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Predation on <I>Littorina Irrorata</I> (Mollusca:Gastropoda)

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Predation on <I>Littorina Irrorata</I> (Mollusca:Gastropoda) BULLETIN OF MARINE SCIENCE, 26(3): 403-409, 1976 PREDATION ON LITTORINA IRRORATA (MOLLUSCA:GASTROPODA) BY CALLINECTES SAPlDUS (CRUSTACEA: PORTUNIDAE) Paul V. Hamilton ABSTRACT Callinectes sapidlls swim 10 Ihe surface in the intertidal zone during high tide, and remove Littorina irrorata adhering to plant stems near to and above the water line. Snails are carried to the bottom, cracked open, and eaten. Laboratory observations indicate specialized roles for each of the morphologically different chelipeds during shell opening. Concentration of predation on snails of a particular relative size is suggested by analysis of opercula in cardiac stomachs, and may be partly related to maximizing the amount of food obtained per unit time. The movement of L. irrorata up plant stems on an advancing tide probably serves as a defense against predation by C. sapidlls, Melongena corona, and possibly other predators. The marsh periwinkle, Littorina irrorata and Neritina virginea were common items in (Say, 1822), is probably the most abundant the largely molluscan diet of C. sapidus in gastropod occupying the low energy inter- Louisiana. Tagatz (1968) states that Nas- tidal zones of the northeastern Gulf of sarius obsoletus is occasionally found in the Mexico. Sample densities on barrier beaches guts of blue crabs from the St. Johns River. may range from less than 100 snails per m~ Tagatz and Hall (1971) list several refer- at mean sea level, to almost 600 snails per ences to blue crab predation on commer- m2 at mean high water level, thus indicating cially important bivalves. a large potential food source for predators. This paper documents and describes the This snail feeds on the substrate during low frequent pred~tion on L. irrorata by C. tide and ascends stems of the upper inter- sapidus, and considers the function of the tidal plants, Spartina alterniflora (the dom- vertically oriented tidal movements of L. inant vegetation), Salicornia perrenis, and irrorata. ] uncus roemerianus during high tide. They usually remain about 15 cm above the water GENERAL OBSERVATIONS lihe (plant height permitting) during high Periodic daytime observations of blue tide, and often secrete a mucous holdfast crab activity were made along the barrier and seal the shell aperture with the oper- beach intertidal zone of Goose Creek Bay, culum, thus minimizing desiccation (Bing- a portion of the St. Marks National Wildlife ham, 1972). Pettitt (1975) reviews the Refuge, on the northeastern Gulf Coast of predators (including crustaceans) of other Florida. Figure 1 shows a stylized cross species of Littorina. However, Cherr's section of the upper intertidal zone in this (1974) report of many small L. irrorata in area. A detailed ecological treatment is the guts of Fundulus simi/is is the only provided by Kurz and Wagner (1957). reference to predation on this species I Predation on L. irrorata seemed to be the found. major activity of many blue crabs invading The blue crab, Callinectes sapidus Rath- the upper intertidal zone at high tide from bun, also inhabits this area, and while often May to October. For instance, all 34 crabs considered only a scavenger, it is also known encountered along approximately 600 m of as a predator of mollusks. Darnell (1958) shoreline, during a 2-hr period one after- found that the gastropods M elampus coffeus noon, were preying on L. irrorata. Many 403 404 BULLETIN OF MARINE SCmNCE. VOL. 26. NO.3, 1976 Juncul 25 N=Sl ond Sollcarnl. Sp.rlln. 20 15 10 Figure 1. Stylized cross section of the upper inter- tidal zone in the area studied. Arrows indicate 5 area within which the accumulation of tidal debris causes shorter vegetation, and where blue crab o predation was most frequently observed. 20 30 40 50 60 CARAPACE LENGTH Imml more crabs were undoubtedly overlooked. Several crabs were observed for more than Figure 2. Size-frequency histogram for 81 blue 1 hr continuously capturing and eating snails. crabs collected in the upper intertidal zone. Mean carapace length was 43.7 mm. Shaded area indi- Predation was most frequently observed cates crabs which had opercula of Littorina irrorata near the high tide line, where the water is in their cardiac stomachs and unshaded area indi- shallowest and the density of snails is the cates crabs which lacked opercula. highest. All 34 crabs mentioned above were within about 5 m from the mean high water line. Predation was only occasionally ob- thermore, not all ascents to the surface result served near the seaward edge of the Spar tina in the capture or even displacement of a zone, where water depth is usually 80 to snail from the grass. If no snails are encoun- 100 cm at high tide. tered, the crab either returns to the bottom, Blue crabs usually ascend to the air-water walks to an adjacent stem and ascends again, interface solely by swimming, but are some- or just swims on the surface to an adjacent times aided by grasping a plant stem with stem and resumes searching. It is not known the walking legs. Once at or near the sur- if crabs can detect the location of individual face, crabs hold onto a stem with the walking snails from the bottom, but if so, visual legs and remove a snail from the stem, perception would seem to be the probable usually with both chelipeds. This sometimes mechanism. involves extending a cheliped as far as 7 em A size-frequency histogram for 81 crabs above the water line. However, periodic collected by dip net amidst the vegetation killbacks of Spartina caused by the accumu- in the upper intertidal zone is shown in lation of tidal debris (Kurz and Wagner, Figure 2. Forty-seven crabs were males, 26 1957) result in shorter stems near the high were immature females, and 8 were mature tide line. Consequently many L. irrorata females (all sex determinations based only are unable to move above the water line at on abdomen shape). Six of the eight mature high tide, making them more accessible to females were parasitized by rhizocephalans, blue crabs. and hence may not have originally been Successful prey capture usually consists females. Crabs subsequently found to have of holding the snail in the vicinity of the opercula of L. irrorata in their cardiac mouth and descending to the bottom. How- stomachs formed a representative sub- ever, a blue crab frequently knocks snails sample of those collected. Possession of off plant stems or loses its hold on an indi- opercula was not related to whether the vidual before returning to the bottom. Fur- major cheliped (crusher) was on the right HAMILTON: PREDATION ON LlTTORINA BY CALLINECTES 405 100 1.0 " .. MEAN ESTIMATED 80 OPERCULUM .. SHELL WEIGHT LENGTH NUMBER Imgl Imml 6 ., OF .1 11 OPERCULA .2 40 .0 +--A ::, 35 45 55 65 CARAPACE LENGTH 20 Imm) Figure 4. Relationship between carapace length and the mean operculum weight in the cardiac 0 stomachs for the 24 crabs which possessed more 0 40 60 80 than one operculum. Estimated shell lengths were CRAB NUMBER obtained as described in the text. Figure 3. Distribution of 470 opercula recovered from 81 blue crabs, ordered sequentially from the Figure 4 shows the relationship between crab with the most opercula to the crabs with no opercula (A). A similar sequentially ordered ran- carapace length and mean operculum weight dom distribution (B) and a uniform distribution for the 24 crabs having more than one oper- (C) are included for comparison. culum in their cardiac stomachs. Weights were measured to the nearest 0.05 mg with a Mettler H51 balance. The estimates of or left. Eleven of the 81 crabs possessed shell lengths (right scale) in Figure 4 were only one functional cheliped. Five were obtained from a highly significant (P < .005) missing the minor cheliped (cutter) and linear regression line (y ~ .158x - 1.38, where had 38 opercula (18, 13, 3, 3, 1) in their y = operculum weight in mg and x = shell stomachs, while six wen, mIssing the crusher length in mm) based on pairs of shell length and none had opercula. and operculum weight measurements from Figure 3 shows the distribution of the 470 50 snails of various sizes collected at Goose opercula of L. irrorata recovered from these Creek Bay. These data show a significant 81 crabs, ordered sequentially from the crab (P < .005) relationship between crab size with the most opercula to the crabs with no and mean operculum weight (and hence opercula. The marked difference between snail size). the observed distribution (A), and both a sequentially ordered, random distribution of SHELL OPENING 470 opercula among 81 crabs (B), and a Field observations of shell opening by uniform distribution of 5.8 opercula per C. sapid us were hampered by the minimum crab (C), suggests some crabs are heavily approach distance (about 2 m) before caus- involved in predation on L. irrorata, while ing an escape response, and by plant stems others are not involved at all. Crabs with- obscuring the view. However, in those crabs out opercula in their cardiac stomachs typi- clearly seen, the chelipeds manipulated the cally had large amounts of molluscan shell snail shell in the vicinity of the oral append- pieces and parts of small crustaceans. Five ages. The presence of freshly crushed shells of the six crabs having the greatest number on the substrate after most crabs moved of opercula were in the 40.0- to 44.9-mm away indicated the snails were usually eaten size-class interval, and hence were average immediately. Seven clear observations gave sized crabs (Fig. 2). a mean time from capture of individual L. 406 BULLETIN OF MARINE SCIENCE, VOL. 26, NO.3, 1976 600 commonly aperture up and apex towards the crab.
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