BACKGROUND: Maturity of silver carp (Hypophthalmichthys molitrix) ovary takes place at 2-8 years old and it is depends on photoperiod and environmental water temperature. Objectives: The aim of this study was to identify  macroscopical and microscopical structure of silver carp  ovary in  two categories of  body weight, total body length and  at  2 and 4 years old  in Khuzestan province climatic conditions. Methods: 20 silver carp ovaries were studied in two groups: Group 1(G1): 10 fish with total body length 46 ±1.33 Cm and weight 1050±0.09 grand approximately 2 years old. Group 2(G1): 10 fish with total body length 86±3.64 Cm and weight 5460±800 gr and approximately 4 years old. For the histometrical studies, 5-6 µ sections were made by routine paraffin embedding method and stained by H&amp;E and PAS. Results: Microscopic results showed that ovaries of G1 covered by a thin loose connective capsule which numerous of egg-bearingtrabecula extended from capsule into ovaries. These trabecula contained primary germ cells, oogonium, chromatin nocleolus, perinocleolus and cortical alveolus follicles. Ovaries of G1 do not have yolk and mature follicles. But in ovaries of G2, growth and mature follicles were observed and mature follicles were most follicles. Histometrical results showed that there are not a significant differences in diameter of follicles, oocytes and oocytes nucleolus in G1 and 2. The maximum and minimum diameter of follicles were seen in mature and chromatin nucleolus follicles respectively. The Zona plucida thickness was increased from corticalalveolus (1.53±1.11 mm) to mature follicles (6.88±0.21 mm) significantly (p<0.05). ConclusionS: The most interesting finding of this study is that in Khuzestan Province climate, the ovary of silver carp fish in G1, with average weight 1050 ±0.09 gr and total length 46±1.33 Cm and approximately of 2 years old is immature but ovary of G2 with average weight of 5460± 800 gr and total length 86±3.64 Cm and approximately 4 years old is mature.

BACKGROUND: Dactylogyrus is one of the most common external parasites on                the gills of Cyprind fish. These parasites are highly host specific and many species only have a specific host. OBJECTIVES: Since there are reports of silver carp specific Dactylogyrus species isolated from big head carp and vice versa, the investigation of Dactylogyrids have been done in these two fish species. METHODS: 81 silver carp and 82 big head carp were caught from 10 fish farms in Guilan province and after preparing wet mounts of body surface Dactylogyrus parasites divided and fixed by glycerin jelly. In order to perform morphometric assessments on captured images, Image J software was used for 7 point to point distances. Drawing of parasites was done by drawing tube and then compared by identification keys and parasites identified. For molecular investigation the genomic DNA was extracted from one parasite specimen and 28S rDNA region of Dactylogyrus specimens were amplified by related primers in PCR. RESULTS: Sequences were deposited in GenBank with accession numbers MG825611 and MG825765 respectively for D. hypophthalmichthys and D. suchengtaii isolated from Hypophthalmichthys molitrix, and also MH023397 and MH023399 respectively for D. aristichthys and D. nobilis isolated from Hypophthalmichthys nobilis. The phylogenetic tree shows the genetic affinity of isolated parasites from these two fish. CONCLUSIONS: It seems hybrid fish are sometimes produced accidentally in fish reproduction centers of Iran. Racial impurity of silver carp and big head carp is not only the reason of poorer breeding efficiency in fish farms but also these hybrid fish are hosts of more parasitic species.