The role of kelp crabs as consumers in bull kelp forests-evidence from laboratory feeding trials and field enclosures

Abstract

The Northern kelp crab (Pugettia producta) and the graceful kelp crab (Pugettia gracilis) are common primary consumers in bull kelp beds near the San Juan Islands (Salish Sea, NE Pacific). In this system, urchins (often considered the most voracious herbivores exerting top-down control on kelp beds) tend to remain sedentary because of the high availability of detrital macroalgae, but the extent to which kelp crabs consume kelp (and other food options) is largely unknown. I conducted four types of laboratory feeding experiments to evaluate kelp crab feeding patterns: (1) feeding electivity between bull kelp (Nereocystis luetkeana) and seven species of co-occurring local macroalgae; (2) feeding electivity on aged vs. fresh bull kelp; (3) feeding preference between N. luetkeana and small snails (Lacuna sp.); and (4) scaling of feeding rate with body size in P. producta and P. gracilis. In choice experiments, P. producta consumed greater mass of N. luetkeana than of other macroalgal species offered and elected to eat fresh bull kelp over aged. However, P. producta also consumed snails (Lacuna sp.), indicating more generalized feeding than previously suspected. Feeding rates for P. producta exceeded the expected 3∕4 scaling rule of metabolic rates, indicating that larger P. producta may have a disproportionately large impact on bull kelp. A subtidal field experiment, designed to assess the influence of consumers on juvenile bull kelp net tissue gain, found that only fully enclosed (protected) bull kelp increased in wet mass and blade length. Herbivory by kelp crabs, among other consumers, is likely to play a previously unrecognized role in mediating the growth and survival of this annual kelp species within the Salish Sea.

Keywords: Bull kelp; Choice feeding experiments; Herbivory; Nereocystis luetkeana; Pugettia gracilis; Pugettia producta.

Figure 1. Kelp crab feeding in choice experiments.

Change in mass of macroalgae adjusted by change in randomly paired autogenic controls in choice feeding experiments with P. producta. Letters correspond to macroalgal species offered: N, Nereocystis luetkeana; A, Alaria marginata; S, Saccharina latissima; C, Costaria costata; M, Mazzaella splendens; U, Ulva sp.; Sm, Sargassum muticum; Af, Agarum fimbriatum. Box plots show median (dark horizontal line), interquartile range (box), minimum and maximum values (bottom and top “whiskers”), 1st and 3rd quartile (bottom and top of box, respectively), and suspected outliers (dots). Letters indicated statistically significant differences.

Figure 2. Feeding on fresh and aged bull kelp.

Consumption of fresh and aged Nereocystis luetkeana by kelp crabs (Pugettia producta) in a choice feeding trial (including adjustment from randomly paired autogenic controls). * indicates statistically significant difference in consumption of fresh N. luetkeana vs. aged.

Figure 3. Choice and no choice feeding experiments with kelp and snails.

Proportional change (in wet mass) of bull kelp (N. luetkeana) and snails (Lacuna sp.) in (A) choice and (B) no choice feeding trials; p-values from choice and no-choice indicate no significant difference between proportional food consumption in either experiment.

Figure 4. Scaling relationships between feeding rates (g/h) and crab mass (g).

Log–log plots showing scaling relationships (SMA regression) between feeding rate (g/h) and crab mass (g) for two species of kelp crabs; (A) P. producta shows a noticeable departure from 3∕4 scaling rule; (B) P. gracilis shows negative allometry close to the expected 3∕4 rule. Circles and solid line show observed relationship. Dashed line shows the expected slope of 0.75.

Figure 5. Field exclusion cage experiment.

The effect of four experimental treatments on the change in (A) mass and (B) blade length of juvenile N. luetkeana in a subtidal field experiment. The “cage” treatment was fully enclosed to protect the individual kelp from all large grazers; the “crab” treatment was fully enclosed with one adult P. producta included inside; the “open” treatment offered no protection to attached kelp; the “partial” treatment was a procedural control that included caging material over the top and two sides but left the other two sides open. Letters indicated statistically significant differences.