Contributions to the Systematics and Ecology of Aquatic Diptera A Tribute to Ole A. Sæther, 2007, T. Andersen (ed.), pp. 73-79. 2007 The Caddis Press. Hydrobaenus saetheri Cranston, new species, an aestivating, winteremerging chironomid (Diptera: Chironomidae) from California PETER S. CRANSTON 1, GINA M. BENIGNO 2 AND MARIANNE C. DOMINGUEZ 1 1 Entomology Department, University of California Davis, Davis, CA 95616, USA E-mail: pscranston@ucdavis.edu 2 California Department of Water Resources, 3251 S Street, Sacramento CA 95816 USA Abstract. A new orthocladiine midge, Hydrobaenus saetheri Cranston, new species, is described from larvae, pupae and adults, evidently belonging to the genus Hydrobaenus Fries. Larvae aestivate over summer, and commence development as soon as wetted when winter-rains induce flow in seasonal waterways. Larvae of this species dominate in the early flow, and adult numbers can reach nuisance levels in mid-winter. Key words: Chironomidae, Orthocladiinae, Hydrobaenus, new species, California Introduction The chironomid genus Hydrobaenus Fries is restricted to the northern hemisphere and includes several species that emerge as adults in mid-winter having developed from aestivating larval cocoons (Sæther 1976; Grodhaus 1980; Kondo & Okuda 1990). Some species attain nuisance numbers such as Hydrobaenus kondoi (Sæther) in Japan, described by Kondo (1996) as enormous swarms of midges have brought serious nuisances: inhalation of dust from dead adults, disturbance to drivers, and invasions of factories and residences in the area. Other species have been alluded to as emerging in large numbers elsewhere (e.g., for Germany, Steinhart 2000). Amongst reports of aestivating Chiro-nomidae, Grodhaus (1980) mentioned a species of Hydrobaenus from California whose identity has remained unknown. Recently, in a study of fish and their diets in the Yolo Bypass, a winter floodway of the waterways due west of Sacramento (California), a dominant species of chironomid was revealed to emerge abundantly in winter. Indeed, the adult midges are abundant enough to cause nuisance to the vehicle drivers on the river of cars (Gary Snyder, unpublished) on the busy Interstate 80. Rehydration of previously summer-dry sediments has revealed early larval instars emerge post-inundation and develop quite rapidly to mid-winter emergent adults. Life history details are provided here from experimental and field observations. All stages are identifiable readily as belonging to Hydrobaenus, but no life history stage matches any previously described taxon. Here the species is described as Hydrobaenus saetheri new species, taking authorship of the senior author alone. The epithet acknowledges Professor Ole A. Sæther s important resolution of the generic boundaries in this area of previous confusion, and in recognition of his many contributions to systematics of Chironomidae, on the occasion of his 70th birthday. Material and methods All specimens came from sites within the Yolo Bypass waterway (Fig. 1), the primary floodplain for the Sacramento River and associated waterways (Sommer et al. 2001). This seasonally inundated flood-control system extends some 61 kms from north to south, is up to 16 km wide and covers some 24,000 hectares. The floodplain is typically inundated at least once annually from the Sacramento River or smaller tributaries during winter or spring in all but the driest of years. All collections were made between November 2004 and late January 2005 by G. M. Benigno. In November 2004 pre-inundation sediment samples 45 x 30 x 10 cm were collected 73
from the perennial floodplain channel. Adult midges, often with associated exuviae still attached, and pharate adults especially were intercepted by such drift nets. Samples were fixed in 10% buffered formalin containing Rose Bengal dye and transferred to 70% ethanol for preservation within one month of collection. Slides were made by clearing in KOH, neutralization in Glacial Acetic Acid, dehydration in Isopropanol and mounting in Euparal. Adults, especially females, predominantly were teneral with incompletely expanded wings, and were collected into formalin which fails to clear and make poor slides. Some larval mounts were made in Hoyers Mountant and ringed with clear nail varnish. Morphological terminology follows Sæther (1980) and Cranston (1994) except for using taeniate for filamentous. Institutional abbreviations BMNH British Museum (Natural History), Cromwell Rd, London SW7 5BD, UK. USNM United States National Museum, Washington, D.C. 20560, USA. ZMB The Natural History Collections, Bergen Museum, Bergen, Norway. ZSM Zoologische Staatssammlung, Munchen, Germany. Hydrobaenus saetheri Cranston, new species FIG. 1. Map of the Yolo Bypass and lower Sacramento River, Sacramento, California, from Schemel et al. (2001). from summer-dried areas of the Yolo Bypass. Samples were placed in clear plastic bins covered with a 1 mm mesh and flooded to a minimum depth of 10 cm on November 5, 2004. Flooded sediments were exposed to ambient photoperiod and water temperature, which ranged in the bins from 9 14 C. For three weeks after inundation, samples were collected twice weekly using a 100 μm mesh hand net and preserved in 70% ethanol. Initial inundation of the floodplain occurred in late December, 2004. During the first month of inundation, weekly drift net collections were made Type material. Holotype male, slide mounted in Euparal. USA: California, Yolo Co., Causeway, wildlife area, North of I80 freeway, 38 31 27 N 121 35 21 W, 9.xi.2004, G. M. Benigno (USNM). Paratypes: same locality as holotype; 24 L (10 slides, 8 Hoyers), 6 Pe/male, 1 Pe/female, 9 males, 6 females (BMNH, USNM, ZMB, ZSM). One paratype comprises the voucher for the species (and hence the genus) in an ongoing molecular phylogeny project (Morse & Cranston in prep). Male (n = 10) Body length 3.8 5.4 mm, wing length 2.6 3.2 mm. Thorax, legs and abdomen brown-black. Head. Antennal segments 1 12: 185 210 μm long, 13: 300 350 μm long. AR 1.57 1.89. Head with 8 11 uniserial outer verticals / postorbitals, 9 12 clypeals. Palp segment lengths (in μm): 2 5, 45 60, 120 150, 90 160, 100 120. 74
Hydrobaenus saetheri Cranston, new species Thorax. Antepronotal lobes broadly connected medially, with weak notch. Thoracic setation: 4 6 lateral antepronotals,? 0 3 minute acrostichals, 4 9 dorsocentrals, 4 5 prealars, 0 supraalars, 7 9 uniserial scutellars. Wing. Anal lobe slightly protruding, costa very weakly extended. Squama with 14 20 setae, R with 4 5 setae. V.R. c. 1.10. Legs. Midleg with pseudospurs on ta 1. LR 1 0.73 0.96, LR 2 0.54 0.59, LR 3 0.50 0.53. Hypopygium, as in Figure 2A. Gonocoxite 240 260 μm long. Gonostylus 110 118 μm long, with 3 7 apical / subapical megasetae (Fig. 2B) Female (n = 4) Body length 3.0 3.8 mm, wing length 2.2 2.5 mm. Head. Terminal (5th) antennal segment darkened, 120 145 μm long. AR 0.61 0.74. Head with 2 3 frontals, 2 3 strong postorbitals, 9 13 clypeals. FIG. 2. Hydrobaenus saetheri Cranston, new species, adult. A. Male hypopygium. B. Apex of gonostylus. C. Female genitalia, lateral view. D. Female genitalia, ventral view, left side. 75
Cranston, Benigno, and Dominguez Thorax. Setation: 4 6 lateral antepronotals, many small hooked acrostichals, 5 8 dorsocentrals, 3 4 prealars, 0 supraalars, 9 10 uniserial scutellars. Wing. Squama with 18 24 setae, R with 8 10, R 1 with 4 6, and R 4+5 with 1 2 setae. V.R. 1.01 1.05. Legs. LR 1 0.69 0.72, LR 2 0.50 0.51, LR 3 0.50 0.53. Genitalia (Figs. 2C, 2D). Gonocoxite IX protruding, microtrichiose, with 12 19 setae. Tergite X with 15 21 long setae on each of 2 protrusions. Seminal capsules bicolored (dark in oral 2/3), 135 155 μm long, 62 75 μm wide, elongate-ovoid, without distinct neck; seminal duct with 10 15 μm long microtrichia orally, with strong loop. Pupa (n = 7) Length 4.4 5.1 mm. Cephalothorax 1.06 1.22 mm long. Frons with paired warts, frontal setae c. 120 μm long. Thoracic horn (Fig. 3D) spinose throughout, 350 400 μm FIG. 3. Hydrobaenus saetheri Cranston, new species, larva (A C), pupa (D E). A. Mentum. B. Antenna. C. Mandible. D. Thoracic horn. E. Abdominal tergites. 76
Hydrobaenus saetheri Cranston, new species long, maximum width (subapical) 54 68 μm. Antepronotals: 2 dorsal, 90 120 μm long; 1 lateral, 45 55 μm long; 3 pc, 1 thicker and longer, 120 140 μm long; 2 short and thin, 35 45 μm long; dorsocentrals evenly spaced on mid-thorax, dc 2 10 20 μm long, dc 1, 3, 4 50 55 μm long. Abdomen. Tergites as in Figure 3E. L-setae of T IV and V non-taeniate, VI with 3 taeniate L-setae, VIII with 4 (rarely 3) taeniate L-setae; L 4 located very posteriorly on III VII. Anal lobe fringe with 24 30 setae; 3 macrosetae of maximum length 325 400 μm. Genital sacs extending slightly but distinctly beyond anal lobe. Larva (n = 14) Body length 5.9 8.5 mm, color uncertain (preserved stained pink). Head capsule. Length 400 550 μm, yellowbrown with occipital margin dark, mental, mandibular and premandibular teeth brown-black. Mentum (Fig. 3A) 133 188 μm wide, ventromental plate maximum 15 20 μm wide. Antenna (Fig. 3B) segment lengths (in μm): 60 77.5, 12.5 18.75, 5 8, 5, 3.75 5. AR 1.85 2.95. Blade 25 35 μm long. Lauterborn organs subequal to segment 3 in length. Labrum with S I seta c. 50 μm long, with 6 8 coarsely plumose branches; S II stout, 75 μm long, S III minute, S IV short. Premandible with 2 dark apical teeth, 172 205 μm long. Mandible (Fig. 3C) 140 175 μm long. Abdomen. Procercus, brown, darker posteriorly, 60 80 μm high, 48 52 μm wide, bearing 7 anal setae of maximum lengths 600 660 μm. Supraanal setae 270 340 μm long. Posterior parapods about 250 μm long, with 17 22 simple, dark brown claws. Anal tubules subconical, about half length of posterior parapods. Systematics Adult males broadly conform to generic diagnoses (Sæther 1976; Cranston et al. 1989) with additional species (Sæther 1989), except for the virtual absence of acrostichals (a pale elongate oval area in the mid-third of the thorax may enclose few very short, perhaps curved, setae), the virga is indistinct, and the gonostylus, uniquely in the genus, bears several megasetae (Fig. 2B). The female adult conforms to Sæther s (1976) description. The pupa and larvae both key to Hydrobaenus and match completely the diagnoses of Sæther (1976), Coffman et al. (1986, pupa) and Cranston et al. (1983, larva). Six species groups are recognized for adults (Sæther 1976; Cranston et al. 1989), with fewer and less clearly delimited groups for the immature stages. Based on the normal palp and antenna, non-triangular gonostylus without crista dorsalis, weak anal point, and measures / counts (based on essentially overlapping numbers) the adult male keys to the pilipes group. Based on distribution and length of L-setae, smooth genitalic sheath without terminal papilla and mensural features the pupa keys to the rufus / lugubris group, conforming to the latter except for the longest anal lobe fringe seta being longer and posterior-most amongst the macrosetae. Keys to larval species groups are inadequate, but the coarsely plumose SI seta may be indicative of the pilipes group. That H. saetheri does not fit comfortably into any one species group in all life history stages may be unsurprising given the inability to recover these groups in numerical cladistic analyses (Cranston & Humphries 1988). Ecology Hydrobaenus saetheri appears to be a dominant species in the seasonally-inundated floodplains of the Sacramento River. In Yolo Bypass, sampling during 1998 2005 revealed that Hydrobaenus dominates the chironomid fauna during flood. Larval H. saetheri appear rapidly after inundation and are maintained at relatively high levels throughout flood events (Sommer et al. 2004). Abundance trends correlate closely with flow, probably as a result of increased seasonal inundation of terrestrial habitats at higher flows. H. saetheri larvae emerged from 18 of 24 (75%) rehydrated floodplain sediment samples from throughout the Yolo Bypass. Co-occurring Chironomidae, one species each of Limnophyes, Goeldichironomus and Paratanytarsus, were uncommon. Mandible length measurements indicate that larval H. saetheri collected four days after experimental rehydration were second and third instar larvae and fourteen days after rehydration all larvae collected were in fourth instar. This suggests emergence from a desiccation-resistant resting stage (likely a cocoon) upon rehydration is followed by rapid maturation. Emergence of H. saetheri larva from floodplain sediments was observed also in the field. In shallow, slow-flowing floodplain waters, drift net samples two and three weeks after initial inundation of the floodplain collected 183 and 320 larvae per m 3, respectively. In the faster flowing main drainage channel of the floodplain, very high densities (up to 550 per m 3 ) of incompletelyemerged adults were collected from drift nets. Drifting larvae are a major prey item for native fishes, which rely on Yolo Bypass as nursery habitat (Moyle 77
Cranston, Benigno, and Dominguez et al. 2004; Sommer et al. 2001). As a consequence of the exceptional production of Hydrobaenus in the Yolo Bypass, feeding success of young fish is superior to that of adjacent perennial rivers. Hatching of pupae from the drift creates dense swarms of adults, providing additional prey for aquatic and terrestrial food webs. During the drier summer and autumn months, the species evidently is sustained by aestivating larval cocoons in floodplain soils. Several other species of Hydrobaenus can take advantage of temporary aquatic conditions by producing desiccation-resistant cocoons as 2nd or 3rd instar larvae (Sæther 1976; Grodhaus 1980; Kondo & Okuda 1990). An unidentified Hydrobaenus species was reported from cocoons in dried sediments from Northern California vernal pools (Grodhaus 1980). Larvae removed from cocoons were 2nd instar, and laboratory-reared larvae built cocoons when they reached second instar regardless of moisture conditions, suggesting an obligatory diapause period regardless of hydrologic conditions (Grodhaus 1980). This material appears lost, since it is not in the modest Grodhaus collection housed in the Essig Museum, Berkeley. Two visits to a vernal pool Grodhaus studied (CA: Siskiyou Co., Murphy Well, 41 40 N 125 07 W) in mid-summer 2005 revealed no cocoons in apparently suitable (winter-wet vernal) soil, no larvae developed on inundation, and neither were Hydrobaenus present in the farm pond created at the site (apparently since Grodhaus visits). Thus, we cannot equate the new taxon described here with that studied by Grodhaus. Acknowledgements We thank Ted Sommer (California Department of Water Resources) for drawing our attention to this phenomenon, for funding Gina Benigno s ecological studies and for permission to include otherwise unpublished data on the Yolo Causeway ecosystem. Marianne Dominguez s examination of specimens and excellent drawings were funded from the Evert Schlinger endowment for systematic entomology to the senior author. Finally, it gives us pleasure to dedicate this contribution to Professor O.A. Sæther for collaboration, intellectual debate on phylogenetics, and above all for providing the scientific community with a phylogenetic and taxonomic framework within which to make so many observations on Chironomidae. References Coffman, W.P., Cranston, P.S., Oliver, D.R. & Sæther, O.A. (1986) The pupae of Orthocladiinae (Diptera: Chironomidae) of the Holarctic region - Keys and diagnoses. In: Wiederholm, T. (Ed.), Chironomidae of the Holarctic region - Keys and diagnoses. Part 2. Pupae. Entomologica scandinavica, Supplement 28: 147 296. Cranston, P.S. (1994) 2. Morphology. In: Armitage, P.D., Cranston, P.S. & Pinder, L.C.V. (Eds.), The Chironomidae: Biology and ecology of nonbiting midges. Chapman and Hall, London, pp. 11 30. Cranston, P.S. & Humphries, C.J. (1988) Cladistics and computers: a chironomid conundrum? Cladistics 4: 72 92. Cranston, P.S., Oliver, D.R. & Sæther, O.A. (1983) The larvae of Orthocladiinae (Diptera: Chironomidae) of the Holarctic region - Keys and diagnoses. In: Wiederholm, T. (Ed.), Chironomidae of the Holarctic region - Keys and diagnoses. Part 1. Larvae. Entomologica scandinavica, Supplement 19: 149 291. Cranston, P.S., Oliver, D.R. & Sæther, O.A. (1989) The adult males of Orthocladiinae (Diptera: Chironomidae) of the Holarctic region - Keys and diagnoses. In: Wiederholm, T. (Ed.), Chironomidae of the Holarctic region - Keys and diagnoses. Part 3. Adult males. Entomologica scandinavica, Supplement 34: 165 352. Grodhaus, G. (1980) Aestivating chironomid larvae associated with vernal pools. In: Murray, D.A. (Ed.), Chironomidae. Ecology, systematics, cytology and physiology. Pergamon Press, New York, pp. 315 322. Kondo, S. (1996) Life cycle of Hydrobaenus kondoi Sæther (Chironomidae) at the middle reaches of the Kiso River, Japan. Hydrobiologia 318: 79 84. Kondo, S. & Okuda, T. (1990) The larval cocoon of Hydrobaenus kondoi Sæther (Diptera: Chironomidae) for aestivation in the Kiso River, Japan. In: Abstracts, V International Congress of Ecology. Yokohama, Japan, pp. 359. Moyle, P.B., Baxter, R.D., Sommer, T.R., Foin, T.C. & Matern, S.A. (2004) Biology and population dynamics of Sacramento splittail (Pogonichthys macrolepidotus) in the San Francisco Estuary: a review. San Francisco Estuary and Watershed Science 2(2) (May 2004), Article 3. Available from: http:// repositories.cdlib.org/jmie/sfews/vol2/iss2/ art3 (11 February 2006). Sæther, O.A. (1976) Revision of Hydrobaenus, Trissocladius, Zalutschia, Paratrissocladius, and 78
some related genera (Diptera: Chironomidae). Bulletin of the Fisheries Research Board of Canada 195: 1 287. Sæther, O. A. (1980) A glossary of chironomid morphology terminology (Diptera: Chironomidae). Entomologica scandinavica, Supplement 14: 1 51. Sæther, O. A. (1989) Two new species of Hydrobaenus Fries from Massachusetts, U.S.A., and Japan (Diptera: Chironomidae). Entomologica scandinavica 20: 55 63. Schemel, L.E., Sommer, T.R., Muller-Solger, A.B. & Harrell, W.C. (2001) Hydrologic variability, water chemistry, and phytoplankton biomass in a large floodplain of the Sacramento River, CA, U.S.A. Hydrobiologia 513: 129 139. Sommer, T.R., Nobriga, M.L., Harrell, W.C., Batham, W., & Kimmerer, W.J. (2001) Floodplain rearing of juvenile Chinook salmon: evidence of enhanced growth and survival. Canadian Journal of Fisheries and Aquatic Science 58: 325 333. Sommer, T.R., Harrell, W.C., Mueller-Solger, A., Tom, B., & Kimmerer, W. (2004) Effects of flow variation on channel and floodplain biota and habitats of the Sacramento River, California, USA. Aquatic Conservation: Marine and Freshwater Ecosystems 14: 247 261. Steinhart, M. (2000) The life cycle of Hydrobaenus lugubris Fries (1803), a chironomid (Diptera) species dwelling in temporary waters. Internationale Vereinigung für Theoretische und Angewandte Limnologie Verhandlungen 27: 2392 2395. Hydrobaenus saetheri Cranston, new species Accepted: 8 February 2006. 79
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