Ascocotyle (Phagicola) longa, Ransom, 1920

Sarabeev, V., 2015, Mortality Of Juvenile So-Iuy Mullet, Liza Haematocheilus (Teleostei, Mugilidae), In The Sea Of Azov Associated With Metacercaria (Digenea), Vestnik Zoologii 49 (6), pp. 537-550 : 542-543

Ascocotyle (Phagicola) longa

Metacercariae were found predominantly in the oesophagus and intestinal walls, denticulate pharyngeal pad, internal organs and rarely in skeletal muscular tissues of host species. Metacercariae of A. (P.) longa were registered in L. haematocheilus during the whole period of the fish survey beginning from the second month of fish life (table 1). Both the prevalence and the mean helminth abundance indicate that the parasite load on zero year old juveniles was higher in 1997–1999 comparing to 2005–2014 (Mann-Whitney tests, U = 3657, p = 0.004). Metacercariae of A. (P.) longa were not registered in one month old juvenile fish, cysts of this helminth were found in fish beginning from the second month of their life. Both epidemiological indices were more or less constant in the period from the third month to the age of 1+, when they were relatively high with prevalence reaching 65 % and mean abundance of 23 specimens per host. In the older fish in the age of 2+, the parasite load was significantly lower than in younger fish in age from two month to 1+ (t = 4.1, p = 0.5). No relationships were found between the total fish length and the number of helminths per host following the regression analysis (R 2 <0.05, p> 0.05).

There was approximately linear relationships between the exponent k and the prevalence (R 2 = 0.91, p <0.0001), and the VMR and mean abundance (R 2 = 0.74, p <0.001). The age dynamics of the VMR in juveniles was represented by two peaks with the subsequent sharp decrease in its value. Both peaks correspond to the highest mean abundance that declined in the subsequent age group. The VMR increased in adult fish regardless of low mean helminth abundance. The exponent k of the NBD was relatively low with a maximum rate not exceeding 0.25 pointing to the highly aggregated distribution of A. (P.) longa in L. haematocheilus . In other words, it tells us about a relatively low proportion of the fish in the susceptible tail of infection.

Juvenile fish at the age of 3 months to 1+ year old were combined in one data set to apply for Crofton’s truncation model because of the even infection parameters in these age groups. The negative binomial parameters were estimated from the fish with 0–45 parasites. The observed frequency of a remained tail was higher than predicted counting five values (table 2) in the range from 58 to 400 metacercariae per host, but this data was not fit for a further categorized analysis. Comparison of parasite distribution between fish of zero-one year old and at the age of two years was not conducted due to the small sample size of the latter age group (table 1).