Pilumnus harrisii (Gould, 1841); white-fingered mud crab

While much of the previous literature describes this species as intolerant of freshwater conditions, populations have recently been found in near-freshwater conditions. Roback (1955) found this species in rivers emptying into the Gulf of Mexico in salinities from 0.006 to 22.6 ppt (Costlow et al. 1966). However, zoeae (larvae) develop in salinities up to 40 ppt (Costlow et al. 1966). Rhithropanopeus harrisii populations have been found well-established (metamorphosis and reproduction) in Tradinghouse Creek reservoir (Texas), which has a salinity of 0.4–0.5 ppt (Boyle et al. 2010). High percentage of eggs from these Texan reservoirs hatched in the lab at salinity of 0.5 ppt (Richey 2004). Rhithropanopeus harrisii is well established and successfully reproducing in the Northern Lagoon (Panama), with constantly recorded salinities between 0.4 ppt and 0.6 ppt (Roche et al. 2009). Reasons for freshwater tolerance (where none had been found before) could be because the source population is more tolerant to low salinity (Boyle et al., 2010). Texas populations may be from Florida and/or Gulf of Mexico populations, which show greater tolerance to low salinity (Richey 2004).  Louisiana populations are genetically distinct from American East Coast and Europe populations (Projecto-Garcia et al. 2010). Freshwater tolerance could also be a recent result of natural selection, based on the ability of a small number of individuals. 

Turoboyski (1973) reported that populations in the Vistula River, Poland could survive winter temperatures below 1°C and could even survive being frozen in ice for a short time. This is not particularly unusual as populations at the northern edge of their natural range in the Miramichi Estuary, Canada are exposed to salt water near freezing for up to six months of the year (Chadwick 1995; Fowler et al. 2013).  Rhithropanopeus harrisii has been recorded in water with dissolved oxygen concentrations of 0.554 mg/L (Turoboyski 1973).

In its introduced range in Poland, R. harrisii is able to reproduce when the water temperature is above 14°C (Turoboyski 1973); zoeae develop at temperatures below 30°C (Costlow et al. 1966).  Since zoeae are retained in estuaries, they develop in a highly variable environment. Accordingly, they can complete development in a very wide range of temperatures and salinities (Forward 2009). Reid et al. (2007) and Klein et al. (2010) measured rapid declines in dissolved oxygen concentration inside ballast tanks to 2 mg/L within 5 to 7 days, with 90% of initial oxygen content lost within 10 days at temperatures above 20°C.  This species inhabits polluted areas, including the Baltic Sea, which is heavily polluted (though improvements have been made) (Glasby and Szefer 1998).

Rhithropanopeus harrisii feeds on both plants and animals, the proportions between these two major components differing between areas (Kujawa 1957; Turoboyski 1973) and often shifting towards more animal prey as size increases (Aarnio et al. 2015; Zalota et al. 2017). The type of food consumed has been found to be significantly (P<0.05) dependent on the locality inhabited: the greater the biodiversity of the habitat, the richer the dietary consumption. For example, while Rhithropanopeus harrisii dwelling in the Vistula Lagoon fed mainly on Dreissena polymorpha (Pallas, 1771) (cf. Kujawa 1957), the major food items in the Dead Vistula included Nereis diversicolor (0. F. Muller, 1776) Mytilus edulis (Linnaeus, 1758), D. polymorpha, and Cordylophora caspia (Pallas, 1771), as well as the algae Cladophora sp. and Enteromorpha sp. (cf. Turoboyski 1973). An Odra estuary population was found to feed mainly on detritus, which accounted for 61.1% of the gut content; the animal food items, making up 12.9% of the contents, contained remains of appendages of copepods and insects, as well as fragments of the blue mussel (M. edulis) and the zebra mussel (D. polymorpha) (Czerniejewski and Rybczyk 2008). In Baltic coastal waters this species feeds on detritus, and also on animal and plant matter. Remains of Chlorophyta, Amphipoda, Ostracoda, Polychaeta, Gastropoda and Bivalvia were found in the stomachs of the specimens analyzed (Hegele-Drywa and Normant 2009).

Rhithropanopeus harrisii can become a dominant benthic species in areas where it has been introduced.  It is one of the most important benthic species in the Vistula lagoon (Baltic sea), where it is non-native. It contributes heavily to the productivity, matter transformation, and habitat modification of the lagoon (Ezhova et al. 2005).  Rhithropanopeus harrisii appears to occupy the same niche as crayfish, which means crayfish could easily be displaced if Rhithropanopeus harrisii is the better competitor. Observations over three summers at Possum Kingdom State Park in Texas have indicated an abundance of crabs and a paucity of crayfish (Richey 2004).

Rhithropanopeus harrisii typically produces 1000 to 4000 eggs, and up to 7500 eggs per clutch (Morgan et al. 1983). Female R. harrisii individuals are able to release fertilized egg clutches up to four separate times following a single mating. Multiple spawnings may also assure continued reproduction under stressful or hazardous conditions, when mating activity may be reduced (Morgan et al. 1983). Rhithropanopeus harrisii zoeae can detect and move in response to vertical gradients in temperature, salinity, and hydrostatic pressure, which allows for depth regulation and avoidance of adverse environmental conditions (Forward 2009).  After R. harrisii megalopae (first stage of crab morphology) settle out in a suitable habitat, they quickly grow to reproductive size. Rhithropanopeus harrisii is physiologically able to reproduce by the fifth crab stage (Payen 1975), which takes only a month to attain after metamorphosis at 25°C (Morgan et al. 1983).

Introduction of Gulf of Mexico populations of Rhithropanopeus harrisii into inland Texas reservoirs may have occurred through bait bucket or accidental angler and boater release (Howells 2001). Rhithropanopeus harrisii has been found in ten freshwater impoundments in Texas (Boyle et al. 2010). While this species could arrive in the Great Lakes via ballast or hitchhiking/fouling, it seems more likely that a population able to establish in freshwater would arrive via hitchhiking/fouling from the Texan reservoirs, as these populations are already adapted to freshwater. While recreational boat traffic does occur between Texas and the Great Lakes , the frequency would likely be very low given the distance.

A gravid R. harrisii female washas been found in ships arriving from coastal traffic to east Canadian ports following mid-ocean ballast exchange (Briski et al. 2012). Female R. harrisii individuals are able to release fertilized egg clutches up to four separate times following a single mating. Multiple spawnings may also assure continued reproduction under stressful or hazardous conditions, when mating activity may be reduced (Morgan et al. 1983). Rhithropanopeus harrisii larvae can detect vertical gradients in temperature, salinity, and hydrostatic pressure, which are used for depth regulation and avoidance of adverse environmental conditions (Forward 2009). Thus this species is highly likely to survive transit in ballast tanks and reproduce enroute.

Over the past century R. harrisii has invaded over 20 countries, two oceans, ten seas, and ten freshwater inland reservoirs across four continents, which span over 45 degrees of latitude, most likely due to anthropogenic means (Roche and Torchin 2007). Rhithropanopeus harrisii inhabits these locations: Adriatic Sea, Aral Sea, Azerbaijan, Azov Sea, Baltic Sea, Belgium, Black Sea, Britain, Bulgaria, Caspian Sea, Denmark, France, Germany, Iran, Italy, Kazakhstan. Lithuania, Mediterranean Sea, Moldova, Netherlands, North Sea, Panama Canal, Poland, Portugal, Romania, Russia, Spain, Tunisia, Turkmenistan, Ukraine, Uzbekistan, Pacific Coast and Texas lakes of the United States (Jensen 2010; Philipenko 2018).

This species inhabits areas with climates similar to the Great Lakes (e.g., mid-Atlantic and northern European climates have hot summers, cold winters and significant rainfall). Water temperatures in inhabited areas include the range of Great Lakes temperatures, e.g., highs in the Panama Canal (up to 30.3°C) (Roche et al. 2009), and lows in the Miramichi Estuary, Canada (near freezing for up to six months of the year) (Chadwick 1995). While much previous literature describes Rhithropanopeus harrisii as intolerant of freshwater conditions, populations have recently been found in near-freshwater conditions. This species is likely to tolerate suboptimal conditions including a wide range of salinity, temperature and oxygen depletion. This species inhabits polluted areas, including the Baltic Sea, which is heavily polluted (though improvements have been made) (Glasby and Szefer 1998). The ability to tolerate high salinities may give R. harrisii a competitive edge over native species in areas polluted by road salt. Rhithropanopeus harrisii is able to reproduce when the water temperature is above 14°C (Turoboyski 1973).  Rhithropanopeus harrisii thrives on a diverse diet of plants and animals, including zebra mussels (Dreissena polymorpha) and will likely find plenty to eat in the Great Lakes ecosystems.

Rhithropanopeus harrisii is parasitized and sterilized by the rhizocephalan parasite Loxothylaxus panopaei in saline environments. Larvae of L. panopaei survive poorly at salinities below 10 ppt (Reisser and Forward 1991; Forward 2009) which may release the freshwater populations from control by this parasite.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

A number of coastal Rhithropanopeus harrisii (Xanthidae, Panopew and other genera) are reported as major predators on young oysters and hard clams (Gosner et al. 1978). Rhithropanopeus harrisii may have a negative impact upon local unionid populations, but this remains to be determined (Howells 1998). Rhithropanopeus harrisii likely plays a minor role in reduction of Crassostrea virginica spat (Kulp et al. 2011). Along with M. viridis and P. antipodarum, R. harrisii has contributed to habitat modification (Ezhova et al. 2005) but the extent to which is not well documented.

There is little or no evidence to support that Rhithropanopeus harrisii has the potential for significant socioeconomic impacts if introduced to the Great Lakes
In Texas, Rhithropanopeus harrisii has become very abundant in almost freshwater reservoirs with low salinities (≤ 0.5 ppt) and is reported to foul PVC intakes in lakeside homes (Richey 2004). In the Caspian Sea, where it has reached very high densities, Rhithropanopeus harrisii causes economic loss to fishermen by spoiling fishes in gill nets (Zaitsev and Öztürk 2001).

There is little or no evidence to support that Rhithropanopeus harrisii has the potential for significant benefits if introduced to the Great Lakes
Rhithropanopeus harrisii can consume Dreissena polymorpha (Kujawa 1957) but the degree of its potential impact on Dreissenid populations is unknown.

R. harrisii zoeae have been used for a variety of toxicology studies to identify lethal and sublethal concentrations. The attributes of R. harrisii for these studies are that (1) the zoeae are easy to rear in the laboratory with low mortality; (2) they are robust and can complete development in a wide range of temperatures and salinities; and (3) since they are retained in estuaries, they would be exposed to land runoff that could contain pesticides, herbicides, and other potential toxicants (Forward 2009). 

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