Effects of salinity on embryonic and larval development of Chinese mitten crab Eriocheir sinensis(Decapoda: Brachyura) and salinity-induced physiological changes

To investigate the effects of salinity on early development of Chinese mitten crab ( Eriocheir sinensis ), and the salinity tolerance mechanism of embryos, different developmental stages of embryos (gastrula, eyespot and pre-hatching stage), and hatched stage I zoea and megalopa, were exposed to a range of salinities (1, 5, 10, 15 (control), 20, 25, 30, 35 and 40). Hatching, survival and molting were monitored. Effects of 24-hour hypersaline (35) and hyposaline (1) stress on egg diameter, water content, Na +/K +-ATPase (NKA) activity, and crustacean hyperglycemic hormone (CHH) gene mRNA expression in embryos and megalopa, are reported. Embryos are more tolerant of low (≤ 5) than high (≥25) salinities, with optimum ranges for gastrula and pre-hatching stage embryos being 5-20, and for eyespot embryo and stage I zoea, 10-20. Most megalopa can molt to the first juvenile instar by day 5 at salinities between 1 and 40, whereas molting of megalopa stages was delayed at 40. Hypersaline conditions resulted in a loss of moisture, reduction of egg volume, and a signifi cant increase in NKA activity and CHH mRNA expression at some developmental stages. Hyposaline conditions did not affect moisture content or egg volume, but resulted in decreased NKA activity and CHH mRNA expression in embryos. For megalopa stages, NKA activity was significantly upregulated following both hypo- and hypersaline stress. Our results suggest high salinity will inhibit development and hatching of E. sinensis embryos, and low salinity will affect the survival of their stage I zoea. Increased NKA activity in embryos and megalopa stages might indicate a hyporegulation response under hypersaline conditions. These findings provide useful information for spawning ground protection of indigenous E . sinensis and enrich the knowledge of embryonic tolerance mechanisms of hyperregulating crustaceans following osmotic stress.

Ontogenetic niche specialization of the spider crab Libinia ferreirae associated with the medusa Lychnorhiza lucerna

Symbiotic relationships in marine environments are not fixed and can change throughout the animal’s life.This study investigated the ontogeny of symbiosis of the spider crab Libinia ferreirae with the host medusa Lychnorhiza lucerna.We described the type of relationship,the temporal correlation among species,and food habits.More than 50%of the sampled crabs were symbionts,most in early life stages.The highest number of crabs found in a single medusa was 11.Symbiosis was observed throughout most of the year but was more evident in warm periods.The crab has many benefits in this relationship with a medusa.One is the use of food resources captured by the medusa,primarily copepods.Because the crab steals the medusa’s food,it is a kleptoparasitic relationship.There is a niche partition between symbiont and the free-living crabs as they occupy different habitats and use nonoverlapping food resources.Previous research reported that symbiosis first developed during the crab’s last larval phase(megalopa)when crab and medusa are in the same habitat.Observation of the crab’s behavior shows that symbiosis occurs when the crab can grab to the medusa when the host touches the sea bottom.The crab also took advantage of water currents,releasing itself from the substrate and then drifting toward the medusa.The symbiotic relationship that crabs have with the medusa provides then with a nursery,food resources,shelter,dispersion,and decreased competition with free-living adult crabs,all essential for the crab’s survival.