We explore the trophic role that a diverse sympatric group of fishes in the genus Chirostoma play in a large, shallow lake in central Mexico, Lake Chapala. We use δ13C and δ15N stable isotope - based food web analyses to explore how they relate to other components of the Lake Chapala ecosystem. We find five Chirostoma species in top trophic levels of the Chapala food web compared to other fishes, relying on a combination of zooplankton, fish and benthic resources as energy sources. Food web metric analyses showed generally overlapping trophic niches for members of Chirostoma, especially in terms of δ13C. However, C. jordani had lower mean δ15N isotopic values than C. promelas. As a group, "pescados blancos" (C. sphyraena and C. promelas) also had higher δ15N signatures than "charales" (C. consocium, C. jordani and C. labarcae) reflecting greater piscivory, but these differences were not strong for all food web metrics used. Trophic overlap among species of Chirostoma in Lake Chapala raises questions about the forces that might have led to a morphologically diverse but functionally similar and monophyletic group of species.

http://www.scielo.br/pdf/ni/v13n2/1982-0224-ni-20140079.pdf

Stable isotope evidence for trophic overlap of sympatric Mexican Lake Chapala silversides (Teleostei: Atherinopsidae: Chirostoma spp.)

Phylogeographic patterns and species delimitation in the endangered silverside “humboldtianum” clade (Pisces: Atherinopsidae) in central Mexico: understanding their evolutionary history

The morphology of the orobranchial structures in fish generally reflects the feeding habits and food preferences of a species and is key in understanding the process and habits of fish feeding. This work describes the orobranchial structures of four neotropical silversides (Teleostei: Atherinopsidae): the Mexican pike silverside Chirostoma estor, the Mexican blacknose silverside Chirostoma promelas, the Argentinian silverside Odontesthes bonariensis and Patagonian pejerrey Odontesthes hatcheri, and correlates their morphological features with existing information on gut contents and feeding habits in adult fish. Results of the compared and analysed data in the present study portray an integrative view of the specialised habits of the different species. Significant morphological differences, which are related to primary feeding habits of four species, were found. C. promelas shows to be more piscivorous compared to O. hatcheri, which has main benthic habits, while O. bonariensis and C. estor have more zooplanktivorous preferences. This information provides knowledge of feeding habits and ecological interactions of these species and their prey.

The orobranchial structures in four neotropical silversides (Teleostei: Atherinopsidae) related with feeding habits

Summary
The trophic guild, niche breadth, and trophic position of two different ontogenetic phases were measured according to the size of Chirostoma spp. captured in different zones of Lake Patzcuaro, Mexico. Analyses were carried out at the genus level due the difficulties to find diagnostic characters to separate the three small silverside species accurately, since even the molecular analyses of mitochondrial DNA for a subsample of the most morphologically differentiated specimens were conducted without success. Fish were captured with a seine net during the wet (September and November 2009) and dry (February and June 2010) seasons at four environmentally different sites. Gut contents of 480 Chirostoma specimens were divided into size-class intervals of 38–51 mm and 66–71 mm for analyses. The contribution of each food item was quantified using the frequency of occurrence and area percentage. Fish were categorized by size, and diets were compared between fish sizes and sites. Dorsal muscle tissue and zooplankton were obtained for nitrogen isotope signature (δ15N) analyses. The trophic guild was determined using a modified quadrant method for gut content analysis; the importance of each prey item was estimated according to a modified index of relative importance. Niche breadth was calculated by the Levins’ Index. The trophic position was assessed using the TrophLab Program and stable isotope analysis. The index of relative importance among study sites and fish size classes was determined by a multivariate approach. Silversides represent a secondary and tertiary consumer in Lake Patzcuaro that feed mainly on evasive prey, such as copepods (55%) and on non-evasive prey, such as cladocerans (25%) at sites with the lowest transparency in the water column (from 350 mm to 150 mm). Fish scales (11%) and insects (6%) were secondary food items. Ingestion of juvenile silversides was rare (1%). Chirostoma spp. prey on insects on the lake bottom and on water hyacinth roots. Based on δ15N analysis, the trophic position of silversides changes between wet and dry seasons. Their opportunistic and specialized predatory feeding strategy indicates the capability of silversides to resist anthropic changes in the lake.

Feeding strategy of silverside species in eutrophic Lake Pátzcuaro, Mexico

A series of crater lakes exist in the central region of Puebla, Mexico; each one with an endemic species of silverside fishes, with no information about their ecology. Poblana letholepis is an endemic species to the Crater Lake La Preciosa and under human and physical pressure endangering its presence. The weight–length relationship of both sexes and juveniles were not statistically different, and their feeding habits are similar between sexes and juveniles as well as to other species of the same family.

http://www.scielo.org.mx/pdf/rmbiodiv/v87n4/1870-3453-rmbiodiv-87-04-01391.pdf

Length–weight relation and diet composition of Poblana letholepis, an endemic species from Central Mexico

Since Barbour proposed sympatric speciation to explain evolution of silversides in the Lerma-Santiago basin, relatively little subsequent study has been done. We assessed foraging patterns of four sympatric silversides species ( Chirostoma estor , Chirostoma grandocule , Chirostoma attenuatum and Chirostoma patzcuaro ) in Lago de Patzcuaro to understand resource partitioning and their sympatric coexistence. We assessed the abundance of invertebrate prey in three feeding habitats and measured physical and chemical habitat parameters at two study sites. Fish were collected during the wet (September 1987) and dry (March 1988) seasons; a total of 242 gut contents were analyzed. We evaluated the trophic guild of each species using the index of relative importance (IRI), prey selectivity with the Ivlev Electivity Index (E), dietary diversity using Shannon and Wiener diversity index (H’), and diet overlap using Morisita index. All silverside species were determined to be predacious carnivores that feed mainly on nekton and periphyton. Dietary diversity and prey selectivity patterns were similar among species and diet overlap was >70%. Our data do not support the proposition that coexistence of these four fish species is maintained by dietary specialization. We hypothesize that sympatric coexistence of atherinopsids in Lago de Patzcuaro is explained by food resource availability and ontogenetic variation in their diets. This study highlights the importance of analyzing ecological patterns and mechanisms as basic elements for designing conservation strategies of species flocks, especially under habitat loss and introduction of exotic species. Conservation efforts are urgent to preserve the rare evolutionary process of sympatric speciation (habitat segregation) that is occurring in other lakes in central Mexico, and probably already lost in the Lago de Patzcuaro, as a result of poor management and inadequate conservation strategies. KEY WORDS:  Atherinopsidae, feeding interactions, species flocks, evolutionary process, conservation.

/pdf/foraging-patterns-of-four-sympatric-species-of-silversides-24zqiddm3m.pdf

Rafael García-Ortega

Papers

Foraging patterns of four sympatric species of silversides (Atheriniformes: Atherinopsidae) in Lago de Pátzcuaro, Central Mexico