Journal of Oceanography, Vol. 56, pp. 295 to 310, 2000 Emergence Patterns of Small Subtidal Arthropods in Relation to Day/Night, Tidal, and Surface/Bottom Factors: Investigations in the Boreal Sea, Japan (Akkeshi, Hokkaido) MASAYUKI SAIGUSA*, KAZUSHI OISHI, AKIHIRO IKUMOTO, HIROSHI IWASAKI and MICHIHIRO TERAJIMA Department of Biology, Faculty of Science, Okayama University, Tsushima 2-1-1, Okayama 700-8530, Japan (Received 20 January 1999; in revised form 24 September 1999; accepted 8 October 1999) The emergence of small arthropods was studied in the boreal sea, Japan (Akkeshi, Keywords: Hokkaido). In the shallow subtidal zone, two impeller pumps were set in the surface ⋅ Daily (=diel) and bottom waters. The pumps ran continuously for 25 days (22 August–16 Septem- rhythm, ⋅ ber, 1998), and invertebrates were sampled using a nylon net (300 to 500 µm in mesh day/night, ⋅ size). The small arthropods collected with the net belonged to 16 orders comprising dispersal, ⋅ emergence pattern, about 60 taxa. In the dominant 23 taxa, a two-way ANOVA was applied to determine ⋅ Hokkaido whether there was any significant difference in abundance between day and night (Akkeshi), and between surface and bottom. While emergence of 15 taxa (65%) was significantly ⋅ small arthropods, different with regard to the day/night factor, that of the other 8 taxa (35%) was not. ⋅ surface/bottom, As to the difference between the two depths, the distribution of 15 taxa (65%) was ⋅ swimming activity. significantly different. Furthermore, in 25 taxa for which over 100 specimens were collected in each of the two depths of water, emergence patterns were examined with regard to the synchrony with day/night and tidal cycles. There were various noctur- nal patterns, and the strength of the synchrony with the day/night cycle was different for each species or group. Within the same taxa, nocturnal patterns were more clearly manifested in the surface water than in the bottom water. A tidal rhythm of emer- gence was only seen in zoeas of shrimp. Variations of the emergence patterns of benthic crustaceans are accounted for by a hypothesis that the frequency of swimming dur- ing day versus at night is different in each species. On the other hand, the emergence patterns of some zooplankton reflect by the daily rhythm of vertical migration or dispersal in the water column. 1. Introduction tide; in this case the period of their rhythm is about The 12.4-h tidal cycle causes drastic changes of the 24.8 h (double-tidal interval) (e.g., Enright, 1972; Saigusa, environment in intertidal zones and estuaries. A number 1981, 1982, 1997). Some workers have defined the ac- of intertidal and estuarine animals have developed activ- tivity pattern with a double-tidal interval as a “circa- ity patterns synchronized with this predictable cycle lunidian” rhythm (Palmer, 1995; Naylor, 1996). However, (Neumann, 1981; Palmer, 1995). Some animals synchro- since synchrony with the tidal cycle is converted from nize their activity with both of the semidiumal tides; in the single-tidal interval to the double-tidal interval, and this case the period of their rhythm is about 12.4 hr (sin- vice versa (see Saigusa and Akiyama, 1995), it is not nec- gle-tidal interval) (e.g., Saigusa, 1992; Saigusa and essary to distinguish these rhythmic patterns. Both types Kawagoye, 1997). On the other hand, the activity of other of the activity pattern can simply be called a tidal rhythm animals coincides with either the diurnal or the nocturnal (Saigusa and Oishi, 2000). One of major issues in biological rhythm research of marine organisms is the range of environment in which * Corresponding author. E-mail: [email protected] the tidal rhythm is observed. The influence of the tidal Copyright © The Oceanographic Society of Japan. cycle should most strongly affect the organisms in the 295 intertidal zone and the lower part of estuaries. Hence, same meaning as “diel” that has been recently used in intertidal and estuarine organisms would show a well- marine biology (Lalli and Parsons, 1993). In contrast, the demarcated tidal rhythm. However, the influence of the word “daily” is commonly used in the field of biological tidal cycle should decrease in habitats further from the rhythm (e.g., Saunders, 1976). We are better acquainted intertidal zone and from the lower part of estuary; i.e., in with “daily” than “diel”. In the marine environment, how- the habitats such as the subtidal zone and the upper parts ever, “daily rhythm” indicates either the day/night rhythm of estuary. Decreased influence of the tides might weaken or the tidal rhythm, because the tidal rhythm is only a the synchrony of animals’ behavior with the tides. It is variation of the day/night rhythm (Saigusa and Oishi, not yet known what activity patterns animals show in these 2000). “Diurnal” indicates daytime, and “nocturnal” in- habitats. dicates nighttime. To address this issue, the activities of various kinds of animals inhabiting areas ranging from subtidal zones 2. Materials and Methods to the upper part of estuaries must first be surveyed in the field. In addition, since the synchrony with the tidal cy- 2.1 Study site and environment cle is assumed to be decreased in these environments, Invertebrate sampling was carried out at the concrete investigations of the animal’s activities should be carried pier in front of Akkeshi Marine Laboratory, Hokkaido out continuously for a long period to assess whether the University, for 25 days in late summer (0800 h on 22 tide-correlated timing is still present in the activity pat- August–0800 h on 16 September, 1998). Akkeshi Bay terns. Weakly manifested tidal timing has already been (43°N and 145°E) is situated on the eastern coast of reported in the larval release activity of three species of Hokkaido, Japan (Fig. 1A). This bay is shallow and the terrestrial crab Sesarma inhabiting the upper part of eutropic; during the spring and autumn plankton blooms, the estuary (Saigusa, 1981, 1982). In contrast, the present and the water column is weakly stratified by phyto- study focused on surveying the emergence patterns of plankton (Saito and Hattori, 1997). The salinity was con- small arthropods inhabiting a shallow subtidal zone near stant (about 30‰) throughout the sampling period, and the shore. the water temperature fluctuated slightly every day, be- Subtidal animals can be classified as benthos and tween 15°C and 17°C, at the sea surface. plankton for convenience (Lalli and Parsons, 1993). The The tidal pattern at Akkeshi is semidiurnal: the height distribution of zooplankton is not uniform in the water of high tides is only moderately different during the day column. Some species aggregate at the bottom or near and at night, but morning low waters recede much fur- the surface of water by day (Herman, 1963; Anraku, 1975; ther than afternoon low waters. Hamner and Carleton, 1979; Ueda et al., 1983; Saito and Hattori, 1997). Other species are distributed in a patchy 2.2 Collection of small invertebrates manner (Lalli and Parsons, 1993). In contrast, bottom Small invertebrates were collected using two impeller substrates are inhabited by a number of small crustaceans pumps (SX-150; Terada Pump Co., Japan) installed at the (e.g., Nicolaisen and Kanneworff, 1969; Biernbaum, center of a concrete pier which jutted about 100 m from 1979). Furthermore, small crustaceans are attached to the shore into the sea (Fig. 1B). One pump was floated macroalgae in shallow subtidal zones (Norton and Benson, by floats at a depth of 50 cm from the surface and the 1983; Duffy, 1990). Some of these animals are benthic other was fixed 50 cm above the bottom. The position of throughout their lives, but others are swimmers of the the surface pump fluctuated vertically with the tidal water column at certain stages. It is not yet known what height; the difference of depth between the two pumps activity patterns benthic and planktonic animals show on was 170 cm at high tide, but 50–100 cm at low tide. the 24-h time scale. Water was pumped at a flow rate of about 120 liters As shown elsewhere (e.g., Oishi and Saigusa, 1999; per minute. The pump ran continuously, and invertebrate Saigusa and Oishi, 1999), the activities of these inverte- samples were collected from an integrated nylon net brates can be monitored by measuring the temporal (15 × 23 cm; 300 to 500 µm in mesh size) every 30 min changes of the number of animals collected in certain lay- (3600 liters). Invertebrate samples collected were imme- ers of the water column. Some patterns of emergence may diately fixed with 5% (v/v) formalin and stored. be reflected by the daily rhythm of swimming activity in Akkeshi Bay was very stormy for 3 days from 30 each species, but others may be reflected by nonrandom August to 2 September, 1998, because of a typhoon. Al- distribution in the water column. This study reports the though the sampling was continued during this period, a results of a survey conducted in the boreal sea, Japan large quantity of broken seaweed pieces and the muddy (Akkeshi, Hokkaido). substrate entered the nylon net, which made it very diffi- We use the term “daily” here to express the temporal cult to distinguish small animals from these impurities. changes of the number of animals in 24 hrs. This is the 296 M. Saigusa et al. Fig. 1. A: Location of the study site in Akkeshi Bay, Hokkaido (solid arrow). The ocean currents, including Oyashio, are also shown by solid arrows. B: The sampling site in Akkeshi Bay (solid circle and arrow). cp: concrete pier, br: concrete bridge, sw: colony of seaweeds (Laminaria spp.), sb: sand beach, st: stones, weeds, wo: woods.