Many species of shore crabs are omnivorous and will eat algae, invertebrates, and dead organic matter.  On the central California coast around Pillar Point and Montara, high-intertidal rocky areas are subject to regular disturbance from water-borne sediments, which clears areas on the rocks.  Shortly after these patches are created, blooms of ephemeral algae appear and, depending upon season, are colonised by dipteran¹ larvae (mainly midges) that eat the algae.  The algae and larvae are in turn eaten by a variety of herbivores (limpets and littorines) and by omnivorous crabs Pachygrapsus crassipes. 

Research to address the question as to whether the herbivores and crabs are enough to prevent the establishment of the algae/larvae assemblage involves clearing patches on the rock and/or cementing concrete plates to the rock.  Three types of experimental patches are created: one in which crabs and limpets are excluded by plastic baffles and copper-containing plastic paint², respectively; one in which limpets are excluded but not crabs; and one which is left as a control.  Within 2-3 weeks in the CONTROL areas, microscopic³ and macroscopic algae begin to grow, followed about 6wk later by the appearance of dipteran larvae.  The initial algal growth is soon discovered by Pachygrapsus and grazed down to turf (microscopic algae growing in the matrix of Enteromorpha holdfasts).  The turf persists and thrives for several weeks before limpets arrive and eventually graze it down to bare surface (see graph upper Right). 

In the NO LIMPETS areas the same pattern exists but, in the absence of limpets, the turf persists.  The crabs may have difficulty in grabbing the slippery Enteromorpha fronds or they may not have a taste for the alga, which is poor in nutrients and usually associated with freshwater run-off (see graph on Left).

In the NO CRABS, NO LIMPETS areas both macro- and micro-algae thrive and persist over the 18-wk study period (see graph lower Right).

The fly larvae, meantime, thrive in the NO CRABS, NO LIMPETS areas, are eaten by crabs to low levels in the NO LIMPETS areas, and are eaten virtually to extinction in the CONTROL areas.  It is not possible from these results to know if the limpets are directly responsible for this decline.  One guesses that they are not.  However, it appears that crabs prefer the larger fly genus Paraclunio to the smaller genus Nocticanace perhaps, in part, because they are easier to find. That larvae of any kind survive in the NO LIMPETS areas may owe to the presence of the heavy cover of turf algae making them hard for the crabs to find and catch.  Without limpets to crop back the turf, the crabs find it especially difficult to locate the smaller Nocticanace.  The limpets, then, are indirectly affecting survival of the flies.  The author notes that in other disturbance-type studies, community structure is affected through interruption of processes of competition.  Here, the interruption is of processes of grazing and predation. Robles 1982 Oecologia 54: 23. Photo courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE¹  the larvae are members of 2 genera Paraclunio and Nocticanace, the former being larger in the larval stage than the latter. They live for several weeks in tubes attached to the rocks, eat algae, and are covered periodically by the tides.  On emergence, the adults are ephemeral, some living for the duration of only a single low tide, and the adults mostly do not eat

NOTE²  the paint, applied as a border around cleared patches, stops limpet traffic (Lottia digitalis and Lottia scabra are the most common species) but not movement of other invertebrates.  Herbivorous littorines are free to wander, but their numbers in comparison with limpets are insignificant

NOTE³  microscopic species include diatoms, blue-green algae, and spores; macroscopic algae are species of Bangia, Urospora, Enteromorpha, Ulva, and Porphyra

The role of mesograzers¹, notably the hermit crabs Pagurus hirsutiusculus and P. granosimanus, in affecting community dynamics in intertidal regions of Tatoosh Island, Washington, is assessed from the standpoints of feeding rates and densities.  Specifically, the author is interested in how well knowledge of these parameters, especially feeding rates obtained in laboratory experiments, would permit estimates to be made of impact of a given consumer in the field.  Hermit crabs are basically scavengers, but will feed readily on algae including diatoms and certain macroalgae².  Results show, interestingly, that feeding rates on the diatom Isthmia scale allometrically, but the significance of this is unclear³.  On the basis of feeding rates and body size, P. hirsutiusculus, of all the mesograzers tested, is predicted to have the greatest impact on community dynamics.  However, tests in field microcosms (9 x 12cm fenced enclosures) at Tatoosh Island fail to support these predictions, and this hermit-crab species in particular is neither most abundant, nor has the most impact.  The author credits the disparity with failure of laboratory-measured feeding rates to approximate closely per-capita effects under field conditions, mostly based on underestimations that fail to account for alternate food sources, increased search times in the field, interference from other consumers, and other factors.  The study is valuable in its use of a variety of mesograzer species, and its combination of laboratory and field measurements.  Ruesink 2000 J Exp Mar Biol Ecol 248: 163. Photo of P. hirsutiusculus courtesy Ron Long, SFU and photo of P. granosimanus courtesy Dave Cowles, Walla Walla University, Washington and wallawalla.edu.

NOTE¹  a category of consumers based on a size, generally <2.5cm.  The study includes other invertebrate species such as crabs, isopods, amphipods, and polychaetes but, for convenience, only the 2 hermit crabs are considered here

NOTE²  the author uses 2 food species, a diatom Isthmia nervosa and a red alga Odonthalia floccosa, to test laboratory feeding rates.  Both species are abundant intertidally at Tatoosh Island and are readily consumed by the crabs

NOTE³  since MR (metabolic rate) ∞ (Mass)^0.67-0.75 (i.e., MR becomes relatively less with increasing body size), then Feeding Rate (FR) would be predicted to scale the same; however, the author reports an overall slope value for all mesograzers of 1.21 (i.e., FR ∞ Mass^1.21).  What this means is that feeding rate in these mesograzers increases out of proportion to body size, which neither fits the prediction nor is a likely biological event.  The author notes that the measured slope, b, of 1.21 is “indistinguishable” from a 0.67- or 0.75-power-scaling prediction, which clearly makes no sense, as slopes above unity or below unity are completely different biological entities.  No statistical tests for difference from isometry or even significance of slopes appear to have been done, or perhaps are not reported in the paper