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<I>Lobophora Variegata</I> (Phaeophyta

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<I>Lobophora Variegata</I> (Phaeophyta BULLETIN OF MARINE SCIENCE, 68(2): 207–219, 2001 THE IMPORTANCE OF LOBOPHORA VARIEGATA (PHAEOPHYTA: DICTYOTALES) AS A HABITAT FOR SMALL JUVENILES OF PANULIRUS ARGUS (DECAPODA: PALINURIDAE) IN A TROPICAL REEF LAGOON Patricia Briones-Fourzán and Enrique Lozano-Álvarez ABSTRACT The fishery for the spiny lobster Panulirus argus in the area around Puerto Morelos is one of the least productive in the state of Quintana Roo (Mexican Caribbean coast). Density of postalgal juveniles of P. argus (i.e., juveniles ≥ 15 mm carapace length, CL) is low in the shallow areas of the reef lagoon at Puerto Morelos despite high annual indices of puerulus influx into the lagoon as measured with artificial collectors. We hypothesized that a population bottleneck might exist at the algal juvenile phase (juveniles 6–15 mm CL, residing in vegetated shallow areas), owing to a lack of suitable habitats for the settlement of incoming postlarvae and the subsequent survival of algal juveniles. At five sites within the reef lagoon, algal juveniles were collected with an epibenthic net during June 1995 (early summer) and March 1997 (late winter). In March, we estimated, in the same five sites, the density of two species of seagrass and five functional-form groups of macroalgae, as well as the biomass of Lobophora variegata (ruffled form), a drift phaeophyte, which grows in extremely dense mats over a number of substrates. Signifi- cantly more algal juveniles were obtained in March than in June but this temporal pattern was apparently not related to peaks in influx of postlarvae into the reef lagoon. The abun- dance and distribution of algal juveniles in March was not related to the density of seagrass or algae, but was significantly related to the biomass of L. variegata. The ruffled form of L. variegata is very abundant in seagrass meadows throughout the Caribbean; hence, it may represent an important habitat for pueruli and algal juveniles of P. argus in other areas as well. Density of algal juveniles in the reef lagoon was 146 ha−1 in June and 263 ha−1 in March. Considering a conservative average of 150 algal juveniles ha−1, and a residence of 2 mo in the algal habitats, the reef lagoon can harbor around 900 algal juveniles ha−1 in a year, a figure comparable to productive areas for lobster such as the Florida Keys, USA. Therefore, the lobster population bottleneck in Puerto Morelos does not occur at the algal-juvenile phase. The spiny lobster Panulirus argus (Latreille) is a major fishing resource throughout the Caribbean Sea. In Mexico, it is heavily fished on the eastern margin of the Yucatán Pen- insula (State of Quintana Roo). Along the coast of Quintana Roo, some lobster fishing areas are very productive, whereas in others P. argus is rather scarce. The area around Puerto Morelos (20°51'N, 86°55'W) is one of the latter. Because of poor catches in this area, fishermen must travel several kilometers to the north and south of Puerto Morelos to catch lobsters (Padilla-Ramos and Briones-Fourzán, 1997). After a protracted oceanic larval life, the final larval stage of P. argus metamorphoses into a postlarva called puerulus, which returns to the coast and settles in shallow, veg- etated areas where it molts and takes residence for a few months. Reported settlement habitats for pueruli of P. argus include mangrove roots (Acosta and Butler, 1997), seagrass meadows (Buesa, 1965), and especially beds of rhodophyte macroalgae of the genus Laurencia (Marx and Herrnkind, 1985a). Laurencia is considered a prime habitat for settled pueruli and algal juveniles [i.e., juveniles ≤15 mm carapace length, CL, (Butler 207 208 BULLETIN OF MARINE SCIENCE, VOL. 68, NO. 2, 2001 and Herrnkind, 1997)] along the Florida Keys, where it is very abundant. However, al- though Laurencia occurs throughout the Caribbean, the hard bottoms where this alga thrives are not equally common in the Caribbean, where most of the coasts typically have mangrove-fringed shorelines and seagrass beds (Acosta and Butler, 1997). The reef lagoon at Puerto Morelos extends from the coastline out to a coral reef that lies slightly diagonally at a distance of 700–1300 m. Mangroves have no direct contact with the sea because they are separated by a sandbar 100–200 m in width, which forms the shoreline of the reef lagoon. Depth in the lagoon ranges from <1 to 5 m. The water in the lagoon is highly oligotrophic (Ruiz-Rentería et al., 1998). The bottom of the reef lagoon is covered by calcareous sand, which is stabilized by extensive seagrass beds dominated by the turtle grass Thalassia testudinum. The manatee seagrass Syringodium filiforme and a vast array of macroalgal species grow interspersed with T. testudinum. Ruiz-Rentería et al. (1998) divided the lagoon into three zones, based on its vegetation: (1) a narrow coastal fringe, 20–50 m wide, dominated either by S. filiforme or T. testudinum; (2) a broad mid-lagoon zone, 200–1000 m wide, dominated by either T. testudinum with long blades or algae, and (3) an area of backreef vegetation, 100–400 m wide, where S. filiforme is virtually absent and T. testudinum has variable densities of mostly short blades. The monthly influx of P. argus pueruli into the Puerto Morelos reef lagoon has been monitored for the past 9 yrs by means of a set of six floating artificial collectors (Briones- Fourzán, 1994). Pueruli enter the reef lagoon throughout the year, with peaks at different months in different years. Topographic features of the continental shelf and the surface water current patterns drive water from the Yucatán Current into the shallow areas of Puerto Morelos throughout the year (Merino-Ibarra, 1986; Briones-Fourzán, 1994). There- fore, annual indices of puerulus influx (measured as the mean number of pueruli per collector per calendar year) into the reef lagoon at Puerto Morelos are high. For example, they are consistently three to six times higher than at Bahía de la Ascensión (150 km south of Puerto Morelos) where a successful fishery for P. argus exists (Briones-Fourzán, 1994; Briones-Fourzán et al., 1998a). Paradoxically, despite the high influx of postlarvae, the abundance of postalgal juve- niles 15–50 mm CL throughout the Puerto Morelos reef lagoon is extremely low (0–31 lobsters ha−1, Lozano-Álvarez et al., 1998). Because reported settlement habitats for pueruli and subsequent residence of algal juveniles (6–15 mm CL) are scarce or virtually absent from the reef lagoon (e.g., mangrove roots, beds of rhodophytes), we hypothesized that a lobster population bottleneck might exist in this area at the algal juvenile phase. To test this hypothesis, we estimated the density of algal juveniles in the reef lagoon in two different seasons (early summer and late winter) and related these results to the type and distribution of the marine vegetation in the lagoon. MATERIAL AND METHODS The methods herein described were implemented to obtain information on the abundance of the algal juveniles of P. argus, and also of the benthic fauna that would serve as potential food for juvenile lobsters. Results on the benthic fauna appear elsewhere (Briones-Fourzán and Estrada- Olivo, 1998; Monroy-Velázquez, 2000). Five sampling sites were selected throughout the reef lagoon to conduct the study (Fig. 1). Sites 1, 2 and 3 were located in the mid-lagoon zone at depths of 3.0 to 3.5 m. In this zone, typical values BRIONES-FOURZÁN AND LOZANO-ÁLVAREZ: DRIFT ALGAE AS HABITAT FOR JUVENILE SPINY LOBSTER 209 Figure 1. The reef lagoon at Puerto Morelos, Mexico. The large dots indicate the five benthic sampling sites. The small asterisks denote the set of six pueruli collectors. of the standing crop of T. testudinum are 20–35 g m−2 dry weight (DW), with occasional patches up to 80 g m−2 DW, which are nevertheless low compared to other areas throughout the Caribbean (van Tussenbroek, 1995). Blades of T. testudinum reach 20–25 cm long (Reyes-Zavala, 1998). Standing crops of S. filiforme are even lower (2–30 g m−2 DW). Low standing crops (up to 20 g m−2 DW) of Laurencia and other rhodophytes occur only in very small, isolated areas where the hardground is exposed, but other rhizophytic and drift algae abound in the sandy mid-lagoon zone, particularly the phaeophyte Lobophora variegata (Lamouroux) Womersley, which forms extensive mats up to 210 BULLETIN OF MARINE SCIENCE, VOL. 68, NO. 2, 2001 500 m in diameter (Rodríguez-Almazán, 1997) with a maximum standing crop of 100 g m−2 DW (Ruiz-Rentería et al., 1998). Sites 4 and 5 were located in the backreef area at depths of 4 and 3 m respectively (Fig. 1). In this area, the standing crop of T. testudinum is 15–30 g m−2 DW but the maximum height of its blades is 9–10 cm (van Tussenbroek, 1995). S. filiforme and most algae are typically absent, but in certain parts Halimeda spp. occur. We used an epibenthic sleigh-net, with a mesh aperture of 1 mm, to catch the algal juveniles. The mouth of the net was rectangular, 0.57 m wide × 0.25 m high. In each sampling site, 10 net trawls were made during the day (11:00–12:00) and 10 during the night (20:00–21:00). Successive trawls in each site were conducted in such a way as to avoid going over the same place twice. Each trawl had a duration of 1 min at a speed of 1 m s−1, and was visually monitored by a diver to ensure that the net performed properly. The diver measured the distance traveled by the net on 16 different trawls (nine in the mid-lagoon zone and seven in the backreef area) with a 100 m long tape, to obtain the average area trawled by the net in order to estimate the density of algal juveniles.
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