Vol. 469: 195–213, 2012 MARINE ECOLOGY PROGRESS SERIES Published November 26 doi: 10.3354/meps09862 Mar Ecol Prog Ser Contribution to the Theme Section ‘Effects of climate and predation on subarctic crustacean populations’ OPENPEN ACCESSCCESS Ecological role of large benthic decapods in marine ecosystems: a review Stephanie A. Boudreau*, Boris Worm Biology Department, Dalhousie University, 1355 Oxford Street, PO Box 15000, Halifax, Nova Scotia B3H 4R2, Canada ABSTRACT: Large benthic decapods play an increasingly important role in commercial fisheries worldwide, yet their roles in the marine ecosystem are less well understood. A synthesis of exist- ing evidence for 4 infraorders of large benthic marine decapods, Brachyura (true crabs), Anomura (king crabs), Astacidea (clawed lobsters) and Achelata (clawless lobsters), is presented here to gain insight into their ecological roles and possible ecosystem effects of decapod fisheries. The reviewed species are prey items for a wide range of invertebrates and vertebrates. They are omnivorous but prefer molluscs and crustaceans as prey. Experimental studies have shown that decapods influence the structuring of benthic habitat, occasionally playing a keystone role by sup- pressing herbivores or space competitors. Indirectly, via trophic cascades, they can contribute to the maintenance of kelp forest, marsh grass, and algal turf habitats. Changes in the abundance of their predators can strongly affect decapod population trends. Commonly documented non- consumptive interactions include interference-competition for food or shelter, as well as habitat provision for other invertebrates. Anthropogenic factors such as exploitation, the creation of pro- tected areas, and species introductions influence these ecosystem roles by decreasing or increas- ing decapod densities, often with measurable effects on prey communities. Many studies have investigated particular ecosystem effects of decapods, but few species were comprehensively studied in an ecosystem context. A simplified synthetic framework for interpreting eco system roles of decapods was derived from the available evidence; however, more experimental and long-term observational studies are needed to elucidate mechanisms and shed light on the long- term consequences of decapod fisheries. KEY WORDS: Lobster · Crab · Trophic cascade · Keystone · Ecosystem effects · Anthropogenic effects · Species interactions Resale or republication not permitted without written consent of the publisher INTRODUCTION of crustaceans have increased ~5-fold since 1950 and are the only invertebrate group that continues to Large crustaceans are becoming increasingly im- trend upward in recent years (Anderson et al. 2011). portant to coastal and continental shelf fisheries Yet, when compared to finfish, there is a much (Anderson et al. 2011, Steneck et al. 2011). For exam- smaller knowledge base available from which to ple, in eastern North America, a well-documented manage these fisheries, particularly in an ecosystem shift has occurred from groundfish, such as Atlantic context (Anderson et al. 2008). Large mega-deca pods, cod Gadus morhua, to invertebrates that now domi- defined here as decapod crustaceans with a carapace nate commercial landings and value (Worm & Myers length (CL) or width (CW) of >10 cm, such as Ameri- 2003, Frank et al. 2005). Globally, commercial catches can lobster Homarus americanus and snow crab *Email: [email protected] © Inter-Research 2012 · www.int-res.com 196 Mar Ecol Prog Ser 469: 195–213, 2012 Chionoecetes opilio, have become particularly im- Micheli et al. 2002), but few experimental studies por tant commercial species in the Northwest (NW) have focused on large decapods. Decapods are typi- Atlantic region and elsewhere. Decapod crustaceans cally quite mobile, undergo ontogenetic habitat have a global distribution and can be found in most changes over their life cycle, and show a progressive habitats, ranging from intertidal to deep water re - dietary shift with increasing size (Sainte-Marie & gions. Their importance to humans is well docu- Chabot 2002). It is therefore likely that they would mented and frequently discussed (Steneck et al. affect a range of habitat and prey types over their life 2011), however we are only beginning to understand cycle. the role that decapods play in marine ecosystems, In this paper we attempt to synthesize what is and how exploitation might modify this role. known about the role of large benthic decapods (lob- Interest in the ecological effects of fisheries often sters and large crabs) in marine ecosystems. Specifi- tends to focus on large apex predators and their role cally we review their multiple roles as (1) prey, (2) in the ecosystem (e.g. Pauly et al. 1998, Jackson et al. predators and keystone species, as well as (3) non- 2001, Frank et al. 2005, Heithaus et al. 2008, Baum & consumptive interactions. Finally, given the large Worm 2009, Estes et al. 2011). One of the commonly role humans play in modifying natural systems (Estes described ecosystem effects of fishing marine preda- et al. 2011), we ask how anthropogenic factors mod- tors has been an increase in benthic invertebrates, ify the ecosystem role of decapods, and how future including large decapod crustaceans (Baum & Worm research efforts could provide deeper insights into 2009, Boudreau & Worm 2010). On occasion large these questions. decapods may replace large fish (e.g. Atlantic cod) as the dominant predator, the ecological impacts of which are poorly understood (e.g. Steneck et al. METHODS 2011). One well-documented example is a shift in the NW Atlantic ecosystem’s trophic structure in the The above questions are addressed by synthesiz- 1980s to 90s due to the depletion of top predators. ing the existing evidence from mega-decapod popu- Here, an observed decline in groundfish abundance lations worldwide. A literature search was conducted (largely due to overexploitation) was followed by a using the Web of Knowledge database and the fol- large increase in benthic decapods and other prey lowing keywords: decapod ecosystem effect (52 species, likely because of predation release (Steneck results), lobster ecosystem effect (109), crab ecosys- et al. 2004, Worm & Myers 2003, Frank et al. 2005). In tem effect (347), lobster diet (456), and crab diet contrast, the North Pacific Ocean currently yields low (1570). These papers, as well as references cited decapod abundance (e.g. king crab Paralithodes therein, form the basis of this review. Specifically, we camtschaticus, snow crab Chionoecetes opilio, and were interested in case studies of the different roles shrimp Pandalus spp.) due to population collapses in all species of mega-decapod (CL or CW >10 cm) play the early 1980s (Orensanz et al. 1998). These popula- in oceanic ecosystems. However, the majority of the tions have since been slow to recover. As decapod mega-decapods studied in the literature were of stocks were low in Alaska, the biomass of groundfish commercial value, hence this review is necessarily (i.e. Walleye pollock Theragra chalcogramma) in - biased towards those species. creased to an all-time high (Ianelli et al. 2011). This Available publications employed a variety of meth- suggests that there may be a suite of conditions ods including decapod exclusion experiments in the within an oceanic system that, once altered by fish- field (e.g. Quijon & Snelgrove 2005b), experimental ing, are better suited for supporting decapod crus- transplants (e.g. Robles & Robb 1993), tethering taceans or large-bodied groundfish (Worm et al. experiments (e.g. Silliman & Bertness 2002), and diet 2007). Yet, the broader ecological consequences of studies (e.g. Jewett & Feder 1982). These were used such changes in decapod populations on the benthic primarily to gain mechanistic insights into potential ecosystem are less well understood. predator–prey mechanisms and interactions. Experi- Many studies have focused on the diet of large ments in the laboratory were also used, often in con- marine decapods, but little is known about the cert with field observations and surveys. Non-con- strength of predatory interactions, their cascading, sumptive interactions were studied using similar and overall ecosystem effects. Predation generally methods as those listed above, e.g. observations on plays a strong role in structuring marine benthic collected animals (e.g. Dvoretsky & Dvoretsky 2008), communities (Shurin et al. 2002), ranging from inter- field (e.g. Novak 2004) and lab experiments to tidal shores (e.g. Paine 1994) to the deep sea (e.g. observe interactions (e.g. Williams et al. 2006), or a Boudreau & Worm: Ecological role of marine decapods 197 combination of field and lab studies (e.g. Jones & Clawed lobsters discussed here are the American Shulman 2008). At larger spatial and temporal scales, lobster Homarus americanus native to the NW At lan - time series analyses were used to complement tic Ocean and the European lobster H. gammarus in smaller-scale mechanistic studies. These might exa - the northeast Atlantic. Clawless lobsters include sev- mine ecosystem changes occurring in the wake of eral species of spiny lobster, such as Panulirus inter- decapod exploitation (e.g. Lafferty 2004, Shears et al. ruptus (California), P. marginatus (Ha waii), and Ca - 2006) or recovery (e.g. Babcock et al. 1999). rib bean P. argus (Florida, Bahamas), and the spiny rock lobsters Jasus edwardsii (New Zealand), P. cyg - nus (western Australia), and J. lalan dii (South Africa). SPECIES There are