Activities of digestive enzymes and non-specific immune indicators in <italic>Diodon holocanthus</italic> during larval and juvenile stages

Abstract

In order to solve the problem of high mortality rate of longspined porcupinefish <italic>Diodon holocanthus</italic> larvae and juveniles, the changes in digestive enzyme activities and non-specific immune indexes from the newly-hatched larvae to juveniles (0-40 d) were studied. The relationship between total length and daily age of <italic>Diodon holocanthus</italic> larvae and juveniles could be fixed to the exponential functions. The activities of amylase, pepsin, trypsin, and lipase were identified in the newly- hatched larvae. During the whole experiment period, the amylase and trypsin activities showed a trend of increasing initially, followed by a decreasing, but then increased again(with the increased daily age), in which amylase activity reached the maximum at 20 d, trypsin activity reached the peak at 5 d, the overall activity of both increased with the daily age but the trypsin activity increased more obviously. Lipase activity decreased rapidly initially (1-5 d), followed by an significant increase (5-10 d), but then decreased drastically again(15-20 d) and maintained at a low level. The activity of pepsin increased gradually from 1 d to 15 d and reached the maximum at 15 d, after which it slowly reduced to the minimum. The activity of lysozyme (LZM) and immunoglobulin M(IgM) showed a trend of increasing first and then decreasing, in which immunoglobulin M activity reached the maximum at 10 d, while lysozyme reached the maximum at 5 d, and after that both decreased slowly and fluctuated and eventually remained at a low level. The above results indicated that the changes of digestive enzyme activities in the larvae and juveniles of <italic>Diodon holocanthus</italic> were closely related to their developmental stages and food types, and various digestive enzyme activities existed in newly-hatched larvae that had not yet eaten food. The non-specific immune indicators increased significantly during the mixed nutrition period when the larvae and juveniles of <italic>Diodon holocanthus</italic> transitioned from endogenous nutrition to exogenous nutrition, and then decreased slowly to a lower level, indicating that the nutritional transformation process had a greater impact on the immune function of larvae and juveniles, higher non-specific immune indicators increased the disease resistance of larvae and juveniles and enhanced the survival rate of breeding.