Skin defence mechanisms in fish larvae

Abstract

Wound healing and recovery from injury were investigated in eggs and larvae of herring (Clupea harengus L.), plaice (Pleuronectes platessa L.) and salmon (Salmo salar L.). The resistance of herring eggs to mecha.nical damage was first examined. The chorion of eggs before and just after fertilisation could be burst by loads of 4-30 g but eggs 5 h post-fertilisation could withstand over 1000 g without bursting. Resistance remained high until just before hatching when it decreased to 20-680 g. The vitelline membrane, however, showed a lower resistance at all stages. Early herring and plaice larvae were caught and eaten by medusae (Aurelia aurita, Tiaropsis multiserrata, Bougainvillea sp.), hydroids (Sarsia sp.), megalopa larvae of the prawn Nephrops norvegicus and adult mysids. Early herring larvae survived minor stings from an Aurelia ephyra, and also experimentally inflicted lesions such as superficial scratches, suction wounds and amputation of up to 2 mm of the tail in sea water. The caudal region of the primordial fin regenerated within a month when less than 1 mm was cut off. Yoll, sac and first feeding herring also survived an incision of 0.3 mm long through the body ventral to the notochord and dorsal to the gut; in starving larvae survival was poorer in the later stages of starvation. When skin was removed in larvae of all 3 species the mortality depended on the area of the lesion, thp maximum area tolerated increasing with larval size. In sea water the threshold area was 0.1-0.2 mm2 dO.r 6-8 mm long plaice, <: 0.3 mm for 10-13 mm long herring and 0.3-0.4 mm for 14-17 mm long herring. In river water the threshold was 1~2mm for 19':1l2m1m long saImon and 6.5-8 mm for 26-28 mm long salmon. The thresholds w ore about 1-3% of the total body surface area. Tolerance was increased in isosmotic salinities, the threshold area being as high as 10-14% of the body surface in 24-28 mm long salmon in 8%0. Healing of skin lesions was observed in vivo and by histology, the main response being a mass migration of epidermal cells from the periphery of the lesion. Wound areas of 0.1-7 mm closed in 4-12 h, the mean rates of cell migration being 40-110JUm/h at 10_11°C. The rate of migration was temperature dependent. The normal skin structure was restored with regeneration of a new basement membrane and dermis within 3 weeks. Older stage larvae showed an inflammatory response similar to adult fish.