View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Dispatch R593 action-specific reward values in the 13. Rabin, M. (1993). Incorporating Department of Psychological and Brain striatum. Science 310, 1337–1340. Fairness into Game Theory and Sciences and Krieger Mind/Brain 11. Matsumoto, K., Suzuki, W., and Economics. Am. Econ. Rev. 83, Institute, Johns Hopkins University, Tanaka, K. (2003). Neuronal correlates of 1281–1302. goal-based motor selection in the 14. de Quervain, D.J., Fischbacher, U., 3400 North Charles Street, Baltimore, prefrontal cortex. Science 301, Treyer, V., Schellhammer, M., Maryland 21218, USA. 229–232. Schnyder, U., Buck, A., and Fehr, E. E-mail: [email protected] 12. Camerer, C.F. (2003). Behavioral Game (2004). The neural basis of altruistic Theory (Princeton: Princeton University punishment. Science 305, Press). 1254–1258. DOI: 10.1016/j.cub.2006.07.005 Phylogeny: The Continuing chaetognaths, although it bears some resemblance to the nervous Classificatory Conundrum of systems of nematodes, kinorhynchs and priapulids [1]. The Chaetognaths age of the phylum is borne out by fossils from the Cambrian Burgess Shale which show a remarkable The phylogenetic conundrum posed by the Chaetognatha, a cryptic resemblance to modern phylum consisting largely of planktonic predators, is the subject of two chaetognaths [3] (Figure 2), short papers in this issue of Current Biology. These analyses go some suggesting chaetognaths have way towards defining the phylogenetic position of the chaetognaths, existed without major which possess features apparently spanning the protostome/ morphological change for at least deuterostome divide. 500 million years. Chaetognaths are unusual among marine Eldon E. Ball1,* and David J. Miller2 of the chaetognaths (Figure 1A,B) invertebrates in that they are direct is relatively constant across the developers, hatching from the egg It would be difficult to invent a more phylum, although details may vary essentially as small adults rather bizarre group of creatures than the considerably. The structure of the than as a distinct larval stage. chaetognaths or arrow worms, nervous system reveals little about Chaetognaths appear to rely (literally ‘bristle jaws’). The the evolutionary affinities of mainly on vibrations for information chaetognaths are an ancient lineage of invertebrates that shares Figure 1. Chaetognath mor- A BC phology. some characteristics with just Grasping (A) The deep water chaetog- spines about every other major Ciliate corona invertebrate phylum and has nath Eukrohnia sp. from the Sargasso Sea (Photo Collarette consequently puzzled taxonomists courtesy Russ Hopcroft/ Intestine ever since its original description in UAF/NOAA/CoML). In con- 1769. Darwin described trast to most shallow water forms, this species is chaetognaths as ‘‘remarkable for Hair fans pigmented, making the in- the obscurity of their affinities’’ and Muscles they have puzzled a succession of ternal organs easily visible. (B) This labelled drawing of Lateral eminent zoologists ever since [1]. the pelagic chaetognath, fin Though unfamiliar to most Heterokrohnia, provides a good introduction to the biologists, chaetognaths are Ovary typically the most abundant basic features of chaetog- Anus planktonic predators, sometimes nath anatomy. Some au- Posterior septum accounting for more than 10% of thors divide the body into head and trunk, while others Testis zooplankton biomass and being add a tail, dividing the trunk outnumbered only by their major at the posterior septum. Muscles prey, the copepods. Approximately There are three body cavi- Median 20 genera have been described, ties, which, together with mesentery containing more than 100 the cuticle and the longitudi- Caudal species ([2], see also: http:// nal muscles, form a hydro- static skeleton. One or two fin academic.evergreen.edu/t/ pairs of longitudinal fins pro- Current Biology thuesene/chaetognaths/ vide stabilisation, while the chaetognaths.htm). flattened caudal fin provides locomotory power. There is no circulatory system and no Although most chaetognaths specialised excretory system. The anterior grasping spines used in prey capture are are only a few mm in size and shown spread, although they are normally folded under a hood, which presumably planktonic, there are some inshore serves the dual purpose of protecting them and streamlining the head during swimming. Eggs are released into the thoracic coelomic space and sperm into the coelomic space of benthic and deep-water species the tail. Modified from [20], with permission. (C) Drawing of Sagitta hispida showing the that may be much larger, up to 12 closely spaced hair fans which presumably provide the sensory input required for such cm in length. The basic body plan complex behaviours as mating and prey capture. Modified with permission from [4]. Current Biology Vol 16 No 15 R594 Grasping Figure 2. Fossil chaetog- months in the upper 100 m, spines naths. descending to the 100–250 m The assumption that mod- zone during the winter. Transverse ern chaetognaths epitomize muscles All chaetognath species that an ancient and successful Intestine anatomical design, which have been studied are has remained unchanged hermaphrodites with testes in the Ventral ganglion since the Cambrian, is con- tail and ovaries in the posterior firmed by beautiful fossils trunk, the latter maturing only after Ovaries from the Burgess Shale, the tail coeloms are filled with Anus such as this Oesia disjuncta, sperm. Self-fertilization has been in which many modern Lateral described in Sagitta, but it is fins anatomical features have been tentatively identified unclear whether this occurs in B Seminal (A–C). Modified with permis- nature. Mating specimens of vesicle sion from [3]. Spadella cephaloptera become Caudal A fin aligned head to tail in parallel and C simultaneously exchange sperm Current Biology packets [9], although one-way transfer has also been reported. about their environment and they or glass probe vibrating at an The most amazing mating can hunt in darkness. Their appropriate frequency and behaviour, starting with a dance vibration sense is mediated amplitude with the same ferocity as and ending with a leap to through fans of mechanoreceptive it will attack a copepod [6]. deposit the sperm bundle or hairs [4] (Figure 1C) which Chaetognaths swallow their prey spermatophore, has been apparently play critical roles in all whole and will apparently eat described for Paraspadella gotoi behaviours. In addition to these anything that can be [10] (pictured at http:// hairs, chaetognaths have two eyes. accommodated in their mouth. As www.uibk.ac.at/zoology/ultra/ In some genera the eyes are the chaetognath strikes its prey, evol_ecol/former_research/ inverted, with the photoreceptor the hood over the grasping spines paraspadella/mating/ cells pointing inward and is withdrawn, allowing them to fly mating_behaviour.html). This embedded in a single large outward and form a net that complex behaviour is pigment cell while in others the surrounds the prey and brings it probably mediated by the eyes are everted with the into contact with the formidable mechanoreceptive hair fans [4]. photoreceptors pointing outward. teeth around the mouth. As if these Probably the strangest The eyes have relatively few impressive weapons weren’t chaetognath of all is a deep-sea receptor units and lack lenses, so enough, at least some form, Bathybelos typhlops, they apparently are not chaetognaths appear to use the described originally from a single image-forming. However, two potent neurotoxin tetrodotoxin to specimen from a depth of 2,500 m features that appear to be unique stun potential prey [7]. The exact in the Gulf of Mexico. This to chaetognath eyes are the source of the venom was not specimen showed many unusual conical body, for which some identified but it is most likely features including rayless lateral have suggested an optical role, produced by symbiotic bacteria fins, an elongated head and and the unusual lamellar and injected from one of the a dorsal nervous system, in organization of the distal segment structures surrounding the contrast to the usual ventral one. of at least some species [5].In mouth. Bathybelos is apparently addition, their photoreceptor cell is Chaetognaths seem to spend a bottom-living form, living with clearly ciliary, in contrast to the most of their time gliding on their the ventral side pressed into rhabdomeric photoreceptors fins, until eventually they begin to bottom sediments containing very found in many invertebrates. sink. Sinking is counteracted by low oxygen [11]. Chaetognaths Based on this eye structure, swimming with a series of short, lack a circulatory system, so their chaetognaths should have an rapid darts, followed by another tissues are dependent on opsin characteristic of ciliary type glide. Some species show diurnal diffusion for oxygen. Under such eyes, thus adding to the other vertical migrations, tending to conditions, there would molecular evidence against maintain themselves in a constant presumably be selective pressure ecdysozoan affinities. twilight zone. There is also a for the normally ventral nervous tendency for larger chaetognaths system to move dorsally. Behaviour – Feeding, Sexual to occur at greater depths. More specimens of benthic and Other Weirdness According to Marshall [8], Sagitta deep-water chaetognaths are Chaetognaths are remarkable gazellae in the Southern Ocean eagerly awaited. killers that appear to hunt primarily sink deeper in the water column by sensing vibration using the as they mature, releasing their Where Do Chaetognaths Fit in hair fans (http://www.cc.mie-u.ac. eggs at about 1000 m. The eggs the Scheme of Living Things? jp/wsagitta/lab.html). The are buoyant and float upward, Ever since their discovery, the chaetognath Spadella hatching at about 250 m. The phylogenetic position of cephaloptera will attack a metal hatchlings spend the summer chaetognaths has posed Dispatch R595 a conundrum. In particular, it has gene set. The fact that ‘blind’ particularly of benthic deep-water never been clear to which of the analyses based on many other species, are eagerly awaited.