Timoniella imbutiforme (Molin, 1859) Brooks, 1980

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 : 540-542

Timoniella imbutiforme

Metacercariae were found in skeletal muscular tissues and oesophagus wall of host species. Fry and juveniles of L. haematocheilus from the Molochny Estuary were highly infected by T. imbutiforme in 1997 and 1999, while in the subsequent years this metacercaria was not found in samples of so-iuy mullet except a single record of this worm in November 2014 (table 1). Analysis of the age dynamics of prevalence and abundance of T. imbutiforme shows that infection increased with age, until the fish were at age 3 months and 31–40 mm long. After this, both epidemiologic indexes remained more or less constant for zero year old juveniles. Beginning from one year old juveniles, the infection of T. imbutiforme declined and reached minimal values in two years old fish. The parasite survey of adult individuals revealed slightly higher abundance but the occurrence was about twice lower than in two years old juveniles. Mann-Whitney tests revealed the significant difference in the helminth abundance between compared zero year old and one-two years old fish (U = 4558, p <0.0001). The regression analysis of relationships between the total fish length and the number of helminths per host shows a similar picture of dynamics indicating significant decrease of parasite intensity with fish length for one-two years old juveniles (fig. 1).

The present observation of aggregation indexes for metacercariae of T. imbutiforme revealed the approximately linear relationships between the exponent k and the prevalence (R 2 = 0.82, p <0.001), while the VMR is predominantly affected by the mean helminth abundance (R 2 = 0.4, p = 0.07). The distribution patterns of T. imbutiforme in L. haematocheilus well fit to the NBD in all surveyed age groups of fish except in age of two month, two years old juveniles and adult fish (table 1). Estimated over-dispersion pattern, measured by the VMR, in zero year old fish, was at least twice higher than that of one-two year old fish. The VMR was influenced by infections in zero year old fish as they intensity transformed

Log

Log transformed fish total length

Fig. 1. Relationship between the fish total length and the intensity of T. imbutiforme in zero year old (solid line and solid point) and one-two years old (dashed line, open rhomb) of L. haematocheilus . **Referred to a significant level of 99 %.

had a high mean abundance compared with older juveniles thus increasing the variance of the variable. The exponent k of the NBD increased during the first year of life from highly aggregated (k <1) to aggregated (k> 1) distribution and decreased in the older age group up to value of 0.03 in adult fish (table 1). The highest values of epidemiologic indexes combined with relatively high values of both the VMR and k in juveniles of 5–6 months old may indicate a critical age period when the parasite-induced mortality starts to operate on host population. Both the parasite abundance and the VMR substantially decreased in the following age group category (the 95 % confidence interval of the abundance did not overlap between compared groups, F = 7.9, p <0.01 and the VMR in 4.6 times as less in the older group), while the prevalence remained high (89 %) and the k even increased from 1.02 to 1.29. This observation points out that highly infected fish is absent in the group of one year old juveniles, although infection is highly prevalent in the population. Thus, it may indicate the presence of the density dependent process operated during the first year of the fish life that resulted in the loss of highly infected hosts due to the parasite-associated fish mortality. In older juvenile fish, the k decreased together with all other indexes applied here to quantify the infection rate.

Based on the infection dynamics of T. imbutiforme the data set was split in 2 groups, juveniles of 3–6 months, and 1+ –2+ years olds as the pre-mortality and post-mortality periods to conduct the frequency analysis of the parasite distribution. The helminth aggregation is well described by truncated model with the parameters derived from 0 to 4 parasites for juveniles of 3–6 months old and 0 to 2 parasites for young fish of 1+ –2+ years old. The frequency analysis of T. imbutiforme predicted that the helminth distribution in both the pre-mortality and post-mortality periods was truncated. The truncation was calculated to occur within the first few infection categories given that the threshold for parasite-associated mortality begins at the presence of 3–4 metacercariae of T. imbutiforme per fish (table 2, fig. 2 ). The truncation point was actually at the beginning of the distribution tail in juveniles of 3–6 months old, while it was higher towards the tail end in fish of 1+ –2+ years old, indicating that the majority of fish had an abundance of infection above the truncation point in the pre-mortality period vs. below in post-mortality (i. e., to the right vs. left side of the curve; see fig. 2 and table 2). The negative binomial curve for 1+ –2+ years old fish is shifted to the lower position, beginning at the level of 3–4 parasites per fish comparing with both the observed and estimated curves of the helminth distribution in juveniles of 3–6 months old ( fig. 2 ). The difference between pre-mortality and fitted post-mortality curves is shown as the survival curve, which declines towards the tail end of the worm distribution on the so-iuy mullet . The proportion of the recruit loss as a result of the infection of T. imbutiforme is relatively high and reaches 55 %. For infected fish the estimated loss is slightly higher and counts 67 %.

Figures

Fig.2. Frequency distribution of T.imbutiformesplit following the pre-mortality and post-mortality events.Solid line with solid point and dashed line with open square = the observed and predicted pre-mortality frequency distribution; dashed line with solid triangle = the fitted post-mortality frequency distribution; solid double line with crosses and symbols = the estimated percentage survival of fish with that number of parasites.