The recent El Niño phenomenon warmed the waters of Monterey Bay to the point where a substantial population of the predatory cephalospidean Navanax inermis was recruited, which was feeding voraciously on the pretty little aeolid nudibranch Hermissenda crassicornis. While in the bay region, we were pretty impressed with the feeding behavior of Navanax and made this video. The actual strike, where now-you-see-Hermissenda-now-you-don’t, takes less than 200 msec (4 video frames). Others have shown that this happens through a rapid muscular expansion of the slug’s pharynx, causing a vacuum cleaner effect.
Pleurobranchaea one trial learning
The receding El Niño also left a population of the spectacular aeolid nudibranch Flabellina iodinea, or the Spanish Shawl, in Monterey Bay eating the hydroid Eudendrium. By accident, we found thatFlabellina was one of the few animals that Pleurobranchaea will attack, then spit out. R.-C. Huang located the noxiousness ofFlabellina to its bright orange cerata. Aeolids are famous for their habits of eating corals, anemones and hydroids and selectively preserving their stinging cells (nematocysts) for use in their own defense, mysteriously transporting and embedding the nematocysts in the cerata.
As shown in the video, we found that a single trial exposure of Pleurobranchaea to Flabellina can result in prey-avoidance learning in just a single trial: shown are two encounters separated by 1/2 hour. An attack by Pleurobranchaea is followed by rejection of the Spanish Shawl, which goes into escape swimming. The frustrated predator shows repeated cycles of rejection movements with its radula and jaws, then performs a stereotypic avoidance turn and locomotes the hell oudda there. In the next shot, 30 min. later, Pleurobranchaea‘s avoidance turn occurs on contact with the mucus of the nearby Flabellina. The prey-avoidance learning seems pretty selective: while experienced Pleurobranchaea avoid Flabellina, they continue to eat the related aeolid Hermissenda like popcorn and their feeding thresholds to stimulants like trimethylglycine are unchanged. The aeolidCoryphella sp. acts like Flabellina in training Pleurobranchaea in prey avoidance. Occasionally Coryphella comes up in the same trawls with Pleurobranchaea, indicating that they are sharing the neighborhood in the abyss, the Pleurobranchaea avoidCoryphella without further laboratory exposure.
Pleurobranchaea feeding # 1
Feeding behavior in Pleurobranchaea consists of initial searching displayed by locomotion, “sniffing” orienting movements of the proboscis and extension of the oral veil. Once food is localized, the mouth opens to expose teeth which bring the food to the mouth. Proboscis protracts and retracts repeatedly to facilitate the motion of the food to the mouth and the gut. On the initial phase of proboscis retraction swallowing occurs.
In this clip Pleurobranchaea performs the initial searching behavior as it moves down the tank wall. It then orients on a piece of fresh shrimp on the base of the tank and proceeds to consume it.
Pleurobranchaea feeding #2
Feeding behavior in Pleurobranchaea is as above but without the prominent searching behavior.
Pleurobranchaea‘s locomotion relies heavily on mucus secretion which facilitates a ‘pedaling’ behavior of cilia on the foot.
Sexula behavior of Pleurobranchaea consists of 4 phases:
1. mate detection
3. body orientation (side by side facing in opposite directions)
4. engagement of male and female sexual organs (the animals are hermaphroditic)
During the last 3 phases, the animals assume a specific and highly characteristic sexual posture, consisting of:
– upward curling of the right edge of the mantle to expose gill and sexual organs
– unfurling and rearward extension of the gills
– partial to complete extension of the sexual organs
See Davis and Mpitsos, J. A. vergl. Physiologie 75, 207-232, 1971.
There are at least two types of turning behaviors observed for Pleurobranchaea. The orienting turn is usually slower and consists of twisting the foot by contracting the bodywall muscles. Avoidance turning is more vigorous; higher turning speed is achieved by raising the front portion of the foot and swinging around “on the heel”.