted in contradictory observations in the literature. In most field studies on the feeding of this genus, ciliates and dinoflagellates have emerged as the main components of its natural diet (Atkinson 1995; Nakamura and Turner 1997; Broglio et al. 2004), whereas diatoms and nanoflagellates may occasionally appear in the diet, often as minor contributors (Castellani et al. 2005a, 2008; Atienza et al. 2006). However, Calbet et al. (2000), in waters off Kaneohe Bay (Hawaii), reported high feeding rates of Oithona nana andOithona simplex (up to, 55% body C d21) on 2–5 mm flagellates. The pioneering work of Lampitt and Gamble (1982) on O. nana in the laboratory also reported ingestion of nanoflagellates and diatoms, whereas Uchima and Hirano (1986) reported only ingestion of motile prey by Oithona davisae and the absence of feeding on diatoms. We think that some of the abovementioned disagreement might be, in part, a consequence of likely presence of trophic cascade effects in the feeding experiments with natural prey assemblages, which may mask the actual diet of Oithona (Nejstgaard et al. 2001; Saiz and Calbet 2011). Such effects would be particularly important in this genus, given that its highest clearance rates have been found for the top predators of the microbial communities (i.e., ciliates and heterotrophic dinoflagellates). However, the few laboratory feeding experiments on the genus reported in the literature are difficult to compare, as they focused on different conspecifics and a small variety of prey types. This lack of knowledge on the genus is even more pronounced if the juvenile stages (nauplii) are considered, as most of the studies on copepod vital rates typically focus on the adult or late copepodite stages. The few available studies onOithona nauplii assessing their feeding rates and functional response to food availability in the laboratory (Henriksen et al. 2007; Almeda et al. 2010a) have only considered two dinoflagellates and a diatom as potential prey. The aim of the current study is, therefore, to determine the feeding functional response of the nauplii and adult females of the ambush cyclopoid copepod O. davisae. We have assessed the feeding rates of the species and its prey size spectrum using a variety of prey (algae, ciliates, and copepod nauplii). To our knowledge, a similar approach has only been pursued once in marine copepods, in the study of Berggreen et al. (1988), where ontogenetic changes in the feeding rates of the calanoid copepod Acartia tonsa were studied at saturating concentrations under a variety of *Corresponding author: [email protected] Limnol. Oceanogr., 59(6), 2014, 2077–2088 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.6.2077