FILAMENTOUS ALGAE AN ADDITIONAL FOOD FOR THE PREDATORY ANOSTRACAN BRANCHINECTA GIGAS Denton Belk and Robert Ballantyne ABSTRACT We observed Branchinecta gigas Lynch of a size known to feed by raptorial predation feeding on a clump of filamentous algae in the genus Spirogyra. A review of feeding in the Anostraca Downloaded from https://academic.oup.com/jcb/article/16/3/552/2418853 by guest on 28 September 2021 points to great diversity and suggests there is much yet to be learned. The knowledge that all Anostraca start and behavior of a second predatory anos- life as filter-feeders and seem to have rather tracan, Branchinecta xerox (H. Milne Ed- similar phyllopodous feeding appendages wards). Mertens et al. (1990) and Dumont obscures what more careful investigation et al. (1994) clearly demonstrated that B. demonstrates to be a group of animals open gigs and B. ferox are not the only preda- to considerable diversity in feeding. Fryer tory anostracans. They showed that the fil- (1966) explained how the phyllopods of the ter-feeder Streptocephalus proboscideus largest anostracan, Branchinecta gigas (Frauenfeld), effectively uses suction cur- Lynch, 1937, are uniquely modified for rap- rents to capture and then eat rotifers, cili- torial predation. White et al. (1969) de- ates, the colonial flagellate Volvox sp., and scribed the hunting and capture behavior the nematode Caenorhabditi.s elegans used by this giant fairy shrimp in feeding (Maupas). The failure of S. proboscideus to on its smaller congener Branchinecta mack- capture the cladocerans or copepods offered ini Dexter and copepods of the genus Diap- in experimental containers by Mertens et al. toms. These authors observed that water (1990), while Fryer (1983) reported that B. currents produced by the trunk appendages ferox fed on such animals in its native hab- of B. gigas were not sufficiently strong to itat, can easily be explained by size differ- overcome the escape reactions of the prey ences. Streptocephalus proboscideus is a animals used in their study. This observa- moderate-sized anostracan, 10-25 mm, tion indicates that B. giga.s does not make while B. ferox is the second largest species use of the suction current predation method known, growing to around a length of 70 discussed below and refutes the hypothesis mm. Size influences suction-current of Anderson (1970) that B. gigs probably strength. Size also influences the strength, uses filter-feeding to capture smaller non- dimensions of potential escape openings, anostracan crustaceans. Daborn (1975) and volume of the cage formed around prey studied the life history of B. giga.s, and by the phyllopods. found it to be a filter-feeder up to about 8 In addition to straining and capturing mm in length. At all larger sizes he consid- food of various types from open water, ered the species to be "exclusively preda- many anostracans scrape surfaces or stir up ceous"; a conclusion which does not agree bottom materials. Lowndes (1933) wrote with Fryer's (1966) observation that 40- concerning feeding in Chirocephalus dia- and 50-mm specimens contained gut ma- phanus Prevost "The animal certainly does terials, suggesting that they had been feed- extract from the water minute particles, and ing by scraping the sediments. it does this by means of a very wonderful Fryer (1983) described the suction cur- filter apparatus, but the greater part of its rent-based predatory feeding morphology food consists of material that can only be —> Fig. 1. Ventral view of the head of a 29-mm female Branchinecta gigas with the labrum removed. The mouth is near the upper left-hand corner of the photograph under the remaining portion of the proximal end of the labrum and just anterior to the right mandible (M). Spirogyra sp. is in the food groove between the paragnaths (p), maxillules (ml), and maxillae (m2). Scale bar = 500 u.m. Fig. 2. Transverse section of the gut of a 29-mm female Branchinecta gigas showing three algal filaments. Scale bar = 50 lim. Downloaded from https://academic.oup.com/jcb/article/16/3/552/2418853 by guest on 28 September 2021 taken from the bottom of the pond, except with their ventral surfaces pressed against a during those periods when the water is large clump of filamentous algae. They muddy." Fryer (1966) discussed detrital were near shore in water about 10 cm deep feeding in anostracans. He noted Branchi- and seemed to be pulling at the algal mass necta packardi Pearse spent as much as with their phyllopods. Belk captured these 35% of the time during which he observed three individuals and immediately fixed it actively scraping. Daborn (1978) ob- them in 70% ethanol. He also collected and served males of Branchinecta paludosa preserved a sample of the alga. The SEM (Muller) to scrape mud or detritus from a photographs below show that these anostra- pool bottom, while females more often cans were feeding on a clump of SpirogyraDownloaded from https://academic.oup.com/jcb/article/16/3/552/2418853 by guest on 28 September 2021 scraped rock surfaces. Daborn (1979) re- sp. ported differences in limb form and setation METHODS AND MATERIALS to be the most likely factors influencing dif- ferences in feeding mode. The differences Robert Ballantyne prepared the above mentioned occurred both among species and between specimens for SEM study. Keeping each in 70% eth- anol, he used a microdissection kit to dissect away the sexes in several species. labrum, and a razor blade to make various sections, Mertens et al. (1990) cautioned that gut exposing the gut. He dehydrated the selected samples content analysis may at best only partly re- in a graded ethanol series followed by critical-point veal anostracan diet by missing soft-bodied drying, coating with gold, and viewing with a Hitachi animals such as ciliates. These they could S430 Scanning Electron Microscope. not find by such analysis, even though their RESULTS study methods indicated that these animals were being eaten. They also demonstrated We found filamentous algae (Spirogyra that Streptocephalus proboscideus ingested sp.) under the labrum in the food grove be- exuviae of Daphnia sp. which then showed tween the mouth parts of the B. gigas col- up in gut analysis, but that this anostracan lected in what appeared to be the act of was unable to capture and eat Daphnia. feeding on an algal mass (Fig. 1). We also With these cautionary observations in found filamentous algae in their guts (Fig. mind, we note that several workers have re- 2). ported finding filamentous algae in the guts DISCUSSION of anostracans. Bernice (1971) found such material in Streptocephalus dichotomus Based on our observations, Branchinecta Baird. Fryer (1966) reported finding frag- gigas of a size known to feed by raptorial ments of a filamentous alga in the gut of a predation (Daborn, 1975) can intentionally small individual of the predator Branchi- feed on filamentous algae. While we do not necta ,ferox. He did not consider this very know how common filamentous algae may informative, since these algal filaments be in the diet of this species, we suspect could have been taken inadvertently along that the food habits of B. gigas are more with other detrital material. Lowndes varied than generally believed. Daborn (1933) found filamentous algae in the gut (1975) noted that in captivity large B. gigas of Chirocephalus diaphanus. Not stopping will apparently eat any living material it is here, Lowndes went a step further and ob- able to catch between its legs. served that live C. diaphanus, which he ACKNOWLEDGEMENTS tethered for ease of observation under a dis- secting microscope, dragged algal filaments Denton Belk thanks Wanda Deal for showing him fairy shrimp habitats at Edwards Air Force Base and offered to them down into the food grove giving him the opportunity to make the observations using the endites of their legs. Once in the that became the subject of this paper. Robert Ballan- food grove the algal filaments were passed tyne thanks Dr. Douglas Alexander for introducing him along to the mouth. to vernal pool ecosystems and Dr. Richard Demaree for technical support. While looking at anostracans in ephem- eral pools and playas at Edwards Air Force LITERATURE CITED Base, California, U.S.A., on 2 February 1995, Denton Belk observed three 29-mm Anderson, R. S. 1970. Predator-prey relationships and predation rates for crustacean zooplankters from individuals (two females, one male) of the some lakes in western Canada.-Canadian Journal raptorial fairy shrimp Branchinecta gigas of Zoology 48: 1229-1240. Bernice, R. 1971. Food, feeding and digestion in . 1983. Functional ontogenetic changes in Streptocephalus dichotomus Baird (Crustacea: An- Branchinecta ferox (Milne-Edwards) (Crustacea: ostraca).-Hydrobiologia 38: 507-520. Anostraca).-Philosophical Transactions of the Roy- Daborn, G. R. 1975. Life history and energy relations al Society of London, B. Biological Sciences 303: of the giant fairy shrimp Branchinecta gigas Lynch 229-343. 1937 (Crustacea: Anostraca).-Ecology 56: 1025â�� Lowndes, A. G. 1933. The feeding mechanism of 1039. Chirocephalus diaphanus Prevost, the fairy â��â��â�� . 1978. Distribution and biology of some shrimp.-Proceedings of the Zoological Society of nearctic tundra pool phyllopods.-Verhandlungen London (B): 1093-1118. der internationale Vereinigung fiir theoretische und Mertens, J., N. Munuswamy, C. De Walsche, and H. angewandte Limnologie 20: 2441â��2451. J. Dumont. 1990. On predatory tendencies in the â��â��â�� . 1979. Limb structure and sexual dimorphism feeding ecology of the fairy shrimp Streptocephalus Downloaded from https://academic.oup.com/jcb/article/16/3/552/2418853 by guest on 28 September 2021 in the Anostraca (Crustacea).-Canadian Journal of proboscideus (Frauenfeld, 1873) (Crustacea: Anos- Zoology 57: 894-900. traca).-Hydrobiologia 198: 119-123. Dumont, H. J., A. J. Ali, S.