Active prey rejection in the filter‐feeding appendicularian Oikopleura dioica

Abstract

We describe different modes of prey rejection in a filter‐feeding appendicularian, Oikopleura dioica. Nonselective prey rejection occurs by intermittent rejection through reversal of the feeding current of all particles when the pharyngeal filter is overloaded, and by accidental loss through the spiracles of all particles that have entered the house when the pharyngeal filter occasionally breaks. In addition, selective prey rejection of individual particles may occur in the mouth by reversal of the ciliary current in the spiracle: the unwanted particle is expelled with a small amount of water. Up to four rejections of individual particles may occur per second. This active rejection is based on both the size and the chemical characteristics of the individual particle. A significantly higher rejection rate was found for toxic dinoflagelate (68%) compared to similar sized nontoxic ones (34%); exponentially growing algae were less rejected than senescent ones; and nutrient‐depleted algae were more rejected than nutrient‐replete cells. For nontoxic particles, prey size relative to the size of the appendicularian was the main prey selection criterion and the optimal prey size (> 80% acceptance) for O. dioica is between 0.4% and 2% of their own size, whereas the minimum and maximum particle sizes that they can ingest range from 0.04% to 20% of their own size. This prey size spectrum is much broader than that found in most other planktivorous organisms and demonstrates that appendicularians are simultaneous microphageous and macrophageous planktonic grazers.

The first definitive record of the giant larvacean, Bathochordaeus charon, since its original description in 1900 and a range extension to the northeast Pacific Ocean

“…Because the entry to the spiracles of B. charon is close to an order of magnitude smaller than the diameter of their mouth (Fig. 3), evacuating undesirable objects through their spiracles as do other oikopleurids (Alldredge 1977;Lombard et al 2011), is not an option. If a particle enters the mouth, the only way out seems to be through the gut to the rectum.…”

Morphology, ecology, and molecular biology of a new species of giant larvacean in the eastern North Pacific: Bathochordaeus mcnutti sp. nov.

“…Exclusively marine organisms, larvaceans are holoplanktonic pelagic tunicates. As a group, they are important members of the planktonic community, and are capable of having a grazing impact that exceeds that of copepods (Alldredge 1981; Deibel 1988; Fenaux et al 1998; Gorsky and Fenaux 1998; Lombard et al 2011). A few species have been well studied, particularly those that are easily kept alive in the laboratory, like Oikopleura dioica .…”

“…Because they can transform small, suspended particles into pellets or aggregates that sink quickly, organic carbon processed by larvaceans can shortcut the microbial loop (Deibel 1988; Fenaux et al 1998). Lombard et al (2011) showed that the size spectrum of prey for the oikopleurid Oikopleura dioica ranges from 0.4 to 20% of their body size—a broader range than most planktivorous organisms—as O. dioica feeds on microphagous as well as macrophagous organisms. Resolving the identities of these animals is thus of ecological as well as taxonomic significance.…”

Effect of Type and Concentration of Ballasting Particles on Sinking Rate of Marine Snow Produced by the Appendicularian Oikopleura dioica

“…Appendicularians are planktonic tunicates that use external mucous devices, called houses, to filter, concentrate, and feed on particles ranging in size from 0.2 µm to several hundreds of microns [21,22]. These houses can clog, thus forcing the appendicularian to discard the house and secrete a new one several times per day (up to 27 d -1 [23]).…”