Rusty Crayfish, Super Hybrids!

by Fred Schueler

Giant Orconectes rusticus/propinquus hybrid, captured at Oxford Mills, Ontario Crayfish, on 1 January, 2016

1 January 2016 - Canada: Ontario: Grenville County: Oxford-on-Rideau: below Oxford Mills Dam, Kemptville Creek. (100m around dam), 31B/13, 44.96486N 75.67863W TIME: 2015-2120. AIR TEMP: -1°C, overcast, Beaufort light breeze, snowing. HABITAT: clear brownwater creek at limestone flats below old milldam, water 0°C. OBSERVER: Frederick W. Schueler, Aleta Karstad Schueler, and others. 2016/001/db, Orconectes rusticus HYBRID:Orconectes propinquus (hybrid Rusty/propinquus Crayfish) 1 adult, male, seen, photographed. 40 mm carapace, upside down on gravelly bottom of W sides. Near a small stone - maybe inactive due to first 0°C water of the winter.

There are three native Crayfish species in eastern Ontario: the shy and nocturnal Cambarus bartonii in the Ottawa River and at least formerly in the St Lawrence, the small boldly diurnal Orconectes propinquus in the Ottawa, Mississippi, Rideau, and St Lawrence rivers and their immediate tributaries, and the big pink-and-blue Orconectes virilis in all streams, including those, like upper Kemptville and Cockburn creeks, which dry up in droughty summers.

Crayfish species don't have really vernacular English names, and while O. virilis has gone by "Northern" and "Virile," we prefer to translate the (equally artificial) "Écrevisse à pinces bleues" and call it the Blue-claw Crayfish.

There's an equal number of alien invasive species: the burrowing O. immunis, in the Jock River and the Cassidy Municipal Drain, O. obscurus in the Tay, Mississippi, and Ottawa rivers, and Orconectes rusticus, the Rusty Crayfish, in, to our knowledge, Brassil's Creek and the Ottawa, Jock, Tay, Rideau, Mississippi, and North Branch of the South Nation rivers.

Orconectes obscurus and O. rusticus hybridize with O. propinquus to produce hybrid swarms that presumably combine the favourable characteristics of the invader with the local adaptations of the native parents. We first found O. rusticus in the Rideau River at the mouth of the Jock River in 1987, and a hybrid swarm of O. rusticus x propinquus has since spread well up the Jock River and along the Rideau River

For 38 years we found only Blue-claw O.virilis in Kemptville Creek below its barrier dam in Oxford Mills (it's also the only species in the creek above the dam). Our first databased record here is from August 1975, followed by the one painted for Wild Seasons Daybook, and then a species account in our Crayfish Ontario website in December 1984, followed by records in 1991 (1), 1992 (3), and 1995 (1). Then in 1999 one was found as prey of Necturus, on 8 January 1999, at the First "Mudpuppy Night in Oxford Mills," but no more were seen that winter, and they were common in the summer. Subsequent records are all from Mudpuppy Nights: 4 in 1999-2000, 11 in 2000-2001, 8 in 2001-2002; 2002-2203 was the best year with 48 seen, 12 in 2003-2004, and 5 in 2004-2005. There were none between 11 Feb 2005 and 12 Jan 2007, totalling 3 for 2006-2007, 12 in 2007-2008 (all in Oct and Nov), and 10 in 2008-2009. We saw no Crayfish between 2 Feb 2009 and 3 Oct 2011, but quite a recovery in 2011-2012 when 18 were seen, many of these small.

We see the Crayfish here mostly early in the season, and the date of the first Mudpuppy Night is variable from year to year, so there will be less variability in these results when various factors are corrected by regression, though Crayfish are also prone to population crashes from epizootic diseases. Part of the decline through the winter is due to predation by Mudpuppies, and we have suspected that Mudpuppies control the demography of Crayfish in the creek

We saw our first Rusty hybrid here on 2 Nov 2012, but for the rest of the winter saw only 10 O. virilis. Then in the winter 0f 2013-2014 there were 13 Rusties and 7 Blue-claws, and in 2014-2015 21 Rusties and 5 Blue-claws, and so far this winter 1 Blue-claw and 10 Rusties. So through the winters since the first Rusty hybrid appeared, they've made up 9, 65, 81, and 91% of the Crayfish seen. Early in the fall of 2015 we found about 20 old chelae in a pile on the bottom, which we haven't looked at carefully, they could be due to a predator or a human dump.

Unfortunately, we haven't had time when we've been home during the summer to check the creek downstream of the dam, so we don't know how fast the invaders spread up the creek - our last survey downstream was in 2001 and it found only O. virilis, the Blue-claw.

Orconectes rusticus is a fast-swimming, large-clawed species, whose adults are well able to defend themselves and escape being eaten by all but Otters and the the largest Rock and Black bass. They are active out from under cover during the day, can tolerate high populations of their own species, drive native species from their hiding places, and compete for food, often devastating the bottom vegetation.

A good characteristic to identify Orconectes rusticus is the curved claws of big individuals. Another is a rusty smudge on either side of the carapace, and a third is the pinched-looking "rostrum," the part of the carapace that projects forward between the eyes. The distinctive rostrum feature of O. propinquus is a very thin keel or ridge, called a carina (Latin for keel) in the middle of the rostrum. This old boy (pictured above) has nearly lost the constricted rostum through hybridization, but doesn't show the carina, which many of the Oxford Mills hybrids do. The other conspicuous field mark of O. propinquus is a dark mid-dorsal band down the abdomen, and this individual has this merged with the transverse bands on each segment which are characteristic of O. rusticus. Also, both species are characterized by orange bands on the chelae (claws), which here are reduced to mere tips.

We're four years into the invasion of the creek by the hybrids, and can only wait and see how they may affect things - maybe they'll overpower mother Mudpuppies and eat the eggs they're guarding, maybe predation by Mudpuppies on juvenile hybrids will select for more timorous ways, or maybe things will go on much as they did when O. virilis was the only Crayfish in town.

Fred

SUGGESTED TEXT FOR CITATION: 
Schueler, Fred. 2016. Rusty Crayfish, super hybrids. Doing Natural History http://doingnaturalhistory.blogspot.ca/2016/01/rusty-crayfish-super-hybrids.html 
- posted 13 January 2016 by Aleta Karstad, revision of 16 May 2016.