Lobsters and crabs as potential vectors for tunicate dispersal in the southern Gulf of St . Lawrence , Canada

Following anecdotal reports of tunicates on the carapaces of rock crab (Cancer irroratus) and American lobster (Homarus americanus), we evaluated the role of these species and northern lady crab Ovalipes ocellatus as natural vectors for the spread of invasive tunicates in the southern Gulf of St. Lawrence. Several hundred adult specimens of crabs and lobster from two tunicateinfested estuaries and Northumberland Strait were examined for epibionts. Small patches of Botrylloides violaceus were found on rock crabs examined from Savage Harbour and a small colony of Botryllus schlosseri was found on one lobster from St. Peters Bay. Lobster and lady crab collected in Northumberland Strait had no attached colonial tunicates but small sea grapes (Molgula sp.) were found attached on the underside of 5.5% of the rock crab and on 2.5% of lobster collected in Northumberland Strait in August 2006. Lobster and rock crab clearly represent a vector for the spread of invasive tunicates regionally and wherever living crustaceans are shipped globally.


Introduction
Since about 1998, the viability of the bivalve aquaculture industry in eastern Canada, and particularly in Prince Edward Island (PEI, Figure 1), has been challenged by the colonization and spread of several non-indigenous tunicate species: the clubbed tunicate (Styela clava Herdman, 1882), first detected in 1997; the golden star tunicate (Botryllus schlosseri (Pallas, 1766)) in 2001; the violet tunicate (Botrylloides violaceus (Oka, 1927)) in 2002; and the vase tunicate (Ciona intestinalis (Linnaeus, 1767)) in 2004 (Locke et al. 2007, Ramsay et al. 2008).The rapid spread of these non-indigenous tunicates along the PEI coast has prompted several investigations into the vectors facilitating their dispersal.Much of the emphasis has been on processes related to the bivalve aquaculture industry, recreational boating, and commercial boating for wide-range dispersal (e.g., papers in this issue).Scant attention has been directed towards natural vectors of spread.There are, however, anecdotal reports of tunicates occurring on shells of crabs and lobsters caught in commercial fisheries in these waters.To confirm the possibility that large crustaceans can transport invasive tunicates on their carapaces, we collected and examined adult specimens of crab and lobster collected from two tunicate infested estuaries (St.Peters Bay and Savage Harbour, PEI) and the Northumberland Strait to determine the presence/absence of invasive tunicates and other epibionts.The potential host species examined were American lobster (Homarus americanus H. Milne-Edwards, 1837), Atlantic rock crab (Cancer irroratus Say, 1817), and northern lady crab (Ovalipes ocellatus (Herbst, 1799).Lobster and rock crab were expected to occur in all three study areas, but northern lady crab were expected to be found only in Northumberland Strait (Voutier and Hanson 2008).

Methods
During November 2005, artificial structures in St. Peters Bay and Savage Harbour (Figure 1) exhibited moderate to high infestations of golden star (Botryllus schlosseri) and violet (Botrylloides violaceus) tunicates.Rock crab and American lobster were collected on 3-4 November by means of lobster traps baited with herring.Ten traps were set in each bay (5 traps in each of the inner and outer halves of the bays) for 24 hours.Each crustacean captured was measured (carapace length CL in mm for lobster; carapace width CW in mm for crabs), sex determined, the presence or absence of tunicates and other epibionts determined, and the percent coverage of epibionts estimated.The molting stage of all lobsters was estimated by means of a shell durometer, and the second pleopods were clipped and preserved in 95% ethanol for subsequent analysis in the laboratory (after Comeau and Savoie 2001).
Crab and lobster from Northumberland Strait (Figure 1) were also inspected for tunicates and other epibionts during a bottom trawl survey on 4-5 August 2006 (survey details in Voutier and Hanson 2008).At the time of the survey, B. violaceus was highly abundant and B. schlosseri was in low abundance off Borden (PEI).B. violaceus was found at Cape Tormentine (NB) approximately a month later.When epibionts were detected, the crab or lobster was taken to the laboratory for detailed examination and identification of the epibiont.

Results
No invasive tunicates were found on any of the 112 rock crab from St. Peters Bay.Small patches of Botrylloides violaceus (from 2-9 mm in diameter; Figure 2) were found on the carapaces of 5 of 275 rock crabs from Savage Harbour.However, the most common rock crab epibionts were green, red and brown algae, which were found on 58% and 64% of rock crab examined from St. Peters Bay and Savage Harbour, respectively.Small sea grapes (Molgula sp., Figure 3) were discovered either in clusters or individually on 5.4% of rock crab from St. Peters Bay and on 5.5% of those from Savage Harbour.These Molgula sp.ranged in size from 0.8-10 mm in width and 0.5-12 mm in length.Other epibionts (e.g., Anomia simplex d'Orbigny, 1847, Crepidula fornicata (Linnaeus, 1758), Crepidula plana Say, 1822, and various sponges and bryozoans) occurred on 6.3% and 4.7% of rock crab from Savage Harbour and St. Peters Bay, respectively.The six lobster collected from Savage Harbour were free of epibionts.In contrast, eight of 66 lobster collected from St. Peters Bay had bryozoans on them, and a colony (14 mm in diameter) of Botryllus schlosseri was found on the mid-ventral surface of one large female lobster (92 mm CL, Figure 4), which was in intermolt stage C 4 as described by Comeau and Savoie (2001).
On 4-5 August 2006, 199 lobster and 126 lady crab from Northumberland Strait were examined.Unfortunately, no rock crab were caught.Nonindigenous tunicates were not found on any lobster or lady crab; however, 2.5% of lobster had Molgula sp.attached on their ventral surface, typically between pereopods 2 and 4. Most Molgula could not be identified to species, but the native Molgula citrina Alder and Hancock, 1848 was identified on one of the large (144 mm CL) male lobster (Figure 5).

Discussion
Even though invasive tunicates were found on a relatively low proportion of the rock crab and lobster from Savage Harbour, St. Peters Bay, and Northumberland Strait, these crustaceans are clearly susceptible to colonization by nonindigenous and native epibionts.Moreover, this represents a significant potential vector for spread of invasive tunicates because there are many millions of adult crab and lobster in the southern Gulf of St. Lawrence where thousands of tonnes are landed each year (Department of Fisheries andOceans Canada 2002, 2007).
The composition of the fouling community appeared to differ between crustaceans captured in estuarine versus more open coastal waters.Algae were by far the most common epibionts of rock crab in estuarine waters of Savage Harbour and St. Peters Bay.In contrast, lady crab and lobster from Northumberland Strait had very few algal epibionts; however, Molgula sp. was present on crustaceans from both estuarine and coastal habitats, occurring on 5.5% of rock crab in estuaries and on 2.5% of lobster from Northumberland Strait.The limited information on the fouling community of the snow crab (Chionoecetes opilio (Fabricius, 1788)) that occur in the deeper offshore waters of the southern Gulf of St. Lawrence suggests bryozoans (94.5%) and polychaetes (68.8%) were the most common epibionts, and no tunicates were observed (Savoie et al. 2007).
The extent to which large crustaceans serve as vectors of spread of invasive tunicates (or any other invasive species group) will depend on the distance traveled and the frequency of migration between estuarine and coastal habitats.Unfortunately, relatively little is known about the migrations of commercial decapods in the estuarine and coastal waters of the southern Gulf of St. Lawrence.Lobster travel seasonally as much as 86 km each way in Northumberland Strait (Bowlby et al. 2007) and to a maximum of 24 km off the northern shore of Prince Edward Island (Comeau and Savoie 2002).Preliminary data from an acoustic tagging study conducted in the Montague/Brudenell (PEI) estuarine system suggest that rock crab move seasonally from at least 7 km into the estuary to locations in coastal waters outside of the estuary; however, the potential for movement between estuaries was not investigated (L.Comeau, Fisheries and Oceans Canada, unpublished data).Given these extensive movements, and the large populations involved, further study of the importance of crab and lobster as vectors of regional spread of invasive tunicates appears warranted.
Invasive tunicates may only be able to spread to non-infected areas on large crustaceans if they can remain attached for extended periods.Several defensive mechanisms (including substratum wettability, production of chemical repellants/toxins and mechanical sloughing, grooming and abrasion) have been suggested to diminish the colonization success of epibionts (Wahl 1997).However, surface wettability was not found to be an efficient antifouling defense mechanism for crustaceans in general (Becker et al. 2000) and neither was mechanical grooming for brachyuran crabs (Bauer 1989).Behavioral patterns such as sediment burrowing, hiding in amongst hard substrates, and emersion from water (especially for intertidal species) seem to be more efficient defenses against epibiont colonization (Becker and Wahl 1996).Indeed, there were remarkably few epibionts on the northern lady crab examined in this study, and this is a crab that spends much of its time buried deep in sand (Barshaw and Able 1990;Sponaugle and Lawton 1990).Finally, molting and molt frequency are the most obvious means for crustaceans to rid their exoskeletons of epibionts.
Molting by crabs and lobsters clearly would affect the success of epibiont colonization and the frequency of molting almost certainly affects the rate and extent of spread of epibionts.
Although molting might delay the dispersal of tunicates by these crustaceans, it would most likely not eliminate dispersal since invasive tunicates may be able to survive and proliferate on the discarded exoskeleton, assuming the exoskeleton is not immediately consumed by a lobster or crab, as often occurs.Some studies suggest that crustaceans with longer inter-molt duration (i.e.older individuals or ovigerous females) host more epibionts than those with shorter inter-molt periods (Abelló et al. 1990, Gili et al. 1993, Shields 1992).Consequently, because molting frequency decreases with age, the larger and older lobster and rock crab might represent more of a threat for tunicate dispersal than younger individuals that molt at least annually.
Our results indicate that lobster and rock crab should be considered as likely vectors for regional transport and spread of tunicates within the southern Gulf of St. Lawrence.Since our study was not focused in areas where Ciona intestinalis and Styela clava are abundant, we do not have any indication that these solitary invasive species can be transported by means of epibiosis on crab or lobster.Although Styela clava has been reported to occur on the carapace of rock crab (Thompson and MacNair 2004), it remains to be determined if this is a common occurrence.Lastly, living lobster are extensively shipped between processing plants and holding pens in Atlantic Canada and all parts of the USA.Thus, they may represent a hitherto poorly documented means of spread of hitchhiking alien marine species between marine waters.Given the long distances that live lobster are shipped (indeed, worldwide), a thorough investigation into the identity, prevalence, and invasion potential of the epibiont community on lobster would appear warranted.

Figure 1 .
Figure 1.Map of southern Gulf of St. Lawrence, Canada, showing locations sampled in November 2005 (Savage Harbour and St. Peters Bay), Northumberland Strait (yellow dot) sampled in August 2006 and other place names mentioned in the text.

Figure 2 .
Figure 2. Newly settled Botrylloides violaceus on dorsal surface of a commercial-size (>105 mm in carapace width) rock crab Cancer irroratus collected from Savage Harbour, Canada (Photo: N. MacNair).

Figure 3 .
Figure 3. Patch of ten Molgula sp.clustered together on ventral surface of Cancer irroratus from St. Peters Bay, Canada (Photo: A. Nadeau).

Figure 4 .
Figure 4. Colony of Botryllus schlosseri (14 mm in diameter) on large (92 mm carapace length) female American lobster (Homarus americanus) from St. Peters Bay, Canada.The orange organism to the right of the circled tunicate colony is a sponge (Photo: A. Nadeau).