Life Cycle of Liocarcinus arcuatus (Brachyura: Portunidae) in the Ría de Arousa (Galicia, NW Spain): Role of Beach and Mussel Raft Culture Areas

P.S.Z.N: 1: Marine Ecology, 12 (3): 193-210 (1991) Q 1991 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0173-9565

Acccpted: Deccmbcr IY, 1990

Life Cycle of Liocarcinus arcuatus (Brachyura: Portunidae) in the Ria de Arousa (Galicia, NW Spain): Role of Beach and Mussel Raft Culture Areas J. FREIRE, R. MUIRO,L. FERNANDEZ & E. GONZALEZ-GURRIARAN Dpto. Bioloxia Animal, Facultade de Ciencias. Univcrsidade da Corufia. E-15071-A Coruiia, Spain. With 10 figures and 2 tablcs Key words: Population studies, mussel culture intlucncc, Liocurcinus urcuurus, BruchyNW Spain.

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Abstract. The population biology of the portunid crab Liocurcinus urcuurus is analyzed over a yearly cycle in beach zones and mussel raft culturc areas in the Ria dc Arousa. The breeding cycle of this spccics has two annual pcaks. which result in two annual cohorts that are recruited in different areas. The growth rate is higher in the group settlcd in spring than in autumn, and, especially, in the culture areas compared with the beach zones. There are movements from the beach zones to the raft polygons related to reproductivc and feeding behavior as well as growth. The role of mussel culture in the dynamics of this spccics is discussed.

Problem Liocarcinus arcuatus is a brachyuran crustacean highly abundant in the Galician rias, where it is found in shallow beach areas with sandy bottoms (GONZALEZGURRIARAN & MBNDEZ,1985). In the Ria de Arousa, the site of intensive mussel culture of Myrilus galloprovincialis on rafts, it is one of the dominant decapod species, especially in the beach areas; it also appears in the raft polygons located in the inner ria, where the waters are shallow and river runoff exerts a greater influence (GONZALEZ-GURRIARAN, 1982; ROMERO et al., 1982). Mussel culture in the Ria de Arousa has brought about a major transformation of the bottom due to the production of detritus; changes in the food web & GONZALEZ, 1975; have also been demonstrated on varying levels (TENORE et al., 1982). The zones devoted to mussel culture have a typical TENORE megabenthic community, reaching extremely high biomass values (GONZALEZGURRIARAN, 1982; ICLESIAS, 1981; OLASO,1982). The dominant species have modified their feeding habits to include the new resources - the mussel and its associated epifauna - coming from the rafts (FREIRE er af., 1990; GONZALEZU. S. Copyright Clearance Center Code Statement:

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C t d., GURRIARAN, 1978; GONZALEZ-GURRIARAN ef d., 1989; LOPEZ-JAMAR 1984). They also exhibit population dynamics related to the cultures and to the culture sites; this is evident in the portunid crabs Necoru puber (GONZALEZer al., 1991), the GURRIARAN, 1985a, b) and Liocarcinus depurator (FERNANDEZ most abundant decapods in the mussel culture raft polygons (GONZALEZGURRIARAN, 1982; ROMERO er af., 1982). This paper presents a comparative study on the population biology of L. arcuatus in raft culture and beach areas in order to determine how the mussel culture affects the dynamics of this species through the changes it makes in the environment. Data is also included on the biology of this species (only one previous study has been carried out on this subject, in the Adriatic Sea, S T E V ~ I ~ , 1987).

Material and Methods 1. Sampling Monthly samples wcrc takcn from Octobcr 1978 to Scptcmber 1979 at four stations in the Ria dc Arousa. Thc sampling areas wcre sclccted based on abundancc data for this specics from carlier decapod community studies (ROMERO ei nl., 198'2). Thc arcas choscn had the greatcst L. urcuutus dcnsitics in the ria and represcnt thc two habitats that this spccies occupies. Two stations in bcach zones and two in raft polygon arcas wcrc chosen (Fig. 1). The bcach stations, P 1 and P3. located in thc inncr ria arca. are influcnccd by river runoff (mostly in P I). Thcy havc shallow waters (3-5 m dcpth) and sandy bottoms with an abundancc of grccn scawccds in spring and summcr. Station B I is a raft arca dcvotcd to musscl culturc. It is locatcd in the inncr ria at a depth of IO-15m; the bottom hcrc has clcarly been altcrcd by the culturc (muddy. with an abundance of cpibenthic organisms as wcll as mussels and associated epifauna that fall off thc ropes from the rafts). The other raft station, B 6. is located in a COVC in thc central area of thc ria; although it docs have mussel rafts. most of the rafts are devoted to oyster culturc. This station has a depth or 1Om and, dcspitc the rafts. the bottoms show vcry little change. They are sandy and have a large quantity of green scaweed in spring and summer, as in the beach zones. A semi-balloon trawl with an effective mouth of 4 m and a LO mm wide mesh was used. Two 10 minute trawls wcrc takcn monthly at each station. This is estimated to be the equivalent of an 800mz AR~N O.n board, L. arcuatus were sampling area per trawl (IGLESIAS, 1981; G O N Z ~ L E Z - G U R R I1982). separated and examined in the laboratory to determine total number and weight. Specimens were also measured (carapace width between the fifth pair of anterolateral spines). sexed, and examined to determine if they were in the early stages of post-ecdysis and if the females were ovigerous. Complimentary data were recorded from a sub-sample of 300 individuals for both sexes in order to obtain the biometric relationship cephalothorax width-length and 130 for both sexes for the relationship size-wet weight. The individuals were in the intermolt stage covering the whole range of sizes and showed no signs of external damage (such as loss of appendages, erc.).

2. Data analysis The biometrical relationships were obtained by linear regression and the sexes were compared using covariance analyses (ANCOVA). The residual variances. however, were not homogeneous. For this reason an alternative method was used. consisting of drawing ellipses of joint 95% confidence region for the slope and elevation (DRAPER & SMITH, 1966; CONAN, 1975). The size frequency distributions (SFD) werc regrouped into 2' mm carapace width class ranges for later analysis. The sex-ratio in the L. urcuutus population was studied at each of the stations and months sampled, both for total captures and size class. An x2 test was used to analyze the statistical significance of deviations from the 1 : 1 sex-ratio.

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Fig. 1. Rias Baixas dc Galicia ( N W Spain). Location of mussel raft culture arcas (polygons) and sampling stations (stars) in the Ria dc Arousa. P = beach stations: B = raft stations.

The SFDs obtained for each sampling station were compared in pain for each month and sex using the KOLMOGOROV-SMIRNOV test. Subsequently, factorial correspondence analysis (FCA) of the SFDs in each monthly sample in the different stations was carried out for each sex. For the growth study, the SFDs were analyzed and broken down into their normal components using the method proposed by MACDONALD & PITCHER (1979; program MIX, MACDONALD & GREEN,1985). Each component of the SFDs was considered to constitute a cohort or age group. growth function using the evolution in time of its average size in thc fitting of a VON BERTALANFFY (VBGF):

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L, =: L,,(1; ek('-'"' 1 where L, represents the maximum asyntotic size, k is a growth constant, and t,, is the theoretical age algorithm corresponding to size 0. The fitting was made by non-linear regression using MARQUARDT'S (program FISHPARM, SAILAet al., 1988).

Results 1. Abundance and distribution Liocarcinus arcuatus reached high densities in the Ria de Arousa and at the same time showed a very clearly defined yearly cycle with sharp differences

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between the raft and beach stations (Fig.2). In the beach zones this species showed great fluctuations, with maximum biomass and numbers from September to December, coinciding with the period of trawl recruitment (as high as 1.71g-m-Z in P 3 in December and 0.53 indiv. * m-? in P 1 in September). A sharp drop was registered starting in January. The average densities throughout the year in these areas were 0.16 indiv. * m-2 and 0.72 g . m - 2 in P 3 and 0.14 indiv. m-2 and 0.63 g . m-2 in P 1. The raft areas showed minor fluctuations in L. arcuatus abundance, especially at B 1, which was characterized by the lowest densities throughout the year; the values averaged 0.04 indiv. . m-? and 0.48 g .m-?, with maximum values in September. A t B 6, L. arcuutus reached the greatest mean annual density: 1.73g . m-z and 0.20 indiv. m-?, with maximum values of 3.44g.m-’ in March and 0.54 indiv. m-*in December.

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2. Maturity and breeding cycle One of the criteria used to estimate the size at which female decapods reach sexual maturity consists of estimating (in the month or months having the highest percentage of ovigerous females) the size which corresponds to 50 % of the gravid females by fitting a sigmoid or probability curve to the percentage 1980; WENNER et al., 1974). This method is not data by size group (SOMERTON, suitable for L. arcuutus because the percentage of ovigerous females in the samples corresponding to the two yearly peaks is relatively low and homogeneous for the entire maturity size range. The same is true for L. depurator in this ria (FERNANDEZ et al., 1991). The smallest ovigerous female was captured in April in B 6 (carapace width 14 mm). Sexual maturity may be set in the 21-22mm size class, since the number and percentage of ovigerous females show high values beginning at this size (reaching over 50% in the months with maximums in the breeding cycle). Spatial variations do not exist in this parameter: the four stations sampled have the same population pattern.

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Population biology of Liocarcinus arcuarus

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The analysis of the percentage of ovigerous females in relation to the number of females included within the maturity size range throughout the sampling period points to the existence of a marked reproductive cycle (Fig.3). In all stations, two maxima were observed in the appearance of gravid females: one in February (except in B 6 where it occurred in March) and the other in July (June in P3). The percentage of ovigerous females in these months generally fluctuated between 50 and 60 % ,although the percentage was always higher in the raft areas than in the beach zones. It should be noted that the absolute density of ovigerous females is greater in raft polygon stations, especially in the abovementioned periods with maximum values of gravid females. Thus, in B 6 in March, ovigerous female densities were 0.10 indiv. * m-*, whereas in P 1 in June and July, densities only reached 0.04 indiv. . rn-2. Both of these were the periods having the highest density of gravid females in these zones. % OVIGEROUS

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Fig. 3. Evolution of the percentage of ovigerous fcmales in rcspcct to the total number of maturc fcmales of L.arcmms throughout a yearly cycle For each station sampled.

3. Biometric relationships The regression lines obtained for the carapace width (CW)/carapace length (CL) relationship are the following (Fig. 4): males: CL = 1.355 + 0,74O*CW r2 = 0.992 females: CL = 1.199 0.756*CW r? = 0.990

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The residual variances are not homogeneous (ANCOVA, F298.2~8 = 1.428, P < 0.01). For this reason both equations are compared drawing the ellipses of joint 95 % confidence region for the slope and elevation. Given that the ellipses corresponding to each sex do not overlap (Fig.4), we may conclude that the IengtWwidth relationship is significantly different in L. arcuatus males and females (P