The Effects Of Seaweeds On Two Categories Of Representative Seaweed-associated Amphipods Biological Traits In Mussel-rafts Culture Area

The mussel-rafts culture area is a kind of artificial habitat,and its unique physical environment such as light,temperature and water flow,which enables abundant and diverse marine organisms to inhabit there,including the seaweed who is an important primary producer in the ocean.Substantial studies have shown that seaweeds played an important role in conservation of the growth and reproduction of many marine organisms.Seaweeds were not only the food sources of herbivorous and detrital animals,but also provided shelter for many marine organisms to inhabit and spawn.As the one of the most direct beneficiaries of the seaweed,Seaweed-associated amphipods were small in size and inhabited on the surface or in the gap of seaweed in large numbers,especially during the prosperous and withering period of seaweed,the biomass of the seaweed-associated amphipods usually reached its peak.And effectively attracted other small predators to forage and formed an important food link that based on seaweed as a primary productivity and seaweed-associated animals as food.Therefore,the relationship between seaweeds and algae animals has always been one of the hotspots in the study of the ecological function of seaweeds communities.However,in the past,most of the research around the relationship between seaweeds and amphipods was carried out under the natural benthic seaweed beds,less is known in other habitats,especially in the mussel-rafts culture habitat.As mentioned above,we selected the largest mussel-rafts culture area in Zhejiang Province,as the research area.The investigation was conducted from May 2017 to December 2018.The water temperature and light intensity were measured from January to December 2018.And specimens were sampled in May 2017 and May 2018,respectively.Then biological experiments was conducted in laboratory to obtain relevant data.The results showed that the mean annual water temperature in the study area was 18.45±0.09°C（df=16133,a=0.05）.The illumination intensity was 1680.76±65.21 lux（df=6659,a=0.05）.Also,42 species of seaweeds were collected,such as Ulva lactuca,Grateloupia livida,Pachydictyon coriaceum,and Hypnea boergesenii.And the biomass of algae attached on the rope（Diameter is equal to 3 centimeters）every thirty centimeters was 96.72±23.33 g（df=19,a=0.05）.Moreover,there were large number of seaweed-associated amphipods dominated by Gammaridea and Caprellidae（Abundance>99%）hosted on the seaweeds.The body length of Gammaridea was mainly between 6 and 14mm,and the weight was mainly between 0.007 and 0.046 g,the relationship between body length and weight satisfies the relationship:y=1.5*10^-5x³（r²=0.94,P<0.001）（y indicates the length of the Gammaridea,x indicates the weight of the Gammaridea）.The body length of Caprellidae was mainly between 9 and 15 mm,and the weight was mainly between 0.006 and 0.014 g,the relationship between body length and weight satisfies the relationship:y=3.8*10^-6x³（r²=0.71,P<0.001）（y indicates the length of the Caprellidae,x indicates the weight of the Caprellidae）.The mean abundance of the Gammaridea attached on per gram of seaweeds（wet weight）was 10±3 ind（df=102,a=0.05）,while the biomass could reach as high as 0.051±0.020 g（df=102,a=0.05）.The mean abundance of the Caprellidae attached on per gram of seaweeds（wet weight）was 28±10 ind（df=102,a=0.05）,while the biomass could reach as high as 0.046±0.017 g（df=102,a=0.05）.Chi-square analysis showed that population density and mean body size of Gammaridea attached on different seaweed species were significantly different[c²（10,n=103）=38.509,P<0.001;c²（10,n=1010）=179.82,P<0.001)].Both of highest population density and largest body size of Gammaridea appeared on Pachydictyon coriaceum.And population density and mean body size of Caprellidae attached on different seaweed species were significantly different[c²（10,n=103）=42.485,P<0.001;c²（10,n=1010）=93.443,P<0.001].The highest population density and largest body size of Caprellidae appeared on Ganonema farinosum and Grateloupia constricata,respectively.Moreover,correlation analysis showed that seaweed species with relatively lower biomass tended to support higher population density of Gammaridea and Caprellidae and smaller size of Caprellidae,but had no significant effect on size of Gammaridea.Furthermore,seaweed with higher surface area to biomass ratio tended to support higher density and larger body size of Gammaridea,but had no significant effect on density or body size of Caprellidae.In a word,our investigation indicated that seaweeds are beneficial for the habitation of seaweed-associated amphipods in mussel-rafts culture area.But seaweeds community paly different functional role in structuring seaweed-associated amphipods ecological traits in mussel rafts culture area,which were determined by the diverse seaweed morphology traits.