'. BIOLOGICAL CONTROL OF IOLLER ALGAE, CAULERPA TAXIFOLIA '' Lars WJ Anderson USDA- ARS Exotic and Invasive Weed Research Davis, CA
ABSTRACT Caulerpa taxifolia, a marine alga, has invaded Mediterranean subtidal ecosystems from small founder populations in 1984 to present expanse of about 6,000ha, including coasts of France, Monaco, Italy, Spain and I, Croatia, and Tunisia. It was recently found in Australia and Southern California in 2000. It has successfully displaced several native, benthic communities and can establish on a variety of substrates from sand to rocky shores. Reproduction and spread is clonal via stolons supporting fronds from 0.2 to 3 meters long. The small, California populations have been under vigorous containment and eradication since their discovery under the direction of the Southern California Caulerpa Action Team (SCCAT). Four species of herbivorous gastropods (mollusks) have been examined in Europe for potential reduction of th extensive Mediterranean C. taxifolia populations ( Oxytnoe azuropunctata, Elysia subornatat, Oxynoe olivacea and Lobiger serradifalci) E. suboranata, a tropical species, though unable to survive below 15C, has direct, benthic development (no pelagic larvae), feeds well at higher (e.g.>20C) temperatures, and appears to be the best candidate. However, since it is not native, further testing of host-specificity and potential for switching is required. L, serraedifalci, a native to the Mediterranean, feeds on C. taxifolia, but also tends to produce fragments, which can spread the population further. At present, there is no ' ' proven, effective biological control agent for C. taxifolia.
INTRODUCTION Nearly 20 years ago, one of the most interesting and aggressive marine invasive species, Caulerpa taxifolia, entered Mediterranean waters off Monaco and began its dominance of subtidal habitats there (Meinesz, 1999). This alga has many biological characteristics that facilitate its successful establishment, dispersal, and displacement of native species. C. taxifolia, (Byropsidales) a green siphonous alga, is a true single-celled coenocytic organism possessing adequate architectural strength to form large, persistent colonies in spite of the wave and tidal action of coastal environments (Fig. 1.) This strain can grow in very low light, yet does well in moderately high light of clear, shallow waters. It tolerates a wide range of water temperatures 10 to 31 °C, but has maximum growth in temperatures at 20°C and above (Komatsu et al. 1997, Gacia et al. 1996). Thus, it could become established in coastal waters of Mexico both U.S. coasts. (However, it will not tolerate fresh water.) Note: The genetic relationships of Caulerpa taxifolia was recently reported by Jousson et al. (2001), who showed that the introduction in the US (California) was indeed the same as the Mediterranean strain. Dispersal and spread is through elongation of rapidly growing stolons as well as from fragments of the thalli (fronds) that may be produced from wave action, foraging fish or human activities such as fishing and anchoring. Very small, sub-centimeter sized pieces can produce new plants. However, reproduction appears to be exclusively vegetative in the Mediterranean strain- only male gametes have been observed (Zuljevic and Antolic 2000). Growth rates of one to a few several cm/day have been observed. In addition, Williams and Grosholz (2002) reported the presence of many "proliferations", or second-order branch points on fronds in one of the California populations. The importance of recruitment via fragments was examined by Ceccherelli and Cinelli (1999), who showed, through release of fragments at 3 and 10 m depths during different seasons, that successful establishment was greatest from June to September at the margins of native seagrass, Posidonia oceanica stands. Deep water fragment movement off Monaco (45m to 100 m) was shown by Belsher and Meinesz (1995). This raises the interesting question of how grazers feed, not just how much they feed. For example, grazing in the Mediterranean by a native gastropod, Lobiger serradifalci, appears to disperse the plant by creating fragments (Zuljevic et al. 2001). Displacement of native algae, and general dominance of benthic substrate, is achieved through elongation and proliferation of anchoring stolons, dense canopy formation, and through the presence of the toxin, caulerpenyne, which appears to deter grazing by most organisms.
79 Fig. 2. Locations of Caulerpa ta:xifolia in the USA. (Agua Hedionda and Huntington Harbor)
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61\H of C.t.rxffolil> (Aliff aetlOldna 10 al~ ono yoM) ~'.;r::.4:=UM~'fJS,1.91) 2.:T---..(q l : Z,..,4"$'-Rl~,-(~-1><-9.1) 4 :t-•w-1oo(lf) J : p•ruh s.,.~.. (91) •: fl,l"
Fig. 3. Spread of C. ta:xifolia in the Mediterranean areas.
It is clear that, with the fortuitous advantage of having observed the prolific spread of C. taxifolia in Europe, the USA reaction was both reasonable and offered. the best hope for long-term prevention of further incursions in coastal waters here. It is important to note that in addition to the immediate containment and eradication effort, the California state legislature passed a bill banning the sale, transport and possession of C. taxifolia and eight other "look-alike" species in Caulerpa genus. Thus, from an operational as well as political perspective, the response to these first (last?) populations was very quick.
81 ... .. Table 1. Some characteristics of four potential biological control agents for C. taxifolia. ... Species Provenance Larval Opt. Host-Specificity Feeding Development Temp ("preferences") Characteristics Range for Feedin2 Lobiger Mediterranean Pelagic l 7-25C C.prolifera Incisions lead to serradifaclci C.taxifolia wholes and (w/she/1) fragments; low feedine: rate. Elysia subornata Tropical Direct, 17-25 C C. taxifolia Incisions on all parts; (wlo shell) Benthic <15C is C. racemosa kleptoplasty; stores lethal caulerpenin; high feeding rate ( 1Ox others). Oxynoe Tropical Direct, 17-25C C. taxifolia Incisions on all parts; azuropunctata Benthic C. racemosa stores caulerpenin; (w/shel/) medium feedine: rate. Oxynoe olivacea Mediterranean Pelagic 12-25C C. prolifera Feeds on individual (w/she/1) C. taxifolia pinnules; low feedine: rate.
Caulerpa taxifolia and C. racemosa are not the only siphonous algae that have expanded beyond native ranges, nor which are readily consumed by sacoglossans. The alga, Codium fragile and various sub-species (some of which are now prolific in, but not indigenous to, Europe) have been the subject of a large body of work by Cynthia Trowbridge at Oregon State University. She has examined several aspects of sea slug feeding (e.g. Placida dendritica), reproduction and responses in grazed species, and "switching" behavior of more catholic sea slug foragers. The approaches Trowbridge has taken (see Trowbridge, 199la,b; Trowbridge, 1993; Trowbridge and Todd, 2001; Trowbridge, 2002 ) are models for more in-depth work on potential agents to control invasive Caulerpa species. Clearly, the complexities of host-attractants, deterrents, physical responses to feeding (e.g. location of radula incision and cytoplasm extraction), and group feeding behavior will be important to understand as part of developing a strategy for a Caulerpa biocontrol program.
.Allochtonous species J - - ~ Ernbryonn!c stdge 20 lf;,ys
Direct dcvclop,)111~::11 Fig. 4. Life cycle of Elysia subornata (From Theirry Elysia subomata Thibaut).
Benthic - juvenile Lile time 9 month
r,,einosz et al 1996, Thibaut ot al. 2001
83 Piazzi, L., G. Ceccherelli, and Fr. Cinelli. 2001. Threat to macroalgaldiversity: effects of the introduced green alga Caulerpa racemosa in the Mediterranean. Mar Ecol. Prog. Ser. 210: 149-159. Thibaut, T. and A. Meinesz. 2000. Are the Mediterranean ascoglossan molluscs Oxynoe olivacean and Lobiger serradifalci suitable agents for a biological control against the invading tropical alga Caulerpa taxifolia? C.R. Acad. Sci. Paris. 323:477-488. Thibaut, T., A. Meinesz, P. Amade, S. Charrier, K. De Agnelis, S. Ierardi, L. Mangialazo, J. Melnick and Vidal. 2001. Elysia subornata (Molusca) a potential control agent of the alga Caulerpa taxifolia (Chlorophyta) in the Mediterranean Sea. J. Mar. Bio. Ass. U.K. 81:497-504. Trowbridge, C. D. 1991a. Diet specialization limits herbivorous sea slug's capacity to switch among food species. Ecol. 72: 1880-1888. Trowbridge, C.D. 1991b. Group memberfshop facilitates feed of the herbivorous sea slug Placida dendritica. Eco. 72: 2193-2203. Trowbridge, C.D. 1993. Interactoins between an ascoglossan sea slug andits green algal host: branch loss and role of epiphytes. Mar. Ecol. Prog.Ser. 101: 263-272. Trowbridge, C.D. and C.D.Todd. 2001. Host plant change in marine specialist herbivores: Ascoglossan sea slugs on ... introduced macroalgae. Ecol. Monogr. 71: 219-243 . ' Trowbridge, C.D. 2002. Northeastern Pacific sacoglossan pisthobrancs: Natural history review, bibliography, and prospectus. The Veliger 45: 1-24. Uchimura, M., A. Ribal, A. Mato, R. San deaux, J. Sandeaux, and J.C.Baccou. 2000. Potential use of Cu2+, K + and Na+ for the destruction of Cau/erpa taxifolia: Differential effects of photosynthetic parameters. J. Applied Phycology 12: 15-23. Williams, S. and E. D. Groscholtz. 2002. Preliminary reports from the Caulerpa taxifo/ia invasions in southern California. Marine Ecol. Progress Series (in press). Zuljevic, A. and B. Antolic. 2000. Synchronous release of male gametes of Caulerpa taxifolia (Caulerpales, Chlorophyta) in the Mediterranean Sea. Phycologia 39: 157-159. Zuljevic, A. , T. Thibaut, H. Elloukal, and A. Meinesz. 2001. Sea slug disperses the invasive Caulerpa taxifolia. J. Mar. Biol. Ass. U.K. 81: 343-344.
85 Fig. 1. Examples of Caulerpa taxifolia cultured from original populations found in Agua Hedionda Lagoon (California, USA).
ERADICATE OR MANAGE CAULERPA TAXIFOLIA?
The USA Approach
Even before C. taxifolia was discovered in Carlsbad, CA in June, 2000, its track record had prompted inclusion on the Federal Noxious Weed List, as well as early plans to prevent an introduction (Keppner and Caplen,1999). The draft Prevention Program is comprehensive consisting of nine components, which include: Coordination and Leadership; Dispersal Mechanisms and Pathways Analysis; Surveillance and Detection; Control; Research; Regulatory; Legislation; Education; and International Activities. Successful implementation of these components is dependent on establishing and maintaining effective partnerships among international, federal, state, tribal, private and public organizations. In addition, existing information gaps demand the attention of the scientific research community. Future iterations and updates to the Prevention Program will be necessary to refine prevention strategies and to ensure that implementation is effective, efficient, and environmentally sound (Anderson and Keppner, 2001). Based upon several conditions at the sites of infestation in California, as well as the serious threat to much of the California coastal ecosystems, a consortium of federal, state and private stakeholders opted to quickly initiate containment and eradication treatments at both Agua Hedionda Lagoon (Carlsbad, CA) and Huntington Harbour (near Long Beach, CA) (Figure 2). The Southern California Caulerpa Action Team (SCCAT) took the leadership role in conducting this program. Two years after the discovery, the original 12,000 sq.ft. in Agua Hedionda Lagoon has been tarped and treated with chlorine. There are now just a few scattered plants found occasionally following regular SCUBA surveillance. Similarly, plants in Huntington Harbor have been contained and treated. The cost of the program so far is approximately $1.1 million per year. As part of this effort, an International Caulerpa taxifolia Conference was convened in January, 2002. The attendees included several scientists from European countries along the Mediterranean coasts, who were generally impressed with both the rapid response and the effectiveness of treatments (Anderson, 2002). (Note: The Proceedings of this conference will be available by the fall of2002.)
80 .. The European Approach
As Meinesz points out eloquently (1999), the reaction to spreading C. taxifolia in the Mediterranean was one of "denial", coupled with ignorance, and complicated (as is often the case) by bureaucratic face-saving. The end result is a population that is certainly beyond rational hope of eradication, but one that demands some level of management to protect un-iofested areas and to reduce impacts where it has already become established (Fig. 3). Although some localized successes have been observed with various chemical and mechanical methods, the enormous expanse of populations suggests that biological control may be useful, if some appropriate agent can be found.
BIOLOGICAL CONTROL Over the past 5 to 7 years, various approaches have been taken in searching for either indigenous grazers or potentially exotic herbivores that might be used in a biological control program. As in all such strategies, one hopes for a sufficiently host-specific herbivore that has environmental requirements and tolerances overlapping those of target plant, C. taxifolia. Furthermore, given the dynamics of subtidal ecosystems, successful agents will need to be adept at handling highly variable water flows (wave-energies), not be preyed upon too excessively, and have adequate reproductive and dispersal abilities to proliferate and spread. Table 1 listed the four species of ascoglossans (Mollusca: Opisthobranchia) and some of their biological characteristics relevant to suitability for biocontrol. (Thibaut et al. 2001; Thibaut and Meinesz, 2000, Thibaut, pers. comm.).
The high feeding rate, direct, benthic larval development and reproductive rate of E. subornata makes this I' species the best candidate for biocontrol (Thibaut et al. 2001). (Life cycle is shown in Figure 4). Coquillard et al. (2000) used growth and grazing models to show the potential utility of E. subornata. Thibaut and Meinesz (2000) conducted feeding trials with Oxynoe olivacea and Lobiger serradifalci and concluded that these species will probably not provide adequate foraging damage, nor will they likely have sufficient recruitment capability. Furthermore these authors note that inspite of long-term presence of C. taxifolia in Mediterranean waters, there appears to be no significant feeding impacts by these two Mediterranean sea slugs. Current biocontrol efforts with E. subornata remain at the laboratory level in Europe since approvals to conduct small scale introductions have not met with favorable reaction from governmental regulatory agencies. Thus, the initiation of a biological control program is still very questionable; meanwhile, C. taxifolia continues to spread. Work by Ceccherelli and Cinelli (1999a) and Ceccherelli and Sechi (2002) suggest that influences of nutrient levels on interactions between the native seagrass Cymodocea nodosa and C. taxifolia is not terribly predictable, though in some cases, high nutrients may favor C. taxifolia. Overall, however, neither plant was inhibited by the presence of the other species. Both are likely to co-exist in some circumstances. To complicate matters, Piazzi and Ceccherelli (2002) have reported that, based upon reciprocal transplants studies, another Caulerpa species, C. racemosa, which has been spreading in the Mediterrane;m, may even out-compete C. taxifolia, or at least hold its own with C. taxifolia. Ceccherelli and Piazzi (2001) also demonstrated the importance of fragment-driven dispersal in C. racemosa. Therefore, it seems that a successful biocontrol agent shouldn't be so restrictive that it excludes C. racemosa. This may another good reason to pursue E. subornata, even if it means much more extensive testing to insure lack of feeding upon native plants.
82 SUMMARY The protection of our coastal waters in California requires a multi-million dollar commitment of funds and a pledge from agencies and organizations to support and implement prevention and eradication measures. Research initiatives on the biology, reproduction, ecology and control of C. taxifolia will advance control efforts leading to the implementation of the most cost-effective and environmentally sound strategies. However, California can not wage this battle alone. National leadership is required to provide a framework to protect all coastal habitats vulnerable to invasion, including the Florida, Texas, Hawaii, and Carolina shores. Emergency response protocols . should be established to ensure eradication in a manner consistent with that exemplified by SCCAT. National public education and outreach is necessary to increase awareness to promote early detection and increase the likelihood of successful eradication. Early detection is paramount to the successful control of any invasive species, and for Caulerpa, there are international. Unlike Europe and Australia, the US is not yet (and one hopes will never be) in a position to give up on prevention and eradication as the first priority. However, it is in our interest to support research and development of all reasonably methods for countering C. taxifolia and other invasive algal species. This includes supporting work on biological control. We need to be examining potential temperate-water agents and we need to work more closely with colleagues in Europe who are trying extremely hard to find a solution to the ever-expanding populations of this alga. Alternatives to chlorine also need t be examined as well, as part of the current eradication program (Uchimura et al. 2000). In short, all avenues to prevent introductions and establishment should be explored because the risks of a full-blown proliferation in US coastal and estuarine waters are a reality now.
ACKNOWLEDGEMENTS I thank members of SCCAT for providing some of the information presented here, and Thierry Thibaut for providing graphics on sea slugs and pertinent information in Table 1.
REFERENCES Anderson, L.W.J. 2001. Caulerpa taxifo/ia: New marine algal invader in U.S. Waters. Aquatic Invaders 12, 1-6 Anderson, L.W.J. 2002. Caulerpa taxifolia in the United States: Rapid response and eradication program. Proc. Intematl. Caulerpa taxifolia Conf. San Diego, CA (in press). Anderson, L.W.J. and S. Keppner 2001. Caulerpa taxifolia: marine algal invader provokes quick response in U.S. waters. Aquatic Nuisance Spec. Digest 2, 13-23. Belsher, T. and A. Meinesz. 1995. Deep-water dispersal of the tropical alga Caulerpa taxifolia introduced into the Mediterrane. Aquat. Bot. 51, 163-169. Ceccherelli, G. and F. Cinelli. 1999a. A pilot study of nutrient emiched sediments in a Cymodocea nodos bed invaded by the introduced alga Caulerpa taxifolia. Bot. Marin.42, 409-417. Cechercelli, G. and F. Cinelli. 1999b. The role of vegetative fragmentation in dispersal of the invasive alga Caulerpa taxifolia in the Mediterranean. Mar. Ecol. Prog. Ser. 182, 299-303. Ceccherelli, G. and L. Piazzi. 2001. Dispersal of Caulerpa racemosa fragments in the Mediterranean: Lack of detachment time effect on establishment. Bot. Marina 44, 209-213. Ceccherelli G. and N. Sechi. 2002. Nutrient availability in the sediment and reciprical effects between the native seasgrass Cymodocea nodosa and the introduced rhizophytic alga Caulerpa taxifolia. Hydrobiologie (in press). Coquillard, P. T. Thibaut, D.R.C. Hill, J. Gueugnot, C. Mazel and Y. Coquillard. 2000. Simulations of the mollusc Ascoglossa Elysia subornata populations dynamics: application to the potential biocontrol of Caulerpa taxifolia growth in the Mediterranean Sea. Ecol. Modeling 135, 1-16. Gacia, E., Rodriquez-Prieto, C., Delgado, 0. and Ballesteros, E. 1996. Seasonal light and temperature responses of Caulerpa taxifolia from the northwestern Mediterranean. Aquatic. Bot. 53, 215-225. Jousson, O. , J. Pawlowski, L. Zaninetti, F.W. Zechman, F. Dini, G. Di Guiseppe, R. Woodfield, A. Millar and A. Meinesz. 2001. Invasive alga reaches California. Nature 408, 175-158. Keppner, S.M. and R.T. Caplen. 1999. A Prevention Program for the Mediterranean Strain of Caulerpa taxifo/ia. U.S. Fish and Wildlife Service, Lower Great Lakes Fishery Resources Office, Amherst, NY. Komatsu, T., Meinesz, A., Buckles, D. 1997. Temperature and light responses of alga Caulerpa taxifolia introduced into the Mediterranean Sea. Marine Ecol. Prog. Ser. 146, 145-153. Meinesz, Alexander. 1999. Killer Algae. Univ. Chicago Press, 360 p. (Translated by Daniel Simberloff). Piazzi, L. and G. Ceccherelli. 2002. Effects of competition between two introduced Caulerpa. Mar. Ecol. Prog Ser. 225, 189-195.
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