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Cephalopoda: Octopodidae) BULLETIN OF MARINE SCIENCE. 56(3): 752-766, 1995 DESCRIPTIVE ECOLOGY OF TWO SOUTH AMERICAN ELEDONIDS (CEPHALOPODA: OCTOPODIDAE) J. Angel A. Perez and M. Haimovici ABSTRACT Eledone massyae and Eledone gaucha are small octopods that coexist along the continental shelf and slope of southern Brazil. A comparative study of the population structure, distri- bution and diet of these species was carried out based on bottom trawl material from fishing surveys off Rio Cirande do Sui State (Brazil), between 30043'S and 33°45'S and the isobaths of 10 and I 10m. The two species occurred throughout the year in trawls deeper than 40 m, on both sandy and muddy bottoms of the outer continental shelf and upper slope. E. massyae and E, gaucha were found at bottom temperatures ranging from 10 to 22°C in tropical, subtropical and s'.lbantarctic waters. Young E. massyae appear on the continental shelf in late summer and feed predominantly on small crustaceans such as amphipods. A single cohort of maturing animals remains on the shelf during winter and spring, switching their diet to larger prey, such as portunid crabs and polychaetes. In late summer, males and females migrate to spawning grounds outside the study area. Juveniles of E. gaucha occur on the shelf mainly during summer and autumn. Males and females seem to mature on the shelf throughout the year, although no true cohorts could be defined. In all size e1asses, small benthic crustaceans, such as amphipods and isopods are the predominant food items. There is no evidence of habitat segregation between these two eledonids, but as adults, they seem to exploit different food resources, Cold-water octopods constitute over half of the total number of benthic Octo- poda species reported from the southern Brazilian coast (Haimovici and Perez, ]99] b). These species are commonly found over soft bottoms of the continental shelf and slope, and their northern distribution is associated with the influence of cold waters of the MalvinaslFalkland Current on the central continental shelf (Palacio, 1977; Haimovici et aI., 1989; Haimovici and Perez, 199Ib). Among the most abundant, two congeneric species, Eledone massyae Voss 1964 and E. gau- cha Haimovici 1988, coexist on the outer shelf and slope between Cabo Frio (Lat. 23°S) and Chui (Lat. 34°S) (Haimovici, 1985; Haimovici and Andriguetto, 1986; Haimovici, 1988; Haimovici et aI., 1989; Perez and Haimovici, 1991b). E. mas- syae occurs seasonally on the Rio de Janeiro continental shelf (Costa and Fer- nandes, 1993) where it forms part of the octopod landings in the trawl fishery (Costa and Haimovici, 1990). Its distribution extends southward to Peninsula Val- dez (Lat. 43°S), Argentina (Voss, 1964). Morphologically similar, but smaller in size, E. gaucha has been recently differentiated from E. massyae (Haimovici, 1988; Levy et aI., 1988). The review of several cephalopod collections indicated that the species, formerly misidentified as E. massyae, occurs along the southern Brazilian coast (Perez and Haimovici, 1991a), and has not been recorded south of Brazilian waters. A series of groundfish surveys conducted off the coast of the Rio Grande do SuI State between 1981 and 1987 collected both species on the central continental shelf out to 600 m depth (Haimovici and Andriguetto, 1986; Haimovici and Perez, 1991a). Preserved samples from these surveys have provided information for comparative studies on the reproductive biology of both species in the area (Perez et aI., 1990; Perez and I-Iaimovici, 1991b; Perez et aI., in press). This paper describes the habitat and presents comparative data on the diets and distribution of E. massyae and E. gaucha on the southern Brazilian coast. 752 PEREZAND"!A1MOV1Cl:ELEDONIDOCTOPODECOLOGY 753 Rio Grande do Sui : 30 Brazil 100 km 31 53 • detrit. sandy transit. slope Figure I. Study area. Bottom type distribution according to Martins et al. (1972) is indicated. The an'ow indicates the location of the survey area in the southwest Atlantic Ocean. detrit., banks of shell debris; sandy, sandy bottom type; transit., transitional bottom typc consisted of a mixture of sand and mud; slope, slope bottom type consisted of mud. MATERIAL AND METHODS The material was obtained from 16 groundfish surveys of the RN ATLANTICOSULof the Rio Grande University (Brazil), from 1980 to 1985. The cruises occurred throughout the year and covered the southern Brazilian continental shelf between Solidao (30043'S) and Chuf (33°45'S) at depths ranging from 10 to 160 m (Fig. I). A bottom trawl net with a 52.9 m footrope, 31.3 m headrope and a 50 mm stretch-mesh cod end was used to collect the samples. The trawl hauls were done at 3 kn for 30 to 60 min, between dawn and dusk. The specimens of E. massyae and E. gaucha were separated from the total catch, weighed to the nearest O. I g, the dorsal mantle length (ML) measured in millimeters, and then fixed in 10% buffered 754 BULLETIN OF MARINE SCIENCE. VOL. 56, NO.3. 1995 Table 1. The percentage distribution of tows per depth stratum, bottom type and subarea in the four seasonal cruises considered for analysis of the distribution of E. massyae and E. gaucha No. Depth stratum (Ill) Bottom substrate Subarea of Survey tows. Bottom temp. 0-20 20-40 40-60 60-80 >80 Sandy Trans. Slope North South Summer 42 12.8-22.9 30.9 9.5 23.8 19.0 16.7 61.9 26.2 11.9 35.7 64.3 Autumn 41 15.5-22.6 19.5 21.9 24.4 17.1 l7.1 58.5 29.3 12.2 36.6 63.4 Winter 54 11.3-17.8 16.7 11.1 22.2 24.1 25.9 44.4 31.5 24.1 46.3 53.7 Spring 34 12.6-20.2 20.6 14.7 17.6 20.6 26.5 44.1 29.4 26.5 55.9 44.1 Total 171 11.3-22.9 21.6 14.0 22.2 20.5 21.6 52.0 29.2 18.7 43.3 56.7 formalin. In the laboratory, the specimens were transferred to 70% ethanol, and their reproductive organs and digestive tracts were dissected for reproductive studies (Perez et aI., 1990; Perez and Haimovici, 1991b; P,~rez et aI., in press) and diet analysis. The population size structurc and sex ratio was analyzed for 294 specimens of E. massyae, and 169 specimens of E. gaucha collected in all 16 cruises combined. An additional 126 E. massyae and 13 E. gaucha. obtained from the commercial groundfish fishery, were included in the diet analysis. The contents of the crop and the stomach werc examined under a dissecting microscope, and the food items identified to the lowest taxonomic level possible and expressed by their frequency of occurrence. The patterns of distribution of E. massyae and E. gaucha, were studied in 171 tows of four selected cruises made during 1982 and 1983 (Table I). Due to the frequent occurrence of Eledone spp. entan- gled in the wings of the trawling nets (Haimovici, 1988), the gear was not considered a reliable estimator of abundance and only the frequency of occurrence of the species per tow was used in the analysis of distribution. The cnlises were classified as: summer, January 1982 (cruise 1/82); autumn, April 1983 (cruise 4/83); winter, September 1983 (cruise 9/83) and spring, November 1983 (cruise 13/83). The study area was divided into subarea North (between 30046'S and 32°30'S) and subarea South (between 32°30'S and 34°20'S) (Fig. I), and six depth strata with boundaries at 20, 40, 60, 80 and 100 m were defll1ed. The boltom type was estimated by plotting the tows on a chart of the sediment distribution of southern Brazil (Martins et aI., 1972). The bottom sediments were classified into three main zones: sandy, transitional, and slope. The relationship between the frequencies of occurrence of the two species and the distribution of tows per season, subareas, depth strata and bottom types (Table I), was tested using the Chi-square statistic with a confidence level of 0.05. The coexistence of E. massyae and E. gaucha was assessed by the frequency of tows where both species occurred in relation to the total number of tows with the presence of at least one species. The water masses were classified according to Castello and Moller (1977) from the bottom tem- perature and salinity data obtained after each tow. RESULTS Physical Environment.-The southern Brazilian continental shelf is 125 km wide on average, breaking between the 150 and 185 m isobaths. It reaches its maximum width (180 km) south of Rio Grande (Lat. 33°S) and narrows towards the north (Fig. 1). The bottom of the inner shelf is characterized by a sandy zone that extends down to 1he 50 m isobath. Along this zone, banks of shell debris occur parallel to the coast. Mud rich in silts and clays predominates on the outer shelf and slope between the 100 and 3,000 m depth (slope zone). The transitional zone of variable proportions of sand and mud separates the inner sandy zone from the outer slope zone (Fig. 1). Rocky bottoms occur on the slope, predominantly in submarine canyons. The oceanographic conditions of the southern Brazilian coast are determined by the seasonal influences of four water masses: a. Subantarctic water (with tem- peratures between 4.5 to 16SC and salinities between 33.5 and 34.2%0) is derived from a coastal branch of the Malvinas/Falkland Current, and flows northward reaching its northernmost penetration to around 31-32°S during the winter.
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