Hygrotus (Coelambus) nubilus ( LECONTE , 1855)

Fery, Hans & Challet, Gil, 2015, Hygrotus (Coelambus) nubilus (LECONTE, 1855) on Mauna Kea (Hawaii) - first record of the genus from the Pacific zoogeographical region (Coleoptera: Dytiscidae), Linzer biologische Beiträge 47 (2), pp. 1303-1309 : 1304-1308

Hygrotus (Coelambus) nubilus ( LECONTE, 1855)

Hydroporus nubilus LECONTE, 1855: 298 (orig. descr); CLARK 1862: 178.

Coelambus nubilus ( LECONTE, 1855) ; SHARP 1882: 401; FALL 1919: 15 (designation of lectotype).

Hygrotus nubilus ( LECONTE, 1855) ; BLACKWELDER 1944: 76 (new combination); WALLIS 1973:102; ANDERSON 1983: 189 (description); HILSENHOFF (1994: 283); LARSON et al. 2000: 178.

Hygrotus (Coelambus) nubilus ( LECONTE, 1855) ; NILSSON 2001: 208; 2015: 180 (catalogue)

Type locality: USA, Wyoming,GoshenCo.,FortLaramie(ca. 42.21N 104.52W).

Type material (not studied): male lectotype and single female paratype (" allotype ") stored in the Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, USA (see ANDERSON 1983: 189).

Material studied (material from Hawaii): 1, " Mauna Kea, Hawaii, Lake Waiau , 13,000 '" [printed], " 3970 M, 2.IX.1977 " [handwritten], " F.G. Howarth, Collector, Bishop" [printed], " Hygrotus cf. medialis LeConte, GAS ’04 [= G.A. Samuelson 2004]" [handwritten] ; 1, same labels except the last one " Hygrotus sp. , det. G.A. Samuelson ’04" [handwritten in part] (both specimens in coll. H. Fery) . Further nine specimens with similar label texts in Bishop Museum, Hawaii. Lake Waiau has co-ordinates 19.811N 155.477W and is situated in an altitude of about 3970 m (first record from Hawaii).

Additional material studied: 32 exs from localities in Arizona, Colorado, Iowa, Maryland, New York, Oklahoma, Texas, Wyoming (all USA). Two females have been studied from Mexico (Chihuahua State, 1 mi E Coyame; specimens in coll. H. Fery and coll. G. Challet). BLACKWELDER (1944: 76) beside " U.S.A. " included also " Mexico " in the distribution area, however, without providing any exact data. This is why we consider our data at least as first "reliable" record of H. nubilus from Mexico. Chihuahua State belongs formally to the Neotropical zoogeographical region (as defined in NILSSON 2001: 10 and 2015: 7), but is very close to the border with the USA and, thus, in a transitional zone between the Nearctic and Neotropical regions. Beside Hygrotus (Coelambus) fraternus (LECONTE, 1852) , H. (Coelambus) lutescens (LECONTE, 1852) , H. (Coelambus) wardii ( CLARK, 1862) and H. (Hygrotus) hydropicus (LECONTE, 1852) this is the fifth species of the genus occurring in Mexico.

Descriptive notes: The species belongs to ANDERSON’ s (1983: 184) species group VI. This author provides also a key to species of this group. We refrain from reproducing a detailed description of the species, but instead refer to the original description of LECONTE (1855: 298). Also in FALL (1919: 15), ANDERSON (1983: 189) and LARSON et al. (2000: 178) are given valuable descriptive details. The male genitalia are figured in ANDERSON (1983: 179, figs 26a,b) and LARSON et al. (2000: 178). A photo of the female gonocoxae is given in HILSENHOFF (1994: 280, fig. 8).

Nevertheless, we provide a photo of the habitus ( Fig. 2 ) and want to point on the parameres which in the genus are unique in shape (together with those of H. femoratus (FALL, 1901) ; cf. ANDERSON 1983: 179, figs 25 and 26). Also the female gonocoxae and gonocoxosterna have shapes which are very helpful for identification. This is why we give our own illustrations of the male and female genitalia in Figs 3-6 .

Hygrotus femoratus is extremely similar to H. nubilus (females of both are inseparable on morphological characters) and only two males and one female have ever been collected (see ANDERSON 1983: 188). This is why ANDERSON (l.c.) even suspected that both might be the same. The only difference can be found in the male tibiae and femora which have a normal shape in H. nubilus , but are deviating in H. femoratus ( ANDERSON 1983: 180: "male femora with enlarged apical articulations, tibia pedunculate"; see also figs 38a,b on p. 185).

Distribution According to ANDERSON (1983: 189; see also fig. 4 on p. 175) Hygrotus nubilus is a common species which is widely distributed in boreal America ( USA and Canada) east of the Rocky Mountains (for names of USA states and Canadian provinces see ANDERSON 1983: 189). With the records given in the present paper the distribution area must be widened to northern Mexico and to Hawaii.

Discussion: The finding of H. nubilus on the Hawaii Islands is very surprising. We have no doubt that the species must have been introduced to Hawaii, but have no evidence for how this may have happened. It is also not clear when the species has been introduced because it seems to be very rare and collecting activities before 1977 may simply have failed although the species was already present on Mauna Kea or anywhere else.

In HÄNEL & JÄCH (2013: 275 ff) several possibilities are discussed for the introduction of alien species on the islands of the Tristan da Cunha Archipelago. Among these are any kinds of human activities, but also transport by birds is mentioned: "In considering the possible ways in which... invertebrates could be dispersed naturally, birds have been considered as favoured vehicles, especially young ones that still have downy patches, as these are prone to becoming contaminated with debris."

On the internet we found the website of DENNY (2006) where more than twenty species of ducks, geese, and shorebirds are listed which migrate to Hawaii each winter (on the northern hemisphere). Among these is the Pacific golden plover (= Kōlea in Hawaiian; Pluvialis fulva (GMELIN, 1789)) which is known to fly regularly from Alaska to Hawaii and back. This bird has already been observed in California (which is west of the Rocky Mountains, while H. nubilus occurs only east of these mountains; see also COSTE & SALMON 1998). This shows that it cannot be excluded principally that birds are able to cover such an immense distance.

Another candidate is Taverner’s goose (Branta hutchinsii taverneri DELACOUR, 1951) which has been reported from Kansas (where H. nubilus has been recorded from) as well as from Hawaii "...for all recent reports of extralimital Taverner’s Geese, those we have reviewed suggest that this subspecies does occur as a vagrant in Hawaii and east of the Rockies..." ( MLODINOW et al. 2008: 352).

Figures

Fig. 2: Habitus of Hygrotus (Coelambus) nubilus (LECONTE, 1855); male specimen from Lake Waiau, Hawaii.

Figs 3-6. Hygrotus (Coelambus) nubilus (LECONTE, 1855): (3) median lobe of aedeagus in ventral and in lateral view, (4) left paramere, (5) gonocoxae, (6) gonocoxosternum (both specimens from Lake Waiau, Hawaii).