Size Related Dietary Variations and Morphological Correlates in Three Freshwater Fishes from Shatt Al-Arab River, Basrah, Iraq

Abstract

This study was carried out to investigate the possible relationships of some morphometrics of gill rakers and intestine with size-related dietary variations in three sympatric freshwater fishes. A total of 245 specimens of yellowfin porgy, Acanthopagrus latus, redbelly tilapia, Tilapia zilli and greenback mullet, Liza subviridis (73, 90 and 82 specimens, respectively) were collected from Shatt Al-Arab River from Sep. 2010 to Aug. 2011. To facilitate the follow up of size-related variations, specimens of each fish species were divided into three representative size groups. Results of food analysis using the index of relative importance, IRI, indicated noticeable iter-and intra-specific variations in food habits between the examined species, where A. latus demonstrated evident carnivorous mode of feeding, T. zilli was herbivore and L. subviridis was mostly planktivore. Feeding mode was reflected obviously on gill arch length, gill raker number, length and width. Liza subviridis was characterized by more numerous (41-42), longer (3.38-5.83 mm) and thinner (0.09-0.14 mm) gill rakers, in comparison with other two species. Calculated gill raker gap and filtration area was varied also between species and size groups reflecting distinctive adaptive mechanisms for feeding modes. Filtration area was smaller in the planktivore L. subviridis (108.9-195.5), medium in the herbivorous T. zilli (151.7-267.5) and larger in the carnivore A. latus (174.4-331.2 mm 2). Relative gut length, RGL, showed relatively opposite trend appearing larger in T. zilli (1.86-3.11), medium in L. Subviridis (1.89-2.51) and smaller in A. latus (0.9-1.06). All the studied morphometrics exhibited reasonable correlations with fish size denoting possible ontogenetic food shifts in these species.