BULLETIN OF MARINE SCIENCE, 82(3): 333–344, 2008 BatHYmetric anD SPatial Distribution of DecaPOD Crustaceans on DeeP-WATER SHIPwrecKS in THE Gulf of MEXico Morgan J. Kilgour and Thomas C. Shirley Abstract The decapod fauna of six World War II shipwrecks at depths from 75 to 2000 m was examined in 2004 in the northern Gulf of Mexico. A remotely operated vehicle (ROV) was used to conduct video transects on the wrecks and through the adjacent debris fields. Shipwrecks were used as surrogates for deep-sea drilling platforms to determine if structures in deep waters serve as artificial reefs. We focused on two genera and three species of decapod crustaceans: Munidopsis spp., Munida spp., Eumunida picta S. I. Smith, 1883, Rochinia crassa Milne-Edwards, 1879, and Cha- ceon quinquedens (S. I. Smith, 1879). Differences in abundance m–2 were compared among three locations (on, near, and away from [> 300 m] the wreck) to determine if the presence of the wrecks affected crab distributions. The two most abundant crab species were observed only at four of the six shipwrecks, and two species were observed at a single site. All crabs, except R. crassa, had no spatial correlation to proximity of the wreck; however, crab abundance varied significantly on different substrates. Crab abundance was significantly correlated with depth; some species were stenobathyal while others were eurybathyal, but all species were found within their previously reported depth ranges. Drilling structures act as artificial reefs/ag- gregation sites in the deep sea for some species, but not others. In the Gulf of Mexico, the approximately 4000 oil and gas platforms provide an ad- ditional 12 km² of hard substrate (Stanley and Wilson, 1997). Deep waters in the Gulf of Mexico have high potential for mining, oil and gas exploitation (Roberts, 2002), so the expansion of these artificial substrates into deep waters is likely. Despite deep- sea waters covering 60% of the earth’s surface, reefs in the deep sea, both natural and artificial, are poorly studied because of difficulty of access (Glover and Smith, 2003; Quattrini et al., 2004). Moreover, deep-sea fauna in the Gulf of Mexico is not well known (Powell et al., 2003), thus there is uncertainty with regard to how deep-sea structures will affect invertebrate assemblages P( owell et al., 2003). The goal of this study was to determine if deep-sea drilling structures remaining on the sea floor will serve as artificial reefs, and to what extent they might alter benthic invertebrate assemblages. The Gulf of Mexico is home to the largest artificial reef complex in the world with 4000 structures (Stanley and Wilson, 2000). Over half of the gas and oil leases occur in depths exceeding 200 m (Richardson et al., 2004), and this constitutes 28% of the worldwide deep water drilling structures (Harding and Albaugh, 2003). These oil platforms eventually will reach the end of their useful production and removal will become an issue. Some states, such as Louisiana and Texas, have mitigation programs to turn shallow water oil structures into artificial reefs (Bull and Kendall, 1990). Artificial reefs attract fish and are ideal recreation areas for fishing and SCUBA diving (Bull and Kendall, 1994; Walker et al., 2002; Schroeder and Love, 2004). Ad- dition of artificial reefs, such as shipwrecks or oil platforms, in shallow water affects faunal abundance and distribution (Bohnsack and Sutherland, 1985; Love et al., 2000; Bulletin of Marine Science 333 © 2008 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 334 BULLETIN OF MARINE SCIENCE, VOL. 82, NO. 3, 2008 Walker et al., 2002). Attraction of fauna to reefs is not disputed, but whether reefs in- crease productivity or simply redistribute existing populations has been questioned (Bohnsack, 1989; Bull and Kendall, 1994). Possible reasons for attraction of fauna to reef-like structures include enhanced feeding, enhanced habitat complexity for predator avoidance, and use as nursery areas (Husebø et al., 2002; Mortensen and Fosså, 2006). The role of artificial reefs in affecting the distribution and abundance of inverte- brate assemblages is poorly known. The majority of artificial reef studies have focused on fish assemblages, but a few have also included commercially important inverte- brates (e.g., Love et al., 2000; Castro et al., 2001; Walker et al., 2002). The suitability of artificial reefs for invertebrates has been examined (Fitzhardinge and Bailey-Brock, 1989; Cummings, 1994), but rarely has the focus been on decapod crustaceans or on depths exceeding 30 m. Currently, invertebrate abundance and recruitment to artifi- cial reefs are assumed to be similar to that of fish (Montagna et al., 2002). Anthropogenic effects and assemblages in deep waters create controversy on re- moval of deep-sea structures. Fish populations may utilize deep-sea structures, such as shipwrecks and oil platforms, and the removal of these structures may cause fish populations to decrease (Love et al., 2000; Schroeder and Love, 2004). The removal of oil platforms would contribute to considerable loss of hard substrate in the Gulf of Mexico (Beaver et al., 1997). Our research focuses on one subset of the fauna, decapod crustaceans, to evaluate whether shipwrecks alter crab abundance and dis- tribution, as an indirect way of estimating the effect of removing oil and gas produc- tion structures. As there are few natural deep water reefs in the Gulf of Mexico, shipwrecks were used as surrogates to determine if reef-like structures influence the abundance and composition of the decapod crustacean assemblages. An added advantage of using shipwrecks as surrogates for reefs is that the shipwrecks are of known age; all wrecks were sunk within three months of each other, thereby reduc- ing temporal variability. We asked a number of questions about two genera and three species of decapod crustaceans: (1) Did proximity to the wreck affect density of these crabs? (2) Was their abundance significantly different at different depths? and (3) What substrates did the crustaceans occupy at different distances from the wrecks? The presence of five decapods from transects at each wreck was quantified: Rochinia crassa Milne- Edwards, 1879, Chaceon quinquedens Smith, 1879, Munidopsis spp., Munida spp., and Eumunida picta Smith, 1883. These five decapods were chosen because they were easily recognizable on the video and they occurred in sufficient numbers to be analyzed. Methods Site Description.—Six shipwrecks that sank between May 6 and July 31, 1942 were ex- amined in the Gulf of Mexico. The close temporal proximity of the sinking of the vessels strengthens faunal comparisons by decreasing variability that might be related to time (Table 1), and makes them ideal for studying deep-water structures in the Gulf of Mexico (Church et al., 2007). Field Procedures.—Faunal assemblages were examined on, adjacent to, and away from (> 300 m distance) six WWII shipwrecks at depths from 75 to 2000 m (Table 1, Fig. 1). Data were collected with a Remote Ocean Systems (ROS) color camera mounted on a Remotely Oper- ated Vehicle (ROV), the XL-11, with the HOS Dominator serving as a support vessel from KILGOUR AND SHIRLEY: DECAPOD CRUSTACEANS ON DEEP-WATER SHIPWRECKS 335 Table 1. Wreck depth range to nearest hundred meters, date sank, length of time (hours:minutes) visited, dates visited, number of transects analyzed at each wreck, and vessel type. Depth Dates visited Transects Vessel Wreck range (m) Date sank Hours:minutes (2004) analyzed type VIRGINIA 0–200 May 12, 1942 27:25 7/30–8/1 0 Tanker HALO 100–200 May 20, 1942 45:42 8/1–8/2 10 Tanker GULF PENN 500–600 May 12, 1942 49:45 8/4–8/5 18 Tanker 8/11–8/12 8/13 U-166 1,400–1,500 July 31, 1942 19:55 8/6 11 U Boat ROBERT E. LEE 1,400–1,600 July 31, 1942 49:14 8/7–8/8 16 Passenger freighter ALCOA PURITAN 1,900–2,000 May 6, 1942 38:21 8/9–8/10 14 Cargo freighter July 29 to August 15, 2004 (Table 1). Parallel laser beams 12.7 cm apart were mounted on the ROV and faced down to measure variability in ROV height from bottom. During transects the height of the ROV ranged from the seafloor to several m above the seafloor; lasers could not be seen from the camera at the longer distances. The presence of the ROV and lights, sounds, and vibrations produced by the ROV may have changed behavior of some megafauna (Spanier et al., 1994), and the actual abundance of motile crustaceans may have been higher than recorded in this study. However, ROVs are the most practical tool available for observing organisms at depth. Video was compressed and stored on harddrives for laboratory analysis. Transects were conducted over the surface of the wreck and through the debris field to at least 300 m from each wreck subsequent to wreck identification and construction of a photo mosaic; these transects were labeled as “on”, “adjacent to” (1–80 m), or “away from” (> 300 m) the wreck. At each site Global Positioning System (GPS) coordinates of the ROV and bot- tom depth were recorded every 4–12 s. Oxygen, temperature, conductivity, and density were Figure 1. Locations of the wreck sites in the Gulf of Mexico. The symbol for the Robert E. Lee was moved slightly northeast because the site marker overlapped with the U-166. 336 BULLETIN OF MARINE SCIENCE, VOL. 82, NO. 3, 2008 measured at every shipwreck with a conductivity, temperature, depth (CTD) Seacat profiler mounted on the ROV. Once transects were completed, at least 6 hrs were spent collecting voucher specimens us- ing traps and the ROV. Traps were used to collect voucher specimens for cryptic species that might have evaded the ROV.