The effects of Caulerpa taxifolia on invertebrate abundance in Agua Hedionda Lagoon, California. Karoly Tippets Abstract Sea beds composed of eel grass (Zostera marina), Caulerpa taxifolia and both vegetation types were sampled at a lagoon in San Diego County at the first known invasion site of the Caribbean algae (C. taxifolia) in the United States. The hand-collected samples captured organisms from the surrouding water column, top sediment layer and vegetative matter. The abundance of Isopoda, Amphipoda and Spirorbidae populations were quantified. Weight adjusted samples were analyzed by means of the non-parametric Mann-Whitney U-test. The most significant decrease (p<0.05) in abundance of all three taxa was between the eel grass and Caulerpa samples. The effects of C. taxifolia invasions, and the ramifications on the benthic ecology of nearshore lagoons, could be devastating to the basal levels of the food chain.Introduction The challenges of dealing with invasive species have become of global concern with the relative ease of transcontinental movement (Ricciardi et al, 2000). Marine invasive organisms are more prevalent than their terrestrial counterparts due in part to the unwitting transmission on the hulls and in the ballast water of ships. The accidental introduction of C. taxifolia, an algae native to the Caribbean Sea, into the Mediterranean Sea in 1984 occurred when a small piece of algal material exited out of a flow-through aquarium in Monaco (Meinesz and Hesse, 1991). This exotic alga now covers hundreds of hectares of the Mediterranean seafloor, with patches reported up to hundreds of kilometers from original release point (Joussen et al., 1998). The spread of C. taxifolia is due to an initial lack of eradication efforts and the amazing adaptability of this species to new environmental conditions, including colder water temperatures (Chrisholm et al., 2000). Other characteristics of the algae that hinder containment efforts are the use of fragmentation as a method of dispersal (Ceccherelli and Cinelli, 1999; Smith and Walters, 1999) and the lack of suitable biocontrol organisms (Lemee et al., 1996; Coquillard et al., 2000; Zuljevic et al., 2001; Thibaut et al., 2001). The absence of biocontrol organisms is due to the production of caulerpenyne by C. taxifolia. Caulerpenyne is a naturally occurring neurotoxin that severely retards grazers (Pedrotti et al., 1996; Bitou et al., 1999; Ricci et al., 1999; Pedrotti and Lemee, 1999). Studies correlating the detrimental effects of C. taxifolia on invertebrates have focused on physiological impacts caused by the neurotoxin caulerpenyne (Mozzachiodi et al, 2001). The majority of research focusing on the non-caulerpenyne affects of C. taxifolia on fauna has been conducted on fish. These studies have described qualitative and quantitative changes in fish communities (Relini et al., 1998; Relini et al., 2000) and specific physiological changes in some fish (Uchimura et al., 1999). C. taxifolia is also known to cause a reduction in density and abundance of native sea-grasses in the Mediterranean Sea (De Villele and Verlaque, 1995; Ceccherelli and Cinelli, 1997; Ceccherelli and Cinelli, 1998). C. taxifolia was first discovered in the United States in June 2000 in Agua Hedionda Lagoon, California, and to date there have been no published studies conducted on the new infestation. Agua Hedionda is an ideal study site not only because it allowsobservations of a preliminary infestation of C. taxifolia, but also the lagoon has a pre-established baseline for native populations (MEC, 1995) which may be useful in identifying any opportunistic species. Study of this organism will be important on an ecological level if eradication efforts in the previously infested Huntington Harbor, CA and Agua Hedionda Lagoon are not successful. A decline in certain “keystone” species, including amphipods, may result in an eventual decline in all species dependent upon these organisms for food with ramifications extending up the food chain. The objective of this study is to determine the effects of C. taxifolia upon the abundance of marine invertebrates by comparing population differences in infested areas with C. taxifolia with those colonized only by native sea grasses. Methods Description of study subjects Three taxa were chosen for analysis: Amphipoda, Isopoda and Spirobidae. Amphipods and isopods were chosen because they are two of the most dominate invertebrate phyla represented in Agua Hedionda Lagoon according to a 1995 technical report (MEC, 1995) and form a necessary trophic level near the bottom of the food chain (Borga et al, 2001). The calcareous tube-forming worms, Spirobids, were chosen to represent the effects of C. taxifolia on organisms that use eel grass as a primary substrate upon which they live. Infaunal and epifaunal organisms are reasonable choices as bioindicators owing to their close association with the flora in addition to their relative immobility (Lafont, et al., 2001; Nerbonne and Vondracek, 2001). Data Collection To assess changes in invertebrate abundance caused by the invasion of C. taxifolia, three cover types were chosen from within Agua Hedionda Lagoon, CA. The three types of cover are 100% C. taxifolia, a roughly 50-50 mix of C. taxifolia and eel grass, and 100% eel grass. All sample sites are located in the lagoon on the eastern side of Interstate 5. Sampled sites were within a 150m2 area of the lagoon and all samples were roughly equidistant from each other. The small alcove in which the sampling took place ranges from shallows (< 2 m) to depths of 8 m. During the month of August 2000, 24 one-gallon samples were removed from the alcove. The samples were collected via use of a SCUBA device to diminish production of currents that could fragment and consequently spread the C. taxifolia. One-gallon plasticjars were used to collect the samples at an average depth of 4 meters below the surface of the water. The jars were dug into the sediment deep enough to obtain most of the rhizome mass anchoring the C. taxifolia in the Caulerpa and mix samples. The samples contained all of the vegetative material from each bunch of eel grass or C. taxifolia. Samples were immediately transported to shore, drained of water and filled with formalin. Proper storage of samples should be in 70% isopropyl alcohol. Due to financial and time constraints, samples were stored in formalin for approximately one year. As each of the 24
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