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The role of carapace spines in the swimming behavior of porcelain crab zoeae (Crustacea: Decapoda: Porcellanidae)
Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179
Contents lists available at ScienceDirect
Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe
The role of carapace spines in the swimming behavior of porcelain crab zoeae (Crustacea: Decapoda: Porcellanidae)☆ Anna E. Smith a,⁎, Gregory C. Jensen b a b
Bamfield Marine Science Centre, 100 Pachena Road, Bamfield, BC V0R 1B0, Canada School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195, United States
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 7 June 2015 Accepted 8 June 2015 Available online 16 June 2015 Keywords: Porcellanidae Anomura Larval transport Zoeae Swimming Petrolisthes cinctipes
a b s t r a c t Porcelain crab zoeae display vastly different morphological and swimming characteristics compared to other crab zoeae. Porcellanids have highly elongated anterior and posterior carapace spines and swim with smooth lateral directionality. While long spines have been shown to aid in predator deterrence in crab larvae, there is no empirical evidence regarding their effect on swimming behavior. To investigate this possible association, the carapace spines of 40 zoeae of the porcelain crab, Petrolisthes cinctipes (Randall, 1840), were experimentally altered and changes in swimming behavior, spatial orientation and speed were recorded. When anterior, posterior, or both sets of spines were removed, the larvae swam with significantly reduced lateral speed. Ablation of only the anterior (rostral) spine resulted in a loss of depth control and relegated forward progress to erratic, ineffective lurches. Removal of the posterior spines resulted in an inability to swim backward, and reduced the ability to maintain a straight trajectory while swimming forward. There was no visible pitching of the zoeae in any of the treatment groups, which suggests that balance is not the primary function of the elongated spines. This represents the first experimental evidence linking the unusual elongated carapace spines of porcellanid zoeae to their ability to swim laterally, and may help explain their ability to remain in nearshore coastal waters throughout their development. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The zoeal stages of porcelain crabs are among the most spectacular of crab larvae, having extremely elongated rostrums and long, closely appressed pairs of posterior spines. Many crab zoeae (the first true free-swimming stage of crab larval development) have long carapace spines that have been shown to effectively deter gape-limited predators like small fish (Morgan, 1989). Long spines could fulfill additional functions as well, such as reducing sinking rate and facilitating locomotion (Chia et al., 1984; Gurney, 1902; Weldon, 1889). Gonor and Gonor (1973b) anecdotally attributed the predominately forward and backward swimming of porcellanid zoeae to the configuration of the carapace spines and reported that porcellanid zoeae swam equally well in both directions, demonstrating a high level of maneuverability. In contrast, Foxon (1934) noted that the long rostral spine made them “unwieldy creatures” unable to turn quickly, and that removal of the rostrum allowed them to behave in a manner similar to other anomuran larvae. Morgan (1989) removed spines from Rhithropanopeus harrisii
☆ All authors agree to the submission of this article for publication in the Journal of Experimental Marine Biology and Ecology. ⁎ Corresponding author at: 13814 116 Ave., Surrey, BC V3R 2T2, Canada. E-mail addresses: [email protected] (A.E. Smith), [email protected] (G.C. Jensen).
http://dx.doi.org/10.1016/j.jembe.2015.06.007 0022-0981/© 2015 Elsevier B.V. All rights reserved.
(Gould, 1841) zoeae in relation to predator defense and found no effect on sinking rate or stabilization of zoeae, unlike Foxon (1934) who suggested that the long spines of porcellanid zoeae appear to slow their sinking rate. These contrasting studies highlight the uncertainty surrounding the role of carapace spines in the locomotion of zoeae. The motility of crab zoeae is typically limited to vertical movements, using a combination of active upward swimming and passive sinking (Sulkin, 1984). Some zoeae can control their horizontal position through species- and stage-specific responses to stimuli, such as light or pressure, which enable them to associate with water masses that are moving in a particular direction (Spooner, 1933). For example, the positively phototaxic early zoeal stages of the blue crab, Callinectes sapidus Rathbun, 1896 are exported from their natal estuary via outward-flowing surface water (Epifanio et al., 1984), while the equivalent stages of the estuarine mud crab R. harrisii are retained in the estuary by seeking deeper water (Cronin, 1982). Although such behaviors are adaptive under typical conditions, anomalous weather or current patterns can result in recruitment failure due to larvae being carried too far away from appropriate habitat for settlement. Thus, cancrid crab larvae may find themselves over 100 km offshore by the time they reach the megalops stage, with little or no chance of returning to nearshore habitats to settle (Jamieson and Phillips, 1988). In contrast, porcelain crab zoeae remain relatively close to shore. This tendency has been recorded along the western coast of North
176
A.E. Smith, G.C. Jensen / Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179
America, including northern California (Wing et al., 1998) and off Oregon, where porcellanid larvae were most abundant 1.8–5.5 km from shore and rarely taken beyond 18 km (Lough, 1975). Similarly, zoeae of the flat porcelain crab Petrolisthes cinctipes predominated in plankton samples taken within 8 km of shore in British Columbia, Canada (Jamieson and Phillips, 1988). Samples taken further from shore were dominated by cancrid zoeae which primarily swim vertically. Porcellanid zoeae, however, are strong horizontal swimmers (Gonor and Gonor, 1973b) and it has been suggested that this may account for their ability to remain in nearshore areas (Jensen, 1991). To investigate the possible relationship between the elongated spines of porcellanid zoeae and their ability to swim laterally, P. cinctipes zoeae were subjected to different spine removal combinations. Zoeae with all spines removed were expected to either revert to the erratic vertical swimming of other species of crab zoeae, or to be unable to swim at all. The effects of removing the anterior and posterior spines in isolation were less predictable but it was anticipated that the maneuverability and swimming speed of the zoeae would be impacted if the adaptive significance of porcellanid zoeae spines was linked to locomotion. 2. Materials and methods 2.1. Collection of specimens Porcelain crab larvae were collected from the mouth of Bamfield Inlet, British Columbia, Canada June 22–24, 2012 (48°50.32N, 128°8.25S). The collections were conducted at 02:00 to ensure complete darkness and to allow sufficient time for the zoeae to complete their diurnal migration to the surface. The zoeae were collected over three nights using a 560 μm plankton net towed at the surface. Each collection consisted of five 6-min tows. The samples were then sorted using a dissecting scope and all porcellanid zoeae were isolated. The zoeae were identified according to chromatophore patterns as described by Gonor and Gonor (1973a). Four species were collected: Pachycheles rudis Stimpson, 1859, Pachycheles pubescens Holmes, 1900, Petrolisthes eriomerus Stimpson, 1871, and P. cinctipes. The latter was the most abundant species in the samples and was therefore chosen as the focal species in this study. The zoeae were kept in four circular glass containers, each approximately 30 cm in diameter and containing 10 cm of water, within an 11 °C water table and were fed newly-hatched Artemia nauplii ad libidum before and between experiments. The zoeae were given 24 h to acclimate to the lab conditions prior to any experimentation or manipulations.
fine tweezers. The spines were removed with scissors with the aid of a dissecting scope. The rostral spine was cut 1 mm forward of the carapace to ensure no injury to the first antennae or cephalothorax. The posterior spines were cut in unison, also 1 mm away from the posterior margin of the carapace (Fig. 1). After 5 h of recovery time, the swimming behavior of each zoea was observed with attention to movement ability and technique. 2.3. Swim speed measurement To test lateral swimming speed, each zoea was placed in a circular, 30 cm diameter glass culture dish centered on a template marked with concentric circles with radii of 5, 10, and 15 cm. The swimming behavior was recorded using a digital camera with video capability (Nikon D5100 with an 18–55 mm lens). The videos of each zoea were reviewed and the most direct and continuous passage across two concentric circles was chosen for each individual. The trajectory across the circles was measured to the nearest 0.5 mm. The time elapsed was recorded to the nearest millisecond using the time record on the videos. Velocity v was calculated as distance traveled d divided by time elapsed t and was converted to standard units of m/s. The velocity data for all three treatment groups were not normally distributed, showing a skew to the right and the variances differed among groups by an order of magnitude. A log-transformation of the data corrected both issues allowing the use of a one-way ANOVA. 2.4. Measurement of upright orientation Porcellanid zoeae normally swim with the dorsal part of their carapace facing up (Gonor and Gonor, 1973b). To gauge the possible role of spines in maintaining this orientation, the amount of time each zoea spent on its back (or side) during a period of five minutes was recorded. The same zoeae from the previous tests were used in this experiment. The zoeae were individually pipetted into 30 cm diameter glass dishes and stimulated to swim by gently lifting them into the water column with a pipette. Stopwatches were used to keep track of the time spent in each swimming position. Each zoea was placed into the still water bowl, which was filled to a depth of 10 cm, and the zoea was lifted to a depth of 1 cm below the surface for each individual trial. Only one zoea was tested at a time. The amount of time the zoeae spent upsidedown or sideways was compared among treatments using a Kruskal– Wallis non-parametric test because transformations could not correct the unequal variance and the non-normality of the data. The four groups were then compared against one another using a Dunn's pairwise multiple comparison test.
2.2. Spine removal 3. Results Ten stage II zoeae were randomly assigned to each of four experimental groups to test the effect of spine removal. The control group was left fully intact to gauge normal swimming behavior and speed. The treatment groups consisted of zoeae with (1) the posterior spines removed, (2) the anterior rostral spine removed, and (3) both anterior and posterior spines removed. Individual zoea were placed in a slotted microscope slide containing water and held by the rostral spine with
3.1. Qualitative observations of swimming behavior Intact zoeae exhibited smooth lateral swimming behavior, moving with equal proficiency in both forward and reverse directions. Intact zoeae also displayed quick changes in left–right direction, even when traveling at full swimming speed. They changed their swimming
Fig. 1. Spine removal sites (vertical lines) on a zoea of the porcellanid crab Petrolisthes cinctipes. Scale bar represents 1 cm.
A.E. Smith, G.C. Jensen / Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179
direction by abruptly swimming backwards, forwards and then backwards again while turning horizontally slightly with each movement. The intact zoeae swam straight trajectories and maintained a constant depth in the water column. Zoeae with the anterior rostral spine removed retained the ability to swim backwards but were no longer able to maintain a straight trajectory. Forward swimming was reduced to short, erratic bursts and they appeared to have little directional control. None of the 10 individuals were able to maintain a constant depth in the water column and often spiraled vertically. Uncontrolled trajectories were also present in the swimming behavior of the 10 zoeae with the posterior spines removed. These zoeae, however, were able to maintain constant levels in the water column with limited vertical spiraling. The zoeae exhibited loss of backwards motion and were unable to swim forward in straight lines, instead swimming in large circles. The swimming ability of the 10 zoeae with both anterior and posterior spines removed was reduced to slow spinning upside down on the bottom of the dish. None of the 10 zoeae were successful in leaving the bottom and swimming vertically or laterally. When the zoeae were stimulated to swim by being lifted into the water column, they were unable to maintain the positioning and spiraled back down to the bottom without successful swimming motion. Any attempts to swim upwards resulted in short-lived erratic spinning and no gain in elevation. All of these zoeae died in less than 12 h, whereas all unilaterally ablated zoeae survived and molted normally to the megalops stage. 3.2. Quantitative differences in swimming behavior There were significant differences in lateral swimming speed among the treatment groups (ANOVA, df = 3, p b 0.001; Fig. 2). All groups were found to be significantly different from one another, with the exception of anterior spine versus posterior spine treatments. Intact zoeae were significantly faster than all other treatments, and zoeae with anterior spines removed or posterior spines removed were both significantly faster than those with all spines removed. Significant differences were also present among the four groups in the amount of time the zoeae spent on their back or side (Kruskal–Wallis test, df = 3, p b 0.001; Fig. 3). All groups were found to be significant from one another, except anterior spine removed versus posterior spines removed. 4. Discussion All altered P. cinctipes zoeae swam in an erratic, disordered manner, had difficulty maintaining an upright orientation, and swam more
slowly than intact zoeae. These differences support the hypothesis that the anterior and posterior spines are used in conjunction to maintain the smooth lateral swimming observed in porcellanid zoeae. The morphological and behavioral similarity between the zoeae of several porcellanid species suggests that the findings can be generalized to other porcellanids (Gonor and Gonor, 1973b). The difference in control between zoeae missing the posterior spines and those with the anterior spine removed suggests that the spines are used for different functions in swimming. Zoeae with the posterior spines removed were unable to move in reverse or travel in a straight line and moved in large circles. This suggests that the posterior spines may function in a rudder-like fashion, allowing zoeae to make path corrections. The change in backwards swimming ability suggests that the posterior spines also serve to make the zoeae hydrodynamically streamlined from both the anterior and posterior directions. As there was no visible pitching of the zoeae with the removal of the posterior spines, it is unlikely that the spines are an alternative balancing system. Similarly, there was no pitching seen in the zoeae with the anterior spine removed. These zoeae had difficulty in maintaining a constant depth in the water column, often weaving up and down throughout the levels. This was in sharp contrast to those with only the posterior spines removed that were able to maintain a constant depth. The trajectory of the zoeae with the anterior spine removed was erratic, lacking the smooth motion seen in the intact and posterior spine removed groups. This alteration in swimming behavior indicates that the anterior spine may be responsible for both the trajectory of the zoeae as well as positioning in the water column. The complete loss of swimming ability in the zoeae with both posterior and anterior spines removed is further indication that the spines are connected with horizontal swimming ability. Had the zoeae switched to swimming vertically in a manner similar to zoeae of other crab species, more conclusive connections between lateral swimming and elongated spines could be made. However, it is possible that the behavioral differences observed in this group were the result of the trauma caused by the additional handling and ablation, as none in this group survived longer than 12 h. It is possible that the double ablation resulted in too much hemolymph loss, or that the inability to swim adversely affected respiration. Ideally, this would have been controlled for by including a treatment that involved ablation and re-attachment of anterior and posterior spines. There are additional differences in body structure between these zoeae and non-porcellanids that may also influence their swimming. Porcellanid zoeae have a horizontally angled cephalothorax (Fig. 1), giving them a more elongated profile, whereas other crab zoeae have a rounder, more vertically oriented conformation. Variation in the
0.08
Lateral Swimming Speed (m/s)
a 0.07 0.06 0.05 0.04
b
0.03 0.02
c
0.01 0 Intact
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Anterior Removed
Posterior Removed
Both Removed
Spine Treatment Fig. 2. Lateral swimming speed of zoeae of the porcellanid crab Petrolisthes cinctipes in relation to spine removal treatment. Means are shown ±1 SE. N = 10 in all cases.
A.E. Smith, G.C. Jensen / Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179
Time Spent on Back or Side (s/5 min)
178
350
c 300 250 200 150
b
b
100 50
a
0 Intact
Anterior Removed Posterior Removed
Both Removed
Spine Treatment Fig. 3. Time (seconds) spent on the back or side during a period of 5 min: an assessment of changes in spatial orientation of zoeae of the porcellanid crab Petrolisthes cinctipes in relation to spine removal treatment. Means are shown ±1 SE. N = 10 in all cases.
morphology and action of the swimming apparatus (maxillipeds, telson, and abdominal motions) between species may be contributing factors, or the differences may be mostly due to hydrodynamic effects of the different body conformations. The differences in swimming speed and spatial orientation between control and manipulated zoeae also point to an association between the elongated spines and locomotion. The significant decrease in lateral swimming speed of all treatment groups compared to the control group of intact zoeae provides evidence that the elongated spines contribute to the fast horizontal swimming capabilities of these zoeae. Indeed, the mean lateral swimming speed of the uncut zoeae was over twice the speed previously recorded for crab zoeae (Chia et al., 1984). The removal of either the front or rear spines hampered swimming in different ways but to approximately the same extent in terms of speed. The removal of both spines rendered the zoeae incapable of swimming. The removal of any combination of spines also resulted in a reduced ability to maintain spatial orientation, the most drastic of which was seen in the spineless zoeae. The lack of tilting seen in the two single direction spine ablation groups indicates this reduction in gravitational orientation is more likely due to a reduction in sensory information or gravitational interaction than to a change in balancing ability.
5. Future directions The association between the elongated spines of porcellanid zoeae and lateral swimming could be further examined by studying water flow patterns around the spines. The hydrodynamic impact of the spines on changes in flow patterns with different spine removal combinations could be explored. If spines are aiding in hydrodynamics, removal of these spines should result in a reduction in streamlining and the formation of turbulence around the zoea. The connection between carapace spines and swimming behavior could be widened to investigations of other crab zoeae species to better understand their methods of movement. The forward pitch of the body in relation to the spines may also enable porcellanid zoeae to swim laterally and with straight trajectories (Gonor and Gonor, 1973b). A study altering the length or positioning of spines of other species of crab zoeae to observe the effects on swimming could provide further evidence supporting the correlation between elongated spines and lateral swimming. In addition to swimming studies, the role of the anterior spine in sensory recognition and escape from predators could be examined.
P. cinctipes zoeae are transported by currents north and south along the Pacific coast (Toonen and Grosberg, 2011), and their strong horizontal swimming capabilities may be key to their ability to remain fairly close to shore. This raises intriguing questions about the possible cues they are using for orientation as they maintain their position. Since porcellanids are a diverse and widespread group, it would be interesting to see if species in other parts of the world exhibit similar patterns of nearshore retention. Many unanswered questions remain as to the intricacies of porcellanid swimming behavior and the evolutionary pressures that caused these morphological features to arise. Acknowledgments We would like to thank Helen Yip for her continual willingness to assist at all stages of this research and Isabelle Côté for her valuable support and knowledge. We would also like to express our sincere gratitude to the staff of the Bamfield Marine Sciences Centre for their support and for the use of their facilities. Also thank you to midnight skippers Gordon Byron and Gavin Brackett. Thank you to Trudi Smith for assisting in preparation of the manuscript and to Melinda Smith for digitizing the zoea drawing. We would also like to thank Dr. Sandra E. Shumway and an anonymous reviewer for their insightful and detailed comments, which helped to improve our manuscript. [SS] References Chia, F.-S., Buckland-Nicks, J., Young, C.M., 1984. Locomotion of marine invertebrate larvae: a review. Can. J. Zool. 62, 1205–1222. Cronin, T.W., 1982. Estuarine retention of larvae of the crab Rhithropanopeus harrisii. Estuar. Coast. Shelf Sci. 15, 207–220. Epifanio, C.E., Valenti, C.C., Pembroke, A.E., 1984. Dispersal and recruitment of blue crab larvae in Delaware Bay, U.S.A. Estuar. Coast. Shelf Sci. 18 (01), 1–12. Foxon, G.E., 1934. Notes on the swimming methods and habits of certain crustacean larvae. J. Mar. Biol. Assoc. U. K. (New Ser.) 19 (02), 829–849. Gonor, S.L., Gonor, J.J., 1973a. Descriptions of the larvae of four North Pacific porcellanidae (Crustacea: Anomura). Fish. Bull. 71 (1), 189–223. Gonor, S.L., Gonor, J.J., 1973b. Feeding, cleaning and swimming behavior in larval stages of porcellanid crabs (Crustacea: Anomura). Fish. Bull. 71 (1), 225–234. Gurney, R., 1902. The metamorphosis of Corystes cassivelaunus (Pennant). Q. J. Microsc. Sci. 46, 461–477. Jamieson, G.S., Phillips, A.C., 1988. Occurrence of Cancer crab (C. magister and C. oregonensis) megalopae of the west coast of Vancouver Island, British Columbia. Fish. Bull. 86 (3), 525–542. Jensen, G.C., 1991. Competency, settling behavior, and postsettlement aggregation by porcelain crab megalopae (Anomura: Porcellanidae). J. Exp. Mar. Biol. Ecol. 153, 49–61. Lough, R.G., 1975. Dynamics of Crab Larvae (Anomura, Brachyura) off the Central Oregon Coast, 1969–1971 (Ph. D. Dissertation), Department of Biology, Oregon State University, Corvallis, Oregon (308 pp.).
A.E. Smith, G.C. Jensen / Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179 Morgan, S.G., 1989. Adaptive significance of spination in estuarine crab zoeae. Ecology 70 (2), 464–482. Spooner, G.M., 1933. Observations on the reactions of marine plankton to light. J. Mar. Biol. Assoc. U. K. (New Ser.) 19 (1), 385–438. Sulkin, S.D., 1984. Behavioral basis of depth regulation in the larvae of brachyuran crabs. Mar. Ecol. Prog. Ser. 15, 181–205. Toonen, R.J., Grosberg, R.K., 2011. Causes of chaos: spatial and temporal genetic heterogeneity in the intertidal anomuran crab Petrolisthes cinctipes. In: Koenemann, S., Held,
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C., Schubart, C. (Eds.), Phylogeography and Population Genetics in Crustacea. CRC Press, pp. 75–107. Weldon, W.F.R., 1889. Notes on the function of the spine of the crustacean zoeae. J. Mar. Biol. Assoc. U.K. 1 (2), 169–171. Wing, S.R., Botsford, L.W., Ralston, S.V., Largier, J.L., 1998. Meroplanktonic distribution and circulation in a coastal retention zone of the northern California upwelling system. Limnol. Oceanogr. 43, 1710–1721.
Contents lists available at ScienceDirect
Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe
The role of carapace spines in the swimming behavior of porcelain crab zoeae (Crustacea: Decapoda: Porcellanidae)☆ Anna E. Smith a,⁎, Gregory C. Jensen b a b
Bamfield Marine Science Centre, 100 Pachena Road, Bamfield, BC V0R 1B0, Canada School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195, United States
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 7 June 2015 Accepted 8 June 2015 Available online 16 June 2015 Keywords: Porcellanidae Anomura Larval transport Zoeae Swimming Petrolisthes cinctipes
a b s t r a c t Porcelain crab zoeae display vastly different morphological and swimming characteristics compared to other crab zoeae. Porcellanids have highly elongated anterior and posterior carapace spines and swim with smooth lateral directionality. While long spines have been shown to aid in predator deterrence in crab larvae, there is no empirical evidence regarding their effect on swimming behavior. To investigate this possible association, the carapace spines of 40 zoeae of the porcelain crab, Petrolisthes cinctipes (Randall, 1840), were experimentally altered and changes in swimming behavior, spatial orientation and speed were recorded. When anterior, posterior, or both sets of spines were removed, the larvae swam with significantly reduced lateral speed. Ablation of only the anterior (rostral) spine resulted in a loss of depth control and relegated forward progress to erratic, ineffective lurches. Removal of the posterior spines resulted in an inability to swim backward, and reduced the ability to maintain a straight trajectory while swimming forward. There was no visible pitching of the zoeae in any of the treatment groups, which suggests that balance is not the primary function of the elongated spines. This represents the first experimental evidence linking the unusual elongated carapace spines of porcellanid zoeae to their ability to swim laterally, and may help explain their ability to remain in nearshore coastal waters throughout their development. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The zoeal stages of porcelain crabs are among the most spectacular of crab larvae, having extremely elongated rostrums and long, closely appressed pairs of posterior spines. Many crab zoeae (the first true free-swimming stage of crab larval development) have long carapace spines that have been shown to effectively deter gape-limited predators like small fish (Morgan, 1989). Long spines could fulfill additional functions as well, such as reducing sinking rate and facilitating locomotion (Chia et al., 1984; Gurney, 1902; Weldon, 1889). Gonor and Gonor (1973b) anecdotally attributed the predominately forward and backward swimming of porcellanid zoeae to the configuration of the carapace spines and reported that porcellanid zoeae swam equally well in both directions, demonstrating a high level of maneuverability. In contrast, Foxon (1934) noted that the long rostral spine made them “unwieldy creatures” unable to turn quickly, and that removal of the rostrum allowed them to behave in a manner similar to other anomuran larvae. Morgan (1989) removed spines from Rhithropanopeus harrisii
☆ All authors agree to the submission of this article for publication in the Journal of Experimental Marine Biology and Ecology. ⁎ Corresponding author at: 13814 116 Ave., Surrey, BC V3R 2T2, Canada. E-mail addresses: [email protected] (A.E. Smith), [email protected] (G.C. Jensen).
http://dx.doi.org/10.1016/j.jembe.2015.06.007 0022-0981/© 2015 Elsevier B.V. All rights reserved.
(Gould, 1841) zoeae in relation to predator defense and found no effect on sinking rate or stabilization of zoeae, unlike Foxon (1934) who suggested that the long spines of porcellanid zoeae appear to slow their sinking rate. These contrasting studies highlight the uncertainty surrounding the role of carapace spines in the locomotion of zoeae. The motility of crab zoeae is typically limited to vertical movements, using a combination of active upward swimming and passive sinking (Sulkin, 1984). Some zoeae can control their horizontal position through species- and stage-specific responses to stimuli, such as light or pressure, which enable them to associate with water masses that are moving in a particular direction (Spooner, 1933). For example, the positively phototaxic early zoeal stages of the blue crab, Callinectes sapidus Rathbun, 1896 are exported from their natal estuary via outward-flowing surface water (Epifanio et al., 1984), while the equivalent stages of the estuarine mud crab R. harrisii are retained in the estuary by seeking deeper water (Cronin, 1982). Although such behaviors are adaptive under typical conditions, anomalous weather or current patterns can result in recruitment failure due to larvae being carried too far away from appropriate habitat for settlement. Thus, cancrid crab larvae may find themselves over 100 km offshore by the time they reach the megalops stage, with little or no chance of returning to nearshore habitats to settle (Jamieson and Phillips, 1988). In contrast, porcelain crab zoeae remain relatively close to shore. This tendency has been recorded along the western coast of North
176
A.E. Smith, G.C. Jensen / Journal of Experimental Marine Biology and Ecology 471 (2015) 175–179
America, including northern California (Wing et al., 1998) and off Oregon, where porcellanid larvae were most abundant 1.8–5.5 km from shore and rarely taken beyond 18 km (Lough, 1975). Similarly, zoeae of the flat porcelain crab Petrolisthes cinctipes predominated in plankton samples taken within 8 km of shore in British Columbia, Canada (Jamieson and Phillips, 1988). Samples taken further from shore were dominated by cancrid zoeae which primarily swim vertically. Porcellanid zoeae, however, are strong horizontal swimmers (Gonor and Gonor, 1973b) and it has been suggested that this may account for their ability to remain in nearshore areas (Jensen, 1991). To investigate the possible relationship between the elongated spines of porcellanid zoeae and their ability to swim laterally, P. cinctipes zoeae were subjected to different spine removal combinations. Zoeae with all spines removed were expected to either revert to the erratic vertical swimming of other species of crab zoeae, or to be unable to swim at all. The effects of removing the anterior and posterior spines in isolation were less predictable but it was anticipated that the maneuverability and swimming speed of the zoeae would be impacted if the adaptive significance of porcellanid zoeae spines was linked to locomotion. 2. Materials and methods 2.1. Collection of specimens Porcelain crab larvae were collected from the mouth of Bamfield Inlet, British Columbia, Canada June 22–24, 2012 (48°50.32N, 128°8.25S). The collections were conducted at 02:00 to ensure complete darkness and to allow sufficient time for the zoeae to complete their diurnal migration to the surface. The zoeae were collected over three nights using a 560 μm plankton net towed at the surface. Each collection consisted of five 6-min tows. The samples were then sorted using a dissecting scope and all porcellanid zoeae were isolated. The zoeae were identified according to chromatophore patterns as described by Gonor and Gonor (1973a). Four species were collected: Pachycheles rudis Stimpson, 1859, Pachycheles pubescens Holmes, 1900, Petrolisthes eriomerus Stimpson, 1871, and P. cinctipes. The latter was the most abundant species in the samples and was therefore chosen as the focal species in this study. The zoeae were kept in four circular glass containers, each approximately 30 cm in diameter and containing 10 cm of water, within an 11 °C water table and were fed newly-hatched Artemia nauplii ad libidum before and between experiments. The zoeae were given 24 h to acclimate to the lab conditions prior to any experimentation or manipulations.
fine tweezers. The spines were removed with scissors with the aid of a dissecting scope. The rostral spine was cut 1 mm forward of the carapace to ensure no injury to the first antennae or cephalothorax. The posterior spines were cut in unison, also 1 mm away from the posterior margin of the carapace (Fig. 1). After 5 h of recovery time, the swimming behavior of each zoea was observed with attention to movement ability and technique. 2.3. Swim speed measurement To test lateral swimming speed, each zoea was placed in a circular, 30 cm diameter glass culture dish centered on a template marked with concentric circles with radii of 5, 10, and 15 cm. The swimming behavior was recorded using a digital camera with video capability (Nikon D5100 with an 18–55 mm lens). The videos of each zoea were reviewed and the most direct and continuous passage across two concentric circles was chosen for each individual. The trajectory across the circles was measured to the nearest 0.5 mm. The time elapsed was recorded to the nearest millisecond using the time record on the videos. Velocity v was calculated as distance traveled d divided by time elapsed t and was converted to standard units of m/s. The velocity data for all three treatment groups were not normally distributed, showing a skew to the right and the variances differed among groups by an order of magnitude. A log-transformation of the data corrected both issues allowing the use of a one-way ANOVA. 2.4. Measurement of upright orientation Porcellanid zoeae normally swim with the dorsal part of their carapace facing up (Gonor and Gonor, 1973b). To gauge the possible role of spines in maintaining this orientation, the amount of time each zoea spent on its back (or side) during a period of five minutes was recorded. The same zoeae from the previous tests were used in this experiment. The zoeae were individually pipetted into 30 cm diameter glass dishes and stimulated to swim by gently lifting them into the water column with a pipette. Stopwatches were used to keep track of the time spent in each swimming position. Each zoea was placed into the still water bowl, which was filled to a depth of 10 cm, and the zoea was lifted to a depth of 1 cm below the surface for each individual trial. Only one zoea was tested at a time. The amount of time the zoeae spent upsidedown or sideways was compared among treatments using a Kruskal– Wallis non-parametric test because transformations could not correct the unequal variance and the non-normality of the data. The four groups were then compared against one another using a Dunn's pairwise multiple comparison test.
2.2. Spine removal 3. Results Ten stage II zoeae were randomly assigned to each of four experimental groups to test the effect of spine removal. The control group was left fully intact to gauge normal swimming behavior and speed. The treatment groups consisted of zoeae with (1) the posterior spines removed, (2) the anterior rostral spine removed, and (3) both anterior and posterior spines removed. Individual zoea were placed in a slotted microscope slide containing water and held by the rostral spine with
3.1. Qualitative observations of swimming behavior Intact zoeae exhibited smooth lateral swimming behavior, moving with equal proficiency in both forward and reverse directions. Intact zoeae also displayed quick changes in left–right direction, even when traveling at full swimming speed. They changed their swimming
Fig. 1. Spine removal sites (vertical lines) on a zoea of the porcellanid crab Petrolisthes cinctipes. Scale bar represents 1 cm.
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direction by abruptly swimming backwards, forwards and then backwards again while turning horizontally slightly with each movement. The intact zoeae swam straight trajectories and maintained a constant depth in the water column. Zoeae with the anterior rostral spine removed retained the ability to swim backwards but were no longer able to maintain a straight trajectory. Forward swimming was reduced to short, erratic bursts and they appeared to have little directional control. None of the 10 individuals were able to maintain a constant depth in the water column and often spiraled vertically. Uncontrolled trajectories were also present in the swimming behavior of the 10 zoeae with the posterior spines removed. These zoeae, however, were able to maintain constant levels in the water column with limited vertical spiraling. The zoeae exhibited loss of backwards motion and were unable to swim forward in straight lines, instead swimming in large circles. The swimming ability of the 10 zoeae with both anterior and posterior spines removed was reduced to slow spinning upside down on the bottom of the dish. None of the 10 zoeae were successful in leaving the bottom and swimming vertically or laterally. When the zoeae were stimulated to swim by being lifted into the water column, they were unable to maintain the positioning and spiraled back down to the bottom without successful swimming motion. Any attempts to swim upwards resulted in short-lived erratic spinning and no gain in elevation. All of these zoeae died in less than 12 h, whereas all unilaterally ablated zoeae survived and molted normally to the megalops stage. 3.2. Quantitative differences in swimming behavior There were significant differences in lateral swimming speed among the treatment groups (ANOVA, df = 3, p b 0.001; Fig. 2). All groups were found to be significantly different from one another, with the exception of anterior spine versus posterior spine treatments. Intact zoeae were significantly faster than all other treatments, and zoeae with anterior spines removed or posterior spines removed were both significantly faster than those with all spines removed. Significant differences were also present among the four groups in the amount of time the zoeae spent on their back or side (Kruskal–Wallis test, df = 3, p b 0.001; Fig. 3). All groups were found to be significant from one another, except anterior spine removed versus posterior spines removed. 4. Discussion All altered P. cinctipes zoeae swam in an erratic, disordered manner, had difficulty maintaining an upright orientation, and swam more
slowly than intact zoeae. These differences support the hypothesis that the anterior and posterior spines are used in conjunction to maintain the smooth lateral swimming observed in porcellanid zoeae. The morphological and behavioral similarity between the zoeae of several porcellanid species suggests that the findings can be generalized to other porcellanids (Gonor and Gonor, 1973b). The difference in control between zoeae missing the posterior spines and those with the anterior spine removed suggests that the spines are used for different functions in swimming. Zoeae with the posterior spines removed were unable to move in reverse or travel in a straight line and moved in large circles. This suggests that the posterior spines may function in a rudder-like fashion, allowing zoeae to make path corrections. The change in backwards swimming ability suggests that the posterior spines also serve to make the zoeae hydrodynamically streamlined from both the anterior and posterior directions. As there was no visible pitching of the zoeae with the removal of the posterior spines, it is unlikely that the spines are an alternative balancing system. Similarly, there was no pitching seen in the zoeae with the anterior spine removed. These zoeae had difficulty in maintaining a constant depth in the water column, often weaving up and down throughout the levels. This was in sharp contrast to those with only the posterior spines removed that were able to maintain a constant depth. The trajectory of the zoeae with the anterior spine removed was erratic, lacking the smooth motion seen in the intact and posterior spine removed groups. This alteration in swimming behavior indicates that the anterior spine may be responsible for both the trajectory of the zoeae as well as positioning in the water column. The complete loss of swimming ability in the zoeae with both posterior and anterior spines removed is further indication that the spines are connected with horizontal swimming ability. Had the zoeae switched to swimming vertically in a manner similar to zoeae of other crab species, more conclusive connections between lateral swimming and elongated spines could be made. However, it is possible that the behavioral differences observed in this group were the result of the trauma caused by the additional handling and ablation, as none in this group survived longer than 12 h. It is possible that the double ablation resulted in too much hemolymph loss, or that the inability to swim adversely affected respiration. Ideally, this would have been controlled for by including a treatment that involved ablation and re-attachment of anterior and posterior spines. There are additional differences in body structure between these zoeae and non-porcellanids that may also influence their swimming. Porcellanid zoeae have a horizontally angled cephalothorax (Fig. 1), giving them a more elongated profile, whereas other crab zoeae have a rounder, more vertically oriented conformation. Variation in the
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Spine Treatment Fig. 2. Lateral swimming speed of zoeae of the porcellanid crab Petrolisthes cinctipes in relation to spine removal treatment. Means are shown ±1 SE. N = 10 in all cases.
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Time Spent on Back or Side (s/5 min)
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Spine Treatment Fig. 3. Time (seconds) spent on the back or side during a period of 5 min: an assessment of changes in spatial orientation of zoeae of the porcellanid crab Petrolisthes cinctipes in relation to spine removal treatment. Means are shown ±1 SE. N = 10 in all cases.
morphology and action of the swimming apparatus (maxillipeds, telson, and abdominal motions) between species may be contributing factors, or the differences may be mostly due to hydrodynamic effects of the different body conformations. The differences in swimming speed and spatial orientation between control and manipulated zoeae also point to an association between the elongated spines and locomotion. The significant decrease in lateral swimming speed of all treatment groups compared to the control group of intact zoeae provides evidence that the elongated spines contribute to the fast horizontal swimming capabilities of these zoeae. Indeed, the mean lateral swimming speed of the uncut zoeae was over twice the speed previously recorded for crab zoeae (Chia et al., 1984). The removal of either the front or rear spines hampered swimming in different ways but to approximately the same extent in terms of speed. The removal of both spines rendered the zoeae incapable of swimming. The removal of any combination of spines also resulted in a reduced ability to maintain spatial orientation, the most drastic of which was seen in the spineless zoeae. The lack of tilting seen in the two single direction spine ablation groups indicates this reduction in gravitational orientation is more likely due to a reduction in sensory information or gravitational interaction than to a change in balancing ability.
5. Future directions The association between the elongated spines of porcellanid zoeae and lateral swimming could be further examined by studying water flow patterns around the spines. The hydrodynamic impact of the spines on changes in flow patterns with different spine removal combinations could be explored. If spines are aiding in hydrodynamics, removal of these spines should result in a reduction in streamlining and the formation of turbulence around the zoea. The connection between carapace spines and swimming behavior could be widened to investigations of other crab zoeae species to better understand their methods of movement. The forward pitch of the body in relation to the spines may also enable porcellanid zoeae to swim laterally and with straight trajectories (Gonor and Gonor, 1973b). A study altering the length or positioning of spines of other species of crab zoeae to observe the effects on swimming could provide further evidence supporting the correlation between elongated spines and lateral swimming. In addition to swimming studies, the role of the anterior spine in sensory recognition and escape from predators could be examined.
P. cinctipes zoeae are transported by currents north and south along the Pacific coast (Toonen and Grosberg, 2011), and their strong horizontal swimming capabilities may be key to their ability to remain fairly close to shore. This raises intriguing questions about the possible cues they are using for orientation as they maintain their position. Since porcellanids are a diverse and widespread group, it would be interesting to see if species in other parts of the world exhibit similar patterns of nearshore retention. Many unanswered questions remain as to the intricacies of porcellanid swimming behavior and the evolutionary pressures that caused these morphological features to arise. Acknowledgments We would like to thank Helen Yip for her continual willingness to assist at all stages of this research and Isabelle Côté for her valuable support and knowledge. We would also like to express our sincere gratitude to the staff of the Bamfield Marine Sciences Centre for their support and for the use of their facilities. Also thank you to midnight skippers Gordon Byron and Gavin Brackett. Thank you to Trudi Smith for assisting in preparation of the manuscript and to Melinda Smith for digitizing the zoea drawing. We would also like to thank Dr. Sandra E. Shumway and an anonymous reviewer for their insightful and detailed comments, which helped to improve our manuscript. [SS] References Chia, F.-S., Buckland-Nicks, J., Young, C.M., 1984. Locomotion of marine invertebrate larvae: a review. Can. J. Zool. 62, 1205–1222. Cronin, T.W., 1982. Estuarine retention of larvae of the crab Rhithropanopeus harrisii. Estuar. Coast. Shelf Sci. 15, 207–220. Epifanio, C.E., Valenti, C.C., Pembroke, A.E., 1984. Dispersal and recruitment of blue crab larvae in Delaware Bay, U.S.A. Estuar. Coast. Shelf Sci. 18 (01), 1–12. Foxon, G.E., 1934. Notes on the swimming methods and habits of certain crustacean larvae. J. Mar. Biol. Assoc. U. K. (New Ser.) 19 (02), 829–849. Gonor, S.L., Gonor, J.J., 1973a. Descriptions of the larvae of four North Pacific porcellanidae (Crustacea: Anomura). Fish. Bull. 71 (1), 189–223. Gonor, S.L., Gonor, J.J., 1973b. Feeding, cleaning and swimming behavior in larval stages of porcellanid crabs (Crustacea: Anomura). Fish. Bull. 71 (1), 225–234. Gurney, R., 1902. The metamorphosis of Corystes cassivelaunus (Pennant). Q. J. Microsc. Sci. 46, 461–477. Jamieson, G.S., Phillips, A.C., 1988. Occurrence of Cancer crab (C. magister and C. oregonensis) megalopae of the west coast of Vancouver Island, British Columbia. Fish. Bull. 86 (3), 525–542. Jensen, G.C., 1991. Competency, settling behavior, and postsettlement aggregation by porcelain crab megalopae (Anomura: Porcellanidae). J. Exp. Mar. Biol. Ecol. 153, 49–61. Lough, R.G., 1975. Dynamics of Crab Larvae (Anomura, Brachyura) off the Central Oregon Coast, 1969–1971 (Ph. D. Dissertation), Department of Biology, Oregon State University, Corvallis, Oregon (308 pp.).
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