Suitable length of shrimp meat food sticks for rearing juvenile bigfin reef squid (Sepioteuthis lessoniana)

The bigfin reef squid (Sepioteuthis lessoniana) is a valued species in Thailand that is now cultivated and has great commercial potential. Nevertheless, the correct feed management for cultivation is still debated, particularly at the juvenile stage and for non-living food items. In the present study, shrimp meat sticks (1.5 to 2.0 mm diameter) of various lengths (4, 5, 6, 7, and 8 mm) were given to juveniles and the effects were observed on growth, feed utilization, morphometrics, digestive enzyme activities, and mantle quality. A completely randomized experimental design was formulated, comprising triplicate groups of 10-day-old juvenile squid (n = 8 per tank) that were given food sticks made solely of whiteleg shrimp (Litopenaeus vannamei) meat for two weeks. At the end of the feeding trial, squid that received 4-mm food sticks, compared with squid that received food sticks of other lengths (p < 0.05), showed significantly higher final body weight, weight gain, and protein efficiency ratio, with a significantly decreased feed conversion ratio. Squids fed 5-mm food sticks grew the same as those fed 4 mm, but their protein efficiency ratio was lower. Longer food lengths (6, 7, and 8 mm) had detrimental effects on squid. The morphometric parameters of head length, head width, dorsal mantle width, fin length, and fin width were significantly higher in squid that received 4-mm food sticks, followed by squid that received 5-mm food sticks. Calorimetric analysis of squid mantle tissue showed that the muscles of squid that ingested longer food sticks developed more myosin and actin, which could assist muscle contraction. These changes were consistent with the observed increased specific activities of the proteolytic enzymes chymotrypsin and trypsin. However, across all dietary groups, there were no differences in RNA concentrations and RNA/protein ratios in muscle tissue, although the mantle protein concentration decreased in squid receiving longer food sticks. These findings imply that developing an artificial feed with a length of 4 mm could enhance the growth and feed efficiency of juvenile bigfin reef squid.