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Feeding habits of the convict surgeonfish Acanthurus triostegus (Teleostei: Acanthuridae) on the Los Frailes reef, Baja California Sur, Mexico.

INTRODUCTION

The Acanthuridae or surgeonfish are distributed in tropical and subtropical oceans. There are 80 known species distributed in six genera (Nelson 2006). The convict surgeonfish, Acanthurus triostegus (Linnaeus, 1758), is probably the species with widest distribution, having populations from the Indian Ocean to the eastern tropical Pacific. The wide geographic range of this species has allowed the differentiation of populations. Randall (1956) recognized three subspecies: Acanthurus triostegus sandvicensis for Hawaii, A. triostegus marquesensis for the Marquesas Islands in the central Pacific and Acanthurus t. triostegus for the rest of its distribution, including the Line Islands and eastern tropical Pacific. Genetic studies have shown strong differences among populations, with a high genetic diversity for the Hawaiian populations, which validates the subspecies name sandvicensis Randall, 1956, for the convict surgeonfish in that area (Planes & Fauvelot 2002).

The longevity of convict surgeonfish individuals is of about 40 years (Longenecker et al. 2008); they have separate sexes and reproduce during the day in schools of 200 to 2000 individuals (Craig 1998). Females mature (164 mm; 440 days) later than males (90 mm; 168 days) and reach greater sizes. There are estimates of up to 362 000 eggs per year (Longenecker et al. 2008), and the larval phase can last 44 to 60 days, which favors a wide distribution (McCormick 1999).

In the Gulf of California there are six species from two acanthurid genera, most with an Indo-Pacific affinity (Robertson & Allen 2002), among them the convict surgeonfish A. triostegus. In particular in the Los Frailes reef area, which is within the perimeter of the Marine Park of Cabo Pulmo in Baja California Sur (B.C.S.), Mexico, A. triostegus is a species considered to be common and is among the 12 most dominant species on the reef (Moreno 2009).

However, there is no information on the trophic biology of the species in the area. The few studies on trophic ecology of herbivorous fish in the Gulf of California have been carried out on indigenous species that cohabit with A. triostegus, such as the yellowtail surgeonfish Prionorus punctatus (Montgomery et al. 1980), the giant damselfish Microspathodon dorsalis and the Cortez damselfish Stegastes rectifraenum (Montgomery 1980a, b). However, there are studies carried out in the barrier reef of Aldabra, Indian Ocean (Robertson & Gaines 1986) and in the Hawaiian Islands (Randall 1961), where the trophic spectrum of A. triostegus was characterized.

In the present study we characterize the diet of A. triostegus and consider the diversity, frequency and abundance of algae species consumed by this fish.

MATERIAL AND METHODS

The fish were captured monthly from November 2004 to October 2005 in the rocky reef of Los Frailes (23[degrees]5' N and 109[degrees]5' W), in the southern part of the Baja California peninsula, Mexico. Specimens were captured using a pole spear by a free diver between 10:00 and 16:00 hours, the time of best visibility. For each collected fish the total weight (TW) and total length (TL) were recorded and the stomach was extracted. Gastric contents were put in plastic bags and frozen for later analysis at the Fish Ecology laboratory at the Centro Interdisciplinario de Ciencias Marinas (CICIMAR-IPN) in La Paz, Baja California Sur.

During stomach content analysis we separated food categories (items) according to taxonomic group, identifying each to the minimum possible taxon, depending on digestion stage. For the taxonomic identification of algae we used the keys by Dawson (1944, 1961), Abbott & Hollenberg (1976) and Espinoza-Avalos (1993).

Once the taxonomic work was completed, we analyzed stomach contents quantitatively, for which we used:

The percentage of frequency of occurrence (%FO), referred as the frequency of occurrence of prey items within the total number of stomachs with food.

%FO= No. of stomachs including a prey item-No. of stomachs with food x 100

The gravimetric composition percentage (%W) is the wet weight of prey items found within the total wet weight of stomachs with food:

%W= Weight of prey item-Total weight of prey items x 100

It is worth noting that due to the nature of the feeding habits of this species (algae grazers) it was not possible to employ a numeric method to quantify units. In order to evaluate in an integral manner the importance of each food type in the species' diet, we used the Index of Relative Importance (IRI) of Pinkas et al. (1971), modified by Ojeda & Munoz (1999):

IRI = % FO x %W

We calculated the diet width (Bi) using the standardized Levin's index (Hurlbert 1978) from the absolute values obtained using the gravimetric method. This index takes values from 0 to 1. When Bi values are under 0.6, for example, a predator is considered a specialist, which indicates that it uses a low number of resources and presents a preference for certain food items. When values are closer to one (0.6) the spectrum is of a generalist, meaning that species uses all resources without selection.

RESULTS

Overall we collected 50 fish (all had stomach contents). We identified 35 items, of which 18 were algae from the Class Rhodophyceae, 10 were Chlorophyceae and six were Phaeophyceae (Table I).
Table I. Absolute and percent values of the frequency of occurrence
(FO), gravimetric (W) and index of relative importance (IRI) methods
of the Acanthurus triostegus diet in reef Los Frailes, B.C.S. * NIOM
(non-identified organic matter).

Class          Species                       W     % W       FO

Chlorophyceae  Ulva linza                   94.7   42.1   28.0

               Cladophora spp.              3.44   1.53  16.00

               Derbesia marina              2.11   0.94   7.00

               Ulva lactuca                 1.99   0.88  11.00

               Bryopsis spp.                1.78   0.79  18.00

               Rhizoclonium spp.            1.68   0.75   9.00

               Codium simulans              0.19   0.08   3.00

               Caulerpa racemosa            0.15   0.06   1.00

               Cladophorosis fasciculatus   0.09   0.04   1.00

               Enteromorpha spp.            0.08   0.03   1.00

Rhodophyceae   Gelidiella spp.             27.83  12.36  27.00

               Polysiphonia simplex        16.47   7.32  27.00

               Porphyra spp.                7.56   3.36   7.00

               Amphiroa valonioides         6.12   2.72  22.00

               Laurencia spp.               5.92   2.63  22.00

               Hypnea musciformis           3.58   1.59  12.00

               Champia spp.                 3.30   1.47  14.00

               Jania mexicana               3.28   1.46  11.00

               Gracilaria spp.              2.26   1.00   8.00

               Herposiphonia spp.           1.68   0.75   4.00

               Pitophilium spp.             0.83   0.37   1.00

               Ceramium spp.                0.48   0.21   3.00

               Pterocladiella spp.          0.38   0.17   3.00

               Ahnfeltia spp.               0.20   0.09   1.00

               Centroceras spp.             0.11   0.05   1.00

               Gracilaria velanoae          0.07   0.03   2.00

               Goniotrichum alsidi          0.07   0.03   1.00

               Erythrotrichia spp.          0.05   0.02   1.00


Phaeophyceae   Dictyota crenulata           2.14   0.95  14.00

               Sphaelaria spp.              1.93   0.86  10.00

               Lobophora spp.               1.58   0.70   6.00

               Ectocarus spp.               1.22   0.54   6.00

               Dictyopteris delicatula      0.74   0.33   4.00

               Padina concrescens           0.01   0.01   1.00

               * NIOM                      31.04  13.79  20.00

               TOTAL                         225    100     50

Class          Species                     % FO     IRI    % IRI

Chlorophyceae  Ulva linza                   56.0  2356.4  51.50

               Cladophora spp.             32.00   48.94   1.07

               Derbesia marina             14.00   13.11   0.29

               Ulva lactuca                22.00   19.41   0.42

               Bryopsis spp.               36.00   28.53   0.62

               Rhizoclonium spp.           18.00   13.43   0.29

               Codium simulans              6.00    0.50   0.01

               Caulerpa racemosa            2.00    0.13   0.00

               Cladophorosis fasciculatus   2.00    0.08   0.00

               Enteromorpha spp.            2.00    0.07   0.00

Rhodophyceae   Gelidiella spp.             54.00  667.68  14.59

               Polysiphonia simplex        54.00  395.12   8.64

               Porphyra spp.               14.00   47.05   1.03

               Amphiroa valonioides        44.00  119.54   2.61

               Laurencia spp.              44.00  115.72   2.53

               Hypnea musciformis          24.00   38.19   0.84

               Champia spp.                28.00   41.04   0.90

               Jania mexicana              22.00   32.08   0.70

               Gracilaria spp.             16.00   16.06   0.35

               Herposiphonia spp.           8.00    5.98   0.13

               Pitophilium spp.             2.00    0.74   0.02

               Ceramium spp.                6.00    1.29   0.03

               Pterocladiella spp.          6.00    1.03   0.02

               Ahnfeltia spp.               2.00    0.18   0.00

               Centroceras spp.             2.00    0.10   0.00

               Gracilaria velanoae          4.00    0.12   0.00

               Goniotrichum alsidi          2.00    0.06   0.00

               Erythrotrichia spp.          2.00    0.05   0.00

Phaeophyceae   Dictyota crenulata          28.00   26.61   0.58

               Sphaelaria spp.             20.00   17.14   0.38

               Lobophora spp.              12.00    8.45   0.19

               Ectocarus spp.              12.00    6.52   0.14

               Dictyopteris delicatula      8.00    2.62   0.06

               Padina concrescens           2.00    0.01   0.00

               * NIOM                      40.00  551.70  12.05

               TOTAL                                4576    100


Using the gravimetric method, all items had a total biomass of 225.08 g. The green alga Ulva linza represented 42.08% (94.71 g), non-identified organic matter (NIOM) represented 13.79% (31.04 g), Gelidiella spp. represented 12.36% (27.83 g), Polysiphonia simplex represented 7.32% (16.47 g) and Porphyra spp. represented 3.36% (7.56 g) (Fig. 1).

Using the frequency of occurrence method, the green alga Ulva linza was recorded in 56% of stomachs (28 stomachs) followed by Gelidiella and P. simplex with 54% (27 stomachs) respectively, while the red algae Amphiroa valonioides and Laurencia were recorded in 44% of stomachs (22 stomachs), and NIOM was recorded in 40% of stomachs (20 stomachs) (Fig. 1). According to the IRI, the most important food item was Ulva linza, which represented 51.50% of the A. triostegus diet, followed by Gelidiella spp. with 14.59%, NIOM with 12.06%, and Polysiphonia simplex with 8.64%. The Levin's index showed that the diet width of A. triostegus is small (Bi = 0.10), so that from its trophic behavior it can be considered a specialist.

[FIGURE 1 OMITTED]

DISCUSSION

The convict surgeonfish A. triostegus is a strict herbivore, since all recorded items were algae, confirming the report by Robertson & Allen (2002), who mentioned that acanthurids feed on algae throughout tropical Pacific Ocean reefs. For the Los Frailes area in particular, we identified 35 algae species, which represents 55.7% of the flora diversity reported for the Cabo Pulmo-Los Frailes area by Anaya-Reyna & Riosmena-Rodriguez (1996).

Acanthurus triostegus in the Los Frailes reef fed on a high proportion of green and red algae and on a smaller proportion of brown algae. Robertson & Gaines (1986) detected this same feeding pattern for the species at Aldabra Atoll in the Indian Ocean. In that study the green alga Ulva was the fourth in importance, in contrast to the present study in which Ulva linza was the most important. Likewise, Randall (1961) described the diet of A. triostegus sandvicensis captured in the Hawaiian Islands, and stated that it fed preferentially on the red alga Polysiphonia and on the green alga Enteromorpha, completing its diet with a variety of algae among which the most important were Hypnea, Ceramium, Gracilaria and Rhizoclonium species.

The dominance in the diet of the green alga Ulva linza and the red algae Gelidiella and Polysiphonia simplex, as well as the low Levin's index value (Bi = 0.10), indicated that A. triostegus has a specialist trophic behavior. Among the factors that can determine the food selection patterns by herbivorous fish are availability and relative abundance of the food (Horn 1983), the hardness of the algal thallus, the assimilation efficiency and the nutritional value (Littler & Littler 1980), as well as the presence of secondary compounds present in algae (Targett & Targett 1990). It is interesting to note that in the present study a secondary element found in the stomachs of A. triostegus was sand. This has been reported for other acanthurid species and it is thought that its presence can help the fish more rapidly and efficiently grind up plant tissues, which would optimize digestion and energy assimilation (Randall 1967).

The indigenous herbivorous fish species with which Acanthurus triostegus cohabits in the Los Frailes reef feed on the same algal assemblage. The yellowtail surgeonfish Prionurus punctatus consumes green and red algae (Ulva linza and Gelidiella) (Montgomery et al. 1980), the giant damselfish Microspathodon dorsalis is a non-selective herbivore that consumes a large proportion of red algae Polysiphonia (60.5%), Gracilaria (8.2%) and Ceramium (5.8%) while the Cortez damselfish Stegastes rectifraenum feeds on a wide variety of species, among them red algae such as Gracilaria (36.4%), Ceramium (7.1%) and Polysiphonia (7.2%), as well as green algae Ulva (16.1%) (Montgomery 1980a, b). However, in most cases where a similarity in diets has been reported, the effect of high food abundance can be seen, and when the resource is not abundant other factors come into play such as spatial and temporal segregation, as well as trophic plasticity of the species that interact; therefore, it is considered that these species do not compete for food (Moreno et al. 2009).

Apart from alimentary segregation, such as was shown in this study, fish can also segregate spatially within the same reef. For example, acanthurids form resident mobile schools with diurnal habits, with variable densities. Acanthurus triostegus forms temporal monospecific schools; Prionurus punctatus forms monospecific or mixed schools with P. laticlavius, mainly in shallow areas; while the schools of Acanthurus xanthopterus prefer the reef edges (Montgomery et al. 1980; Thomson et al. 2000). The pomacentrids Microspathodon bairdii, M. dorsalis, Stegastes rectifraenum, S. acapulcoensis and S. flavilatus are territorial species that vigorously defend their refuge from intruders, including conspecifics and other herbivorous fish (Wellington & Victor 1988), which allows the coexistence of several herbivorous species in the area.

According to the results obtained, we confirm that Acanthurus triostegus is a diurnal herbivorous fish and a substrate grazer, which had already been documented for this species and other species from the same genus (A. chirurgus, A. bahianus and A. coeruleus) (Randall 1967).

ACKNOWLEDGEMENTS

We would like to thank the Instituto Politecnico Nacional (IPN) for funds received through COFAA and EDI. Xchel G. Moreno Sanchez, Deivis S. Palacios Salgado and Ofelia Escobar Sanchez thank the Consejo Nacional de Ciencia y Tecnologia (CONACyT) and the Programa Institucional de Formacion de Investigadores (PIFI-IPN).

Received: 23 August 2010 - Accepted: 08 January 2011

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Leonardo A. Abitia-Cardenas, Xchel G. Moreno-Sanchez *, Deivis S. Palacios-Salgado and Ofelia Escobar-Sanchez

Centro Interdisciplinario de Ciencias Marinas (CICIMAR-IPN), Departamento de Pesquerias y Biologia Marina. Apdo. Postal 592. La Paz, Baja California Sur, Mexico. C.P. 23000.

* E-mail: xchel.moreno@gmail.com
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Author:Abitia-Cardenas, Leonardo A.; Moreno-Sanchez, Xchel G.; Palacios-Salgado, Deivis S.; Escobar-Sanchez
Publication:aqua: International Journal of Ichthyology
Article Type:Report
Geographic Code:1MEX
Date:Jul 10, 2011
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