Euphausiidae of the Coastal Northeast Pacific

Adult Euphausiidae of the Coastal Northeast Pacific A Field Guide Eric M. Keen SIO 271 Marine Zooplankton Professor Mark Ohman Spring 2012 Note: “Euphausiid morphology” figure on inside cover was adapted from Baker et al. (1990) TABLE OF CONTENTS How to Use this Guide................................................................. i Introduction.................................................................................. 1 Taxonomy & Morphology............................................................. 1 Special Identification Techniques............................................... 4 Diversity....................................................................................... 3 Distribution.................................................................................. 5 Geographic extent........................................................... 5 Bathymetric extent......................................................... 6 Identification Keys Higher Taxon key........................................................... 9 Generic key..................................................................... 10 Euphausia....................................................................... 15 Nematobrachion............................................................. 18 Nematoscelis................................................................... 19 Stylocheiron.................................................................... 20 Thysanoessa.................................................................... 21 Thysanopoda................................................................... 24 Life History Notes Euphausia....................................................................... 28 Nematobrachion.............................................................. 30 Nematoscelis.................................................................... 31 Nyctophanes..................................................................... 32 Stylocheiron..................................................................... 33 Tessarabrachion.............................................................. 34 Thysanoessa..................................................................... 34 Thysanopoda.................................................................... 38 Append ix I: Species list of worldwide Euphausiids.................... 41 Appendix II: Potential “vagrants”................................................ 42 Literature Cited............................................................................ 45 HOW TO USE THIS GUIDE The intent here was to create a quick-reference handbook for researchers at work in the coastal and nearshore waters of the Northeast Pacific (see map below). The species included here were chosen based upon range information in Kathman et al. (1986) and Brinton et al. (1999). This guide is for adult euphausiid forms only, defined by the presence of fully-developed secondary sexual characteristics and the capacity to reproduce. Including other life phases was beyond the scope of this project. To the knowledge of this author, no identification guide currently exists strictly for this region, at least not in field-ready paper form. Hopefully this will be of use for students with a range of resources available to them, from those with access to little more than a hand lens or dissecting microscope (like this author), as well as to those engaged in fully equipped studies on research-vessels. For those with computer access while in the field or during analysis of your collections, the CD-ROMEuphausiids of the World Ocean (Brinton et al. 1999) is the most up-to-date and authoritative reference for range information, identification keys, and relevant literature. The identification keys were constructed by distill- ing extant keys for worldwide euphausiids to only those species occurring in the Northeast Pacific and adjus-ting the artwork ac- cordingly. This simplified published keys to be more useful, expedient, and in- formative for students of the region’s biological oceanography. The images in this guide were adapted The “coastal Northeast Pacific”, the region of focus in this field from Baker et al. (1990). guide, is delineated in gray shading here. The content is divided into 4 sections, arranged in the order that they will tend to be drawn upon: i) A broad, generalized higher-taxa identification key to distinguish euphausiids from look-alikes (decapods and mysids). ii) A generic key, to identify any euphausiid collected in the Northeast Pacific down to the genus level. iii) Specific keys, which will identify euphausiids of a known genus down to the species level. iv) Notes on the life histories of each species occurring in the Northeast Pacific, including references as entry points into the literature for more regarding that species. The appendices provide cursory information on potentially “vagrant” species into the region of focus in this guide. i INTRODUCTION The Euphausiidae is a family of pelagic shrimp-like crustaceans known more commonly as krill, from the Norwegian word “kril”, mean- ing young fry (Kathman et al. 1986). The etymology of the name “Eu- phausiidae” is a reference to the luminescence produced by large light organs, or photophores, on the bodies of euphausiid species (Baker et al. 1990). Euphausiids are known to dominate coastal mid- to high-latitude zoo- planktonic communities, and the northeast Pacific is no exception. Considered a “pivotal taxon” of pelagic ecosystems (Todd et al. 1996), euphausiids comprise a major portion of the plankton biomass (Kath- man et al. 1986). Most euphausiids are omnivorous filter feeders (Todd et al. 1996), ingesting algae, plankton, and detritus (Kathman et al. 1986). Some abundant species (e.g., E. pacifica, T. inermis) can be ac- tively predaceous (Kathman et al. 1986). Euphausiids serve as vertical and horizontal transporters and distrib- utors of organic matter, micro- and macro-nutrients (e.g. vitamin A), radioisotopes and heavy metals. The detritivorous species are thought to serve as important nutrient recyclers, mixing and ingesting sedi- ments and fecal pellets during vertical migrations (Kathman et al. 1986). This taxon is therefore quite relevant to most branches of oceanography. Taxonomy & Morphology Subphylum Crustacea Class Branchiopoda (e.g., cladocerans) Class Maxillopoda (e.g., ostracods, copepods, branchiurans) Class Malacostraca Subclass Eumalacostraca Superorder Peracarids (e.g., mysids, amphipods, etc.) Superorder Eucarida Order Decapoda (e.g., shrimp) Order Euphausiacea Family Bentheuphausiidae Family Euphausiidae Malacostraca is the largest class of the Crustacea and includes all the shrimp-, prawn-, lobster-, and crab-like creatures. In most malacostracans, up to three anterior pairs of thoracic legs are modified as maxillipeds (Todd et al. 1996). Many malacostracans live on the seafloor and 1 only send up their larvae into the planktonic realm (Hardy 1956, p. 166); however, the Euphausiacea (subclass Eumalacostraca: superorder Eucarida), is entirely pelagic, and therein we find the family Eu- phausiidae. The body plan of eucarids is composed of two main parts: the cephalothorax and a well-developed abdomen. The cephalothorax is covered dorsally and laterally by the carapace, and includes two pairs of bira- mous pre-oral antennae, movable stalked eyes, then mouthparts and thoracic limbs ventrally. The abdomen consists of six segments, each of which carrying a pair of alike pleopods (Baker et al. 1990). At the posterior of the abdomen is the “tail”, consisting of the central telson and two flanking uropods that can fan out to provide excellent propulsion. Euphausiids superficially resemble shrimp within the other eumala- costracan orders Decapoda and Mysida. In fact, euphausiids had once been allied with mysids (O. Mysidacea) in the taxon Schizopoda (Kath- man et al. 1986). Today, they are known to be phylogenetically and morphologically distinct, and fall within different superorders of the Malacostraca (euphausiids are within the Eucarida, mysids are within the Peracarida, or “pouch shrimp”). Among the many differences one could point to, perhaps the most immediately obvious is the lack of exposed gills in decapods and mysids. The gills of the Euphausiacea, by contrast, are readily apparent (Baker et al. 1990). Furthermore, euphausiids are typically larger than mysids; the eucarid carapace is fused to the thoracic segments, while in mysids the posterior carapace can be lifted easily from the body; euphausiaceans boast ventral photophores on their thoracic limb bases and abdominal pleopods; finally, euphausiids do not have balancing organs (statocysts) in their uropods as mysids do (Todd et al. 1996). (See Higher Taxon key, page 9.) Within the superorder Eucarida, euphausiaceans (krill) substantially differ from decapods (true shrimp, crabs, lobsters, etc.). Decapod gills are covered by the carapace; euphausiacean gills are exposed. Deca- pods lack photophores; euphausiaceans typically have 5 pairs of them. The first three thoracopod pairs in decapods are modified as feeding appendages, or maxillipeds, and their posterior 5 thoracopods are uni- ramous; in Euphausiacea, all 8 thoracopods are functionally natatory, although in some species the second or third may be elongate and rap- torial to aid in predation. Decapods also have statocysts, while euphausiaceans do not. The order Euphausiacea contains two families: the Euphausiidae and the Bentheuphausiidae, a deep-sea group whose

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