The distribution of the recessus orbitalis across flatfishes (order: Pleuronectiformes).

The recessus orbitalis is an accessory organ of flatfishes functioning in the protrusion of the eyes. This character, along with cranial asymmetry and a forward insertion of the dorsal fin, have been considered synapomorphies for the Pleuronectiformes. New dissections and examination of images taken in the wild show that the recessus orbitalis is present in all representatives of Pleuronectoidei examined but is absent in the single species of Psettoidei dissected. Psettoidei, the most primitive pleuronectiform lineage, contains three recognized species; thus, the absence of the recessus orbitalis in this whole lineage is unclear without further dissections. Ancestral character estimation at the family level for the recessus orbitalis indicates that the recessus orbitalis was likely absent in the common ancestor of Pleuronectiformes but was most likely present in the common ancestor of the Pleuronectoidei. Given that so few species of flatfishes have been assessed for the recessus orbitalis to date, additional characterization of the distribution of the recessus orbitalis across flatfishes will further inform what states this character may have and if it is a synapomorphy of Pleuronectiformes or simply a derived character state of Pleuronectoidei.

Flatfish monophyly refereed by the relationship of Psettodes in Carangimorphariae.

BACKGROUND: The monophyly of flatfishes has not been supported in many molecular phylogenetic studies. The monophyly of Pleuronectoidei, which comprises all but one family of flatfishes, is broadly supported. However, the Psettodoidei, comprising the single family Psettodidae, is often found to be most closely related to other carangimorphs based on substantial sequencing efforts and diversely analytical methods. In this study, we examined why this particular result is often obtained. RESULTS: The mitogenomes of five flatfishes were determined. Select mitogenomes of representative carangimorph species were further employed for phylogenetic and molecular clock analyses. Our phylogenetic results do not fully support Psettodes as a sister group to pleuronectoids or other carangimorphs. And results also supported the evidence of long-branch attraction between Psettodes and the adjacent clades. Two chronograms, derived from Bayesian relaxed-clock methods, suggest that over a short period in the early Paleocene, a series of important evolutionary events occurred in carangimorphs. CONCLUSION: Based on insights provided by the molecular clock, we propose the following evolutionary explanation for the difficulty in determining the phylogenetic position of Psettodes: The initial diversification of Psettodes was very close in time to the initial diversification of carangimorphs, and the primary diversification time of pleuronectoids, the other suborder of flatfishes, occurred later than that of some percomorph taxa. Additionally, the clade of Psettodes is long and naked branch, which supports the uncertainty of its phylogenetic placement. Finally, we confirmed the monophyly of flatfishes, which was accepted by most ichthyologists.

Metal uptake in Psettodes erumei and Hysterothylacium spp. larvae in the Persian Gulf: Evaluation of larvae as bio-indicator.

This study aimed to compare the concentrations of heavy metals in Psettodes erumei as host fish and larvae of Hysterothylacium spp. as its parasite. Moreover, to evaluate the larvae as bio-indicators the uptake of heavy metals, the infected and non-infected fish were also compared. Fresh P. erumei species (n = 19) were randomly sampled during four months from Bushehr County, Iran. The digestive tract of each fish was examined for nematode parasites using a stereomicroscopy. The isolated nematodes were identified, and content of Fe, Cu, Zn, Co, Ni, Cr, As, Cd, Hg, and Pb were measured using ICP-OES. The metal concentrations were simultaneously analyzed for the muscles of infected fish and their parasites, as well as non-infected ones. Of the 19 P. erumei examined, 13 (68.4%) P. erumei were infected with Hysterothylacium spp. larvae. The parasites had significantly higher level of Fe, Cu, Zn, and Ni (with mean value of 7.59, 0.572, 1.223, and 4.623 mg/kg, respectively) than the muscles of the host fishes (with mean value of 3.29, 0.0010, 0.586, and 0.277 mg/kg, respectively) (p < 0.05). Infected hosts showed significantly lower amounts of As element in their muscles (0.050 mg/kg) than non-infected hosts (0.113 mg/kg) (p < 0.05). The findings emphasize the potential role of Hysterothylacium spp. larvae as bio-indicators for monitoring heavy metals pollution in marine ecosystems.