Predators & defenses
 

Predators of larval oysters include filter-feeding invertebrates and fishes and, as described in the first Research Study below, even some unexpected polychaete larvae.  Predators on adults include snails, sea stars, and crabs.  The strong, sharp edges of the shell plates are effective defenses as is the firm attachment to rocks and other solid substrata, and their exposed intertidal habit of life.

 
Research study 1
 

Although researchers have long thought that polychaete larvae will prey on bivalve veligers in the plankton, experimental studies have been lacking. Laboratory tests of predation by larvae of 5 species of polychaetes (in 4 families) on D-hinge larvae of oysters Crassostrea gigas show that both polynoid (e.g., Arctonoe vittata) and spionid (species unidentified) larvae are capable of ingesting the prey whole.  Some veligers are digested and their shell valves voided in the feces, while others pass through the worms’ guts more or less intact (but do not survive). In cases of polynoids ingesting photograph of a D-hinge oyster larva with a predatory worm Arctonoe vittata within its gut courtesy Johnson & Brink 1998 Biol Bull 194: 297oyster larvae, the gut contents of shelled prey may be voided through large posterior ruptures (which subsequently heal).  Interestingly, predation occurs only if prey and predators are placed together in clear seawater with unnaturally high densities of the oyster-larva prey.  If placed in seawater with ambient background plankton concentrations, or with natural densities of the oyster larvae, no predation occurs.  The authors suggest that the natural plankton may offer a substitute food, or perhaps it obscures the prey from detection. What then explains the numerous reports in the literature that natural predation occurs?  The authors conjecture that it could be an example of “cod-end predation” in the plankton net, where predator and prey are mechanically compressed into a small space and gobbling of one by another is made easy. The authors conclude from their study that a natural relationship may not, in fact, exist.  Johnson & Brink 1998 Biol Bull 194: 297.

NOTE  in the laboratory tests, magelonid and phyllodocid larvae do not eat oyster larvae.  Most polychaete larvae used in the study are collected from plankton tows at Coos Bay, Oregon, while polynoid (Arctonoe) larvae are cultured from specimens collected in San Juan Island, Washington.  The oyster larvae come from an Oregon oyster farm

NOTE  naturally occurring plankton assemblages found in whole, unfiltered seawater at ambient concentrations

 
Research study 2
 

photograph of crab Cancer oregonensis Johnson & Brink 1998 Biol Bull 194: 297Young oysters Crassostrea gigas, especially those in trays being grown by mariculturists, are susceptible to being eaten by crabs Cancer oregonensis.  The crabs apparently enter the trays as megalops larvae and can grow to 3cm carapace width in 1yr.  They have powerful claws with strong molar teeth and sharp tips, and can easily crush oysters half again larger than themselves. The authors report no difference between male and female crabs with respect to oyster-crushing ability (see graph). Behrens Yamada et al. 1993 J Shellf Res 12: 89. Photo courtesy Ron Long, SFU, Burnaby.

 
Research study 3
 

graph showing mortality of oysters in Willapa Bay, WashingtonRecovery of natural beds of Olympia oysters2 Ostrea conchaphila in Willapa Bay, Washington may be limited by predatory activities of 2 species of introduced oyster drills.  The predators, the Japanese drill Ocinebrina inornata and the eastern drill Urosalpinx cinerea, were likely introduced to the Bay in commercial oyster shipments prior to 1965 and the early 1900s, respectively. Investigations by researchers at the University of Washington on the severity of the depredations indicate that both predators prefer Pacific oysters Crassostrea gigas over Olympia oysters of similar size, and both prefer smaller sizes of oysters.  Use of field enclosures shows that each predator kills twice as many Pacific oysters than Olympia oysters in a given period.  Interestingly, when densities of  Pacific oysters are increased in mixed prey-species assemblages, then predation on both types of oysters actually goes down2 (see graph).  Conversely, when densities of photograph of oyster drill Ocinebrina inornata courtesy Buhle & RuesinkOlympia oysters are increased, there are only weak, probably non-significant responses, by both species (data not shown).  This positive effect on survival of Olympia oysters in such circumstances is offset by another finding, that of asymmetric competition between the oyster species.  Thus, Pacific oysters reduce photograph of Olympia oysters courtesy Buhle & Ruesink, University of Washingtongrowth and survival of Olympia oysters, but not vice versa.  The authors note that by experiment end about half of all drills in the enclosures are killed by crabs, likely juvenle Cancer3.  Despite the relatively large impacts of the predators in the enclosures, Olympia oysters translocated to intertidal sites throughout the Bay appear to be little affected by oyster drills, and mortality owes mainly to other sources.  Thus, predation by drills is likely only one of several factors that has limited population recovery by O. luridaBuhle & Ruesink 2009 J Shellf Res 28: 87.

NOTE1   at present the dominant oyster species in the Bay is the Pacific oyster Crassostrea gigas

NOTE2  this type of indirect facilitation of one species by another is known as a Type II functional response

NOTE3  the authors have misidentified the crab species, so it is not clear whether the predator is C. magister or C. productus


Japanese oyster drill Ocinebrina inornata
(above 2X) and Olympia oyster (below)

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