J. Mar. Biol. Ass. U.K. (2006), 86,1187^1197 Printed in the United Kingdom

Brittle stars (Echinodermata: Ophiuroidea) in the German Bight () diversity during the past 130 years

O Karin Boos* and Heinz-Dieter Franke* *Biologische Anstalt Helgoland, Foundation Alfred Wegener Institute for Polar and Marine Research, PO Box 180, O 27483 Helgoland, Germany. Corresponding author, e-mail: [email protected]

In this study, an overview of the current species composition of ophiuroids o¡ Helgoland, German Bight (North Sea) is given. In addition, abundance and distribution of ophiuroids on di¡erent types of soft bottom sediments taken by van Veen grab samples around Helgoland were recorded. The literature was reviewed in to outline the diversity of ophiuroid species over the past 130 years in the inner German Bight. In the historical literature, quantitative references often apply to verbal descriptions and thus make comparisons to modern data and assessment of possible changes in abundance rather subjective. In total, six ophiuroid species were identi¢ed o¡ Helgoland: brachiata, ¢liformis, squamata, fragilis, and . The species found in this study had also been reported by previous investigators and are regarded as ‘common’ species in the German Bight. Occasional ¢ndings in this area refer to , aculeata, Ophiocten a⁄nis and Ophiura sarsi, which are regarded here as ‘rare’ species. These ¢ndings, however, do not indicate sustainable changes in the species diversity over time. Apart from , a newcomer in the 1970s, ¢ndings of the common species mentioned above can be dated back to 1875. Therefore, a fairly stable composition of brittle stars is represented in the inner German Bight during the past 130 years.

INTRODUCTION MATERIALS AND METHODS Species spectrum and abundances in the North Sea Processing of samples and literature data apparently have undergone profound and rapid changes From June to August 2003 van Veen grab sampling in recent years. The reasons for this may be diverse: (0.1m2) was performed at eight di¡erent stations on soft ¢shery, eutrophication, introduction of non-indigenous species, climate change (Reise et al., 1999; Franke & Gutow, 2004; Reichert & Buchholz, 2006). The echino- derm fauna of the German Bight has been described along with various studies dealing with the species diversity of di¡erent faunal associations (Stripp, 1969; Ziegelmeier, 1978; Salzwedel et al., 1985; Thatje & Gerdes, 1997). Hagmeier (1925) was the ¢rst to present quantitative data on the faunal composition of benthic communities in the German Bight, while earlier investigations in the central and southern North Sea including the German Bight focused on qualitative data (Mo«bius & Bu«tschli, 1875; Meissner & Collin, 1894; Su«bach & Breckner, 1911). In a comprehensive investigation on the fauna of the central North Sea, Ursin (1960) presented data from the Danish grab surveys (1932^1955) and compared them with the results of a number of previous studies from adjacent coastal areas, including the German Bight. Gerdes (1977) gave a summary of the distribution of in the German Bight comparing his results with those of previous investigations as well. Both authors suggested little change in species composition over time. The objective of the present study was to give an overview of the current species composition of ophiuroids o¡ Helgoland on di¡erent types of sediment as well as of the Figure 1. Sampling stations (1^8) around Helgoland and the German Bight and to assess changes in the diversity of location in the inner German Bight (North Sea) (area of ophiuroid species over time. investigation); modi¢ed after Benoit, 1998.

Journal of the Marine Biological Association of the United Kingdom (2006) 1188 K. Boos and H.-D. Franke Brittle stars in the German Bight

Figure 2. (A^F) Ophiuroid species found o¡ Helgoland, German Bight (North Sea) in dorsal view. (A) Acrocnida brachiata; (B) Amphiura ¢liformis;(C);(D);(E)Ophiura albida;and(F)Ophiura ophiura.

bottom sediments in the vicinity of the Island of The stations ranged within a radius of approximately 5 Helgoland, German Bight (North Sea) (Figure 1). to 6 nautical miles around Helgoland and were located in Additional dredging was performed for complementation the area of 54813 ’^54803’N7845’^8803’E. The grab of the current spectrum of species. Due to lack of quanti- samples covered a total of 0.3 m2 per station. One litre of tative dredge sampling, however, information on sediment from each grab sample was retained for sedimen- abundances was estimated in categories ‘low, medium and tary analysis. On board the research vessel, the samples high’. were passed through a 1-mm sieve and the retained fauna

Journal of the Marine Biological Association of the United Kingdom (2006) Brittle stars in the German Bight K. Boos and H.-D. Franke 1189

Table 1. Characteristics of stations (position, depth, sediment) and abundance of ophiuroids per station in grab samples (total area of 0.3 m2 each); categories in parentheses (low, medium, high) refer to abundances estimated from dregde samples.

Stations 1 2 345678

Position 548080N/ 548080N/ 548090N/ 548120N/ 548030N/ 548050N/ 548090N/ 548130N/ 078550E 078560E 078580E 088010E 078560E 088020E 088030E 078460E Depth (m) 47.5 42.9 39.5 10.5 33.0 23.0 23.0 33.8 Sediment structure medium sand, medium sand, mud and clay blended with ¢ne sand black clayed shells, stones partly gravel mud Content of organic 2.0 2.1 1.3 0.1 5.5 5.3 3.2 2.6 matter (%) Ophiuroid species: Acrocnida brachiata ^ ^ ^ ^ 1 (low) 1 (low) ^ (low) Amphiura ¢liformis ^52^3(medium)^789(low) Amphipholis squamata 1^^^^^^1 Ophiura albida 11 22 (high) 10 2 35 (high) 86 (high) 45 (low) Ophiura ophiura ^^^^^6(high)^^ Ophiothrix fragilis (low)^^^^^^^

was maintained in separate £ow-through containers for between 500 and 250 mm, occasionally blended with further processing. Sorting was carried out in the labora- coarser sediment fractions (Figure 1; Table 1). A particular tories of the Marine Station ‘Biologische Anstalt composition of medium sands, shells and stones was deter- Helgoland’ (BAH). Ophiuroids were identi¢ed to species mined for the ‘Helgola«nder Tiefe Rinne’ (Stations 1 and 2), level and their respective abundances per station recorded. a depression south of Helgoland reaching a unique depth For photographic documentation, the individuals were in the German Bight of about 60 m. Conversely, Stations anaesthetized in isotonic magnesium sulphate solution, 5^8 were basically composed of muddy and clayed sedi- ¢xed in 70% ethanol and dried. ments with grain sizes 5125 mm, partly blended with ¢ne The sediment samples retained from each station were sand. Basically, the stations can be divided in showing dried and sieved to fractions covering a series of seven either coarse sediment types (Stations 1^4) or ¢ne mesh sizes between 3600 and 125 mm. Subsequently, the substrata (Stations 5^8), respectively. The sediment char- sediment types were de¢ned according to the Wentworth acteristics de¢ned in this study are in accordance with grade classi¢cation (Holme & McIntyre, 1984). According those given by Stripp (1969) and Salzwedel et al. (1985). to these authors, fractions exceeding 2000 mm relate to Content of organic matter was three times higher in cobbles, pebbles and granules, whereas fractions between muddy substrata from Stations 5^8 (4.2 1.5%) than in 2000 and 125 mm refer to di¡erent grades of sands. sandy sediments from Stations 1^4 (1.4 0.9%) (Table 1). Fractions below 125 mm apply to very ¢ne sands, silt and clay, respectively. The dominating fraction (referring to at least 25% in each sample) was used for identi¢cation of Species composition sediment type. Ash free dry weight of sediment was used In total, six ophiuroid species belonging to three to calculate the content of organic matter in each sample. families (, Ophiothrichidae and ) Literature dating back to 1875 was reviewed for qualita- were identi¢ed o¡ Helgoland (Table 1). Two burrowing tive and quantitative data on ophiuroids recorded for the species, Acrocnida brachiata (Montagu, 1804) (Figure 2A) inner German Bight (an area of approximately and Amphiura ¢liformis (Mu«ller, 1776) (Figure 2B), as well 40,000km2, limited by the German North Sea coast, lati- as three epibenthic brittle stars, Amphipholis squamata tude 558050N and longitude 6840’E; Figure 1). The (Delle Chiaje, 1829) (Figure 2C), Ophiura albida Forb e s, comparison of literature data is problematic because the 1839 (Figure 2E) and Ophiura ophiura (Linnaeus, 1758) data were not collected for this special purpose. In the (Figure 2F) were found in various grab samples at historical literature, references of abundance are often di¡erent stations. Apart from A. squamata, all species were given verbally, and thus imply a high amount of subjective sampled by dredging as well. Ophiothrix fragilis assessment of the actual situation. Furthermore, the inves- (Abildgaard, 1789) (Figure 2D), however, was exclusively tigations widely di¡ered in temporal and spatial coverage found in dredge samples at the Tiefe Rinne (Station 1). as well as in sorting method. Due to limitation on The most common species, Ophiura albida, occurred on comparative analyses of the species’ quantities over time, all types of sediment. Highest abundances of this species, the individual references of abundance are discussed however, reaching a maximum of 286 ind m72 (Station separately. 6), were recorded on ¢ne (muddy) substrate (Stations 5, 6 and 7) rather than on coarse (sandy) sediments (Table 1). RESULTS Dredge samples generally revealed high densities of O. albida. Amphiura ¢liformis was highly abundant on Sediment characteristics muddy substrata as well. While coarser sediments revealed At Stations 1^4, analysis of sediment revealed mainly low numbers of individuals, a maximum of about ¢ne and medium sands with dominating grain sizes 300 ind m72 was found on a muddy location north-west

Journal of the Marine Biological Association of the United Kingdom (2006) 1190 K. Boos and H.-D. Franke Brittle stars in the German Bight ) tschli, « Continued ( bius & Bu « bach & 1875 1894 Breckner, 1911 « . Data cited from references are ^ 1872 Mo 1938 Caspers, 1938 1923^1924 Hagmeier, 1925 1889^1890 Meissner & Collin, 1902^1907 Su verasstated. many very 3^6 ind, ¼ ¼ ¼ 1^2 ind, are given (no ¼ 2 7 categories available Rinne’, 35 stations were sampled in a total of 4 di¡erent faunal associa- tions. Mean values calculated per faunal associa- tion SD available) associations in the German Bight. Mean values per m ind, very frequent many ind abundance: rare medium frequent positions are in the given ranges of the present work CommentNo legend to Yearof study Reference Study of faunal Categories of 21 of 170 sampled in grab ^ Study site: ‘Tiefe 2 7 (found at 4 stations in the given range) ind m samples from 3 of 5 di¡erent faunal areas (found at 6 stations in the given range) at 3 positions in the given range) Rare to frequent Between 1 and 17.8 Rare to medium Frequent (found 2 7 in 9.6) 2 7 92.9) ind m frequent (found at 5 stations in the given range) 46.0 ind m grab samples from 3of5di¡erent faunal areas in grab samples as well as in dredges (no precise data) and 136.7 ( (found at 13 stations in the given range) (found at 11 positions in the given range) ‘Common species’ Medium to Between 7.5 ( Very frequent Very frequent 1 station in the given range) (no precise data); further descrip- tion: ‘not as frequent as the other brittle stars found in this study’ 1 station in the given range) but patchy distributed in the North Sea (found at 8 positions in the given range) Found in dredges Rare (found at ^ Rare (found at ^ ^ Between 0.6 and ^ Very frequent, ) 2 found in a single van Veen grab (0.1 m 1 station in the given range) 15 individuals Rare (found at in 2 7 233.7) in grab 2 7 10.0) and 1 station in the given range) grab samples from 4of5di¡erent faunal areas samples stations in the given range) ind m ( 632.2 ( (found at 7 positions in the given range) 583.0 ind m Very frequent Records of ‘common’ ophiuroid species from the German Bight (North Sea) between 1872 and 2003, and their abundances; absence of species is shown as ^ Medium (found at ^Rare(foundat3 ^ Between 5.0 ^ Between 0.4 and position along the borders of the given range) Table 2. partly pooled mean values, and partly calculated mean values over the given sampling period and area (see comments). Verbal descriptions were taken o Acrocnida brachiata Amphiura ¢liformis Amphipholis squamata Ophiothrix fragilis Ophiura albida Ophiura ophiura Rare (found at 1

Journal of the Marine Biological Association of the United Kingdom (2006) Brittle stars in the German Bight K. Boos and H.-D. Franke 1191 ) Continued ( 1985 1938 Caspers, 1950 1975 Salzwedel et al., 1969^1976 Gerdes, 1977 1965^1966 Stripp, 1969 1950^1974 Ziegelmeier, 1987 1984^1988 Niermann, 1997 ‘Austernbank’. 44 stations were sampled. Mean values are calcu- lated from a small number of stations presented by Caspers for the German Bight (from 4 di¡erent faunal associations) given by Gerdes for 4 stations and from 4 di¡erent faunal associations (with 17 to 34 stations each) faunal associations (between 7 and 26 stations per association) values for 9 stations sampled during the given time period on transects in the North Sea. Mean values for all stations sampled in the given range and time period Pooled mean values Comment Yearof study Reference Pooled mean values Mean values per Pooled mean Study site: stations 9.9) (no 2 153.4) 1and on 4 of 5 on 3 of 7 ^ Study site: 2 2 2 2 51.8) 5 7 7 7 7 13.5) ind m and 9.0 ( ind m 150.9 ( ind m SD available) ind m 5stationsandin 3 of 4 faunal associations faunal associations 23.3 ind m Between ‘rare’ Between 13.9 ( 15.52 ( 2 7 (no 2 297.6) 333.5) on 5 of 5 on 4 of 7 2 2 2 67.1) 7 7 7 7.9) ind 2 63.1) ind m 31.4) and 45.6 11.0) and 7 ( 263.1 ( ind m ( ind m m ( 232.5 ( ind m in grab samples, found in dredges as well (no precise data) SD available) 5 stations and in 3 of 4 faunal associations faunal associations ‘Common species’ Between 29.0 84.1 ind m Between 3.7 Between ‘rare’ and (21 on 2 2 2 7 7 ^5.4( found in dredges (no precise data); ‘in great quanti- ties between oyster shell fragments’ 1indm individuals found on a rock) of 5 faunal associations 1indm faunal associations 5 5 ‘Rare’ in 1 of 4 on 2 (no 2 2 7 ^10individuals ^ ^ 70.72 ( 2 7 7 44.7) ind 0.03) on 1 of 5 2 7 1 ind m of 5 faunal associations m ind m SD available) stations 0.4 ind m 5 42.0 ( 0.01 ( (no 2 343.8) 144.9) 1and on 4 of 5 in grab 7 19.0) ind 2 2 2 2 5 7 7 7 7 11.1) 0.03) on 2 of 5 2 28.8) and 7 ind m SD available) samples faunal associations stations and in 2 of 4 faunal associations ind m 296.6 ( ( 123.0 ( ind m 30.0 ( m ind m 46.0 ind m Between 8.24 ( (no (Continued). 2 ^0.01( ^ Between 34.4 ^ Between ‘rare’ and 2 7 7 26.9) ind on 1 of 5 10.9) 2 7 SD available) faunal associations m ind m Table 2. 10.9 ( Acrocnida brachiata Amphiura ¢liformis Amphipholis squamata Ophiothrix fragilis Ophiura albida Ophiura ophiura 2.3 ind m 6.5 (

Journal of the Marine Biological Association of the United Kingdom (2006) 1192 K. Boos and H.-D. Franke Brittle stars in the German Bight hne & Rachor, « 1996 1991 Ku 2003 Present study 1988^1989 Berberich, 1989 1969^2000 Schroeder, 2005 grund’; 6 stations sampled (absolute data not available) Rinne’; mean values per 25 stations sampled 8 stations sampled from 4 stations for the given time period, given by Schroeder Study site: ‘Stein- Comment Yearof study Reference Mean values from Pooled mean values 1and ^ Study site: ‘Tiefe 5 124) ind at 4 stations in grab 7.1) ind 2 2 7 7 species in dredges at 4 stations (Dominance: 2.07%) 97 ( m m samples and generally high numbers in dredges Characterizing Between 2.5 ( 99) 15) at 2 2 7 7 95.9) ind 79.4) in grab 2 7 and 176 ( ind m m ind m species in dredges at 6 stations (dominance: 1.28% and 7.40%) 4 stations samples and generally high numbers especially in dredges ‘Common species’ 87.9 ( Characterizing ^44.8( dredges stations Low numbers in ^^Between5( 2 376.1) 7 1.5) ind in grab 2 7 ind m m samples 0.83 ( 193.6 ( Present Found at 2 of 6 1and in grab 1072) ind 2 2 5 7 7 102.4) at 4 stations 20.6) 2 7 1234 ( m ind m ind m samples and low to medium numbers in dredges 3 of 6 stations Between 44.2 ( 6.4 ( Low numbers at at 3 2 (Continued). 7 2 7 in grab 2.0) 1.5) ind 2 7 of 4 stations samples and low numbers in dredges ind m m of 6 stations 1indm Table 2. Acrocnida brachiata Amphiura ¢liformis Amphipholis squamata Ophiothrix fragilis Ophiura albida Ophiura ophiura 0.4 ( Low numbers at 3 5 SD, standard deviation. 0.8 (

Journal of the Marine Biological Association of the United Kingdom (2006) Brittle stars in the German Bight K. Boos and H.-D. Franke 1193 tschli, 1875 « bius & Bu hne & Rachor, 1996 « « bach & Breckner, 1911 « . Data cited from references are ^ 1872 Mo ver as stated. 1969^2000 Schroeder, 2005 1923^1924 Hagmeier, 1925 1889^1890 Meissner & Collin, 1894 1902^1907 Su very ¼ many ind, 3^6 ind, ¼ ¼ 1^2 ind, are given (no SD ¼ 2 7 4 stations for thetime given period, given by Schroeder tions in the German Bight. Mean values per m available) available rare medium frequent very frequent many ind tions are in theranges given of the present work Comment Yearof study Reference No legend to categories ^ Categories of abundance: ^ 21 of 170 sampled posi- ‘Rare species’ 1 station in the given range) ^ Medium (found at 1 station in therange) given in the given range) ^ ^ ^ Pooled mean values from ^ ^ ^ Study of faunal associa- ^ Rare (found at 2 positions 1 station in the given range) at 1 of ^ ^ ^ ^ 2003 Present study ^^^^^^^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 1938 1950^1974 1938 1965^1966 1969^1976 Ziegelmeier, Caspers, 1987 1938 1984^1988 Stripp, 1975 Caspers, 1969 1950 1988^1989 Gerdes, 1977 Niermann, 1991 1997 Berberich, Salzwedel 1989 et al., 1985 Ku ^Medium(foundat ^ ^ Very frequent (found at in grab 2 Records of ‘rare’ ophiuroid species from the German Bight (North Sea) between 1872 and 2003, and their abundances; absence of species is shown as 2 7 7 4stations samples from 1 ofdi¡erent 5 faunal areas (sandy substrata) (no SD available) 1 ind m in the given range) partly pooled mean values, and partly calculated mean values over the given sampling period and area (see comments). Verbal descriptions were taken o SD, standard deviation. Table 3. Amphiura chiajei Ophiocten a⁄nis Ophiura sarsi 5 Rare (found at 3 positions 0.6 ind m

Journal of the Marine Biological Association of the United Kingdom (2006) 1194 K. Boos and H.-D. Franke Brittle stars in the German Bight of Helgoland (Station 8). Dredges revealed low to medium temperatures down to 3^48C, entire populations might densities. Low numbers in grab samples as well as in perish if temperatures drop below this limit. Extinctions dredges and only at a few sampling sites were recorded due to extreme winters, though, are apparently followed for Acrocnida brachiata, Amphipholis squamata, Ophiothrix by a rapid recovery of populations (Ursin, 1960). This is ophiura and O. fragilis (Table 1). ensured by larval transport from abundant populations of this species in the warmer south-west part of the North Sea. Thatje & Gerdes (1997), for example, regarded an DISCUSSION observed patchy distribution and high numbers of juvenile Our qualitative results are in agreement with those of A. ¢liformis during their study as a direct consequence of an previous investigations in the inner German Bight over ongoing recruitment process. the past decades (Mo«bius & Bu«tschli, 1875; Meissner & Ophiura albida was recorded continuously on various Collin, 1894; Su«bach & Breckner, 1911; Hagmeier, 1925; types of sediment by all authors listed in Table 2. Though Caspers, 1938, 1950; Stripp, 1969; Gerdes, 1977; Salzwedel abundances might di¡er between ‘rare’ and very high et al., 1985; Ziegelmeier, 1987; Berberich, 1989; Ku«hne & densities (Table 2; Stripp, 1969), this species appears to be Rachor, 1996; Niermann, 1997; Schroeder, 2005; Tables 2 highly abundant and very common in the German Bight. &3). Although presumably not as abundant as O. albida, Su«bach & Breckner (1911) recorded Acrocnida brachiata O. ophiura is found regularly in moderate to high densities from the outer limits of the inner German Bight in 1904. on various substrata. In the Tiefe Rinne and at the former After a long gap in time and the literature, it was recorded ‘Helgola«nder Austernbank’, however, this species was again in 1975 by Gerdes (1977). Since then it has been lacking (Caspers, 1938, 1950; Berberich, 1989). Low abun- found regularly, although in fairly low to moderate dances of O. ophiura in the present study refer to the rela- abundances on muddy or very ¢ne grained substrata tively small area covered by grab samples (Table 2). The (Salzwedel et al., 1985; Berberich, 1989; Ku«hne & actual situation is probably re£ected better by the dredge Rachor, 1996; Niermann, 1997; Schroeder, 2005; this samples revealing high abundances (Table 2). Opposed to study). Ursin (1960) refers to temperature as an important their amphiurid relatives, both epibenthic ophiurid species factor limiting this species’ distribution. The author appear to be quite robust and rather mobile brittle stars. describes its absence from regions with summer tempera- They exhibit a wide range of di¡erent feeding tures below 108C and winter temperatures below 38C. In mechanisms, e.g. deposit feeding, taking advantage of the inner German Bight the lowest monthly mean the high amount of associated organic matter, scavenging temperature (usually in February) basically has not or (Feder, 1981; Warner, 1982). Further, dropped below 2.78C in the past 20 years (Franke et al., Sko«ld (1998) identi¢ed a variety of di¡erent defence 1999) and has been signi¢cantly higher than in previous mechanisms against potential predators, e.g. burrowing decades (Wiltshire & Manly, 2004). Thus, increased and rapid escape. These features make them rather winter temperatures might have promoted a range expan- opportunistic towards di¡erent conditions, and thus may sion of A. brachiata. In their studies, Webb & Tyler (1985), explain for their success in comparably high densities over Bourgoin & Guillou (1990) and Bourgoin et al. (1991) time. identi¢ed delimited annual spawning processes as well as Amphipholis squamata was reported on di¡erent sediments shortened larval stages and abbreviated or even missing less regularly and in very low numbers (Meissner & pelagic phases in the life cycle of A. brachiata.Thismight Collin, 1894; Caspers, 1938; Stripp, 1969; Gerdes, 1977; help in explaining the slow but steady expansion and Salzwedel et al., 1985; Ziegelmeier, 1987; present study). colonization toward the inner German Bight observed Berberich (1989), however, recorded a mean value of over the past 30 years. nearly 200 ind m72 from 25 stations in the Tiefe Rinne, Abundances of Amphiura ¢liformis are apparently very which is characterized by a mixture of various types of inconsistent over time (Table 2).This species was recorded substrata. Amphipholis squamata is a hermaphrodite and in highest abundances with mean values between brooding species. It releases its o¡spring at a crawl-away 297 ind m72 (Salzwedel et al., 1985) and increased juvenile stage and therefore does not disperse through densities of 590 ind m72 (Thatje & Gerdes, 1997) on planktonic larvae. Accordingly, slow and irregular re- muddy substrata of the Amphiura ¢liformis association sensu colonization processes may take place, for example after Salzwedel et al. (1985). Stripp (1969), however, revealed extreme mortality events. Various authors suggest that only 5.1 ind m72 in his equivalent dispersal occurs through passive transport on other (Pennant, 1777)^Amphiura ¢liformis association. Similar , e.g. medusae or by rafting on macroalgae values for A. ¢liformis were reported in the same study (Highsmith, 1985; Edgar, 1987; O’Hara, 1998; Thiel & from all other faunal communities on di¡erent types of Gutow, 2005). The species’ mode of reproduction as well sediment as well. In addition, Berberich (1989) found as its passive dispersal may explain the irregular ¢ndings only 16 specimens on ¢ve of 25 stations sampled in the and generally low abundances in the inner German Bight, Tiefe Rinne. Variations in abundance of A. ¢liformis although A. squamata is distributed world-wide. Further- certainly refer to the di¡erent locations under study. The more, its preference for cryptic habitats such as crevices species’ infaunal lifestyle in ¢ne grain-sized sediments may in rockpools or holdfasts of macroalgae may a¡ect its explain the low densities or absence on coarser sediments occurrence on soft bottom sediments. or hard bottom substrata. Apart from sedimental aspects, The occurrence of Ophiothrix fragilis in the inner however, variability in abundance over time may also be German Bight is comparatively variable as well. Few due to sensitivity to cold winter temperatures (Ursin, studies revealed high numbers. Generally though, abun- 1960). Even though this species may tolerate winter dances are rather low or refer to single specimens often

Journal of the Marine Biological Association of the United Kingdom (2006) Brittle stars in the German Bight K. Boos and H.-D. Franke 1195 found on hard bottoms. In addition, it seems as if abun- 1992). Due to irregular currents, wave action, tidal e¡ects dances were somewhat higher prior to 1950. This and heavy beam trawl ¢shery, substrata are resuspended to however, is di⁄cult to interpret, since references of abun- a large extent and carried away. Suboptimal physical dance are given in verbal expression for the respective conditions might partly explain the species’ absence from period of time and thus do not allow for an objective the inner German Bight. comparison. Ophiothrix fragilis is a rheophilic suspension Since almost all records of A. chiajei, Ophiopholis aculeata, feeder and therefore dependent on high-current habitats Ophiocten a⁄nis and Ophiura sarsi date back to the period where it can form dense aggregations exceeding prior to 1924, these species most probably can be consid- 2000 ind m72 (Warner, 1971). These aggregations act as ered as incidental visitors in the inner German Bight local barriers to strong currents creating fronts of perhaps due to unusually favourable conditions at that suspended matter, which allow for enhanced feeding time. Su«bach & Breckner (1911) provided very detailed success and eased fertilization (Hughes, 1998). Ophiothrix descriptions of the species they found. Nonetheless, in the fragilis can be found in (micro-) habitats such as rocks, historical literature misidenti¢cations and misleading stones, sponges or corals (e.g. Alcyonium digitatum Linnaeus, synonyms must be taken into consideration. However, 1758).These habitats allow for an elevated positioning and according to the species’ overall short-term occurrence as the avoidance of decelerated bottom-near currents. Where well as their apparently low abundances, they are referred £ow velocity is high enough, this species can be found on to as ‘rare’ species in the inner German Bight. soft bottom sediments as well. These conditions, however, Apart from Amphipholis squamata which has a cosmopo- seem to be rather rare on the prevailing soft bottom sedi- litan distribution along the littoral zones of warm and ments in the inner German Bight. Hence, specimens of temperate as well as arctic and antarctic waters (Unno, O. fragilis presumably occur only sporadically in the inner 2000), all ophiuroid species recorded for the inner German Bight as isolated individuals or in small patchy German Bight are common representatives of the distributed populations. boreo-lusitanian fauna (Hyman, 1955) showing wide Five out of ten species recorded in previous investiga- distributional ranges. tions in the inner German Bight over the past 130 years, Due to a strong continental in£uence, coastal waters, were con¢rmed in this study. From this point of view, e.g. those of the inner German Bight, show a higher Acrocnida brachiata must be treated as a newcomer since annual variability in temperature and salinity compared the late 1970s, possibly as an e¡ect of climate change. Yet, to the central North Sea (Ursin, 1960). Suitable winter due to its continuous occurrence since then, all ophiuroid temperatures may occur in the inner German Bight for species recorded in the present study can be referred to as all of the species mentioned in Tables 2 and 3. Conversely, ‘common’ and regularly found species in the inner unfavourably high summer temperatures might play an German Bight. Our quantitative results correspond important role in reproduction and thus in distribution roughly to the general patterns of abundance depicted by of some species with northerly focused ranges, e.g. previous investigators. Increasing or decreasing trends in Ophiopholis aculeata. This might explain the broad absence abundance over time, however, cannot be inferred from for some of the ‘rare’ titled species from the inner German the reviewed data due to the way they were presented. Bight. Most of the ‘common’ representatives, such as Non-systematic and sporadic investigations over longer Ophiura albida, O. ophiura and Amphiura ¢liformis are highly periods of time do not allow for an unequivocal inter- abundant species in the German Bight. They have been pretation of changes in abundance (Franke & Gutow, described over time as characterizing and even domi- 2004). However, di¡erent anthropogenic or biological nating di¡erent faunal associations. Their high abun- factors may explain irregularities in records as well as in dances and share in biomass as well as their broad abundance over the reviewed period of time. ecological ranges indicate very low extinction probabilities Information on further species in the inner and thus account for a more or less steady ophiuroid German Bight refer to Amphiura chiajei Forb e s 18 43, species composition over time. Ophiopholis aculeata (Linnaeus 1767), Ophiocten a⁄nis Rees et al. (1999) showed a decrease of benthic bio- (Lu«tken 1858) and Ophiura sarsi Lu«tken 1858, and was diversity in the southern North Sea following the broad provided mainly for the period 1872 to 1924 by Mo«bius & coastlines from the English Channel into the south- Bu«tschli (1875), Meissner & Collin (1894), Su«bach & eastern areas of the North Sea. While sediment analyses Breckner (1911) and Hagmeier (1925). Again, verbal revealed higher amounts of coarser grounds along the expression in reference to abundance makes their densities Channel, the south-eastern parts of the North Sea are uncertain to assess, but seemingly refers to scarce or characterized by mainly sandy and muddy areas. The moderate occurrence. This is particularly true for two authors suggested substratum type to be the ‘main struc- singular records of A. chiajei in 1977 and 1983 with only turing force’ in distribution. From this point of view, the two individuals each found in the entire sampling proce- inner German Bight may be an unsuitable area for poten- dures (Schroeder, 2005). Amphiura chiajei has ecological tial arrivals of many species from the Channel area. demands very similar to A. ¢liformis. While the latter Di¡erent authors, however, pointed out, that various species preferably feeds as a suspension feeder and relies investigations on benthic communities in the inner on horizontal transport of suspended matter, A. chiajei is German Bight revealed serious shifts in species composi- considered a deposit feeder (Duineveld et al., 1987; tion over time due to anthropogenic or natural changes in Hollertz et al., 1998). Therefore, A. chiajei depends on the environment (Thatje & Gerdes, 1997; Franke & habitats, which allow for a deposition of ¢ne particulate Gutow, 2004; Reichert & Buchholz, 2006). In addition to matter (Buchanan, 1964). The surface sediments in the changes in the species spectrum, a general trend towards inner German Bight are fairly mobile (Becker et al., increased biomass and density was shown in a review of

Journal of the Marine Biological Association of the United Kingdom (2006) 1196 K. Boos and H.-D. Franke Brittle stars in the German Bight long-term changes in the North Sea benthos by Kro«ncke Edgar, G.J., 1987. Dispersal of faunal and £oral propaguels asso- (1995) and by Thatje & Gerdes (1997). The authors ciated with drifting Macrocystis pyrifera plants. Marine Biology, particularly refer to small short-lived species such 95, 599^610. as many , bivalves, ophiuroidsespecially Feder, H.M., 1981. Aspects of the feeding biology of the brittle Ophiura texturata. Ophelia A. ¢liformisand echinoids. Anthropogenic e¡ects such as star , 20, 215^235. Franke, H.D. & Gutow, L., 2004. Long term changes in the eutrophication were considered most important. While macrozoobenthos around the rocky island of Helgoland changes in the ophiuroids abundances can be observed, it (German Bight, North Sea). Helgoland Marine Research, 58, is yet di⁄cult to distinguish between e¡ects of anthropo- 303^310. genic and natural factors, and between mere £uctuations Franke, H.D., Gutow, L. & Janke, M., 1999. The recent arrival of and serious long-term trends in ecosystem structure and the oceanic isopod Idotea metallica Bosc o¡ Helgoland function. (German Bight, North Sea): an indication of a warming Qualitative changes in the species composition of trend in the North Sea? Helgola«nder Meeresuntersuchungen, 52, ophiuroids over time refer to the arrival of Acrocnida 347^357. brachiata in the 1970s. Some occasional ¢ndings of rare Gerdes, D., 1977. Zur Verteilung der Echinodermen in der Deutschen titled species have been reported during the time period Bucht. Diploma thesis, Institut fu«r Meereskunde, Kiel, investigated. These ¢ndings, however, cannot account for Germany, unpublished. Hagmeier, A., 1925. Vorla«u¢ger Bericht u«ber die vorbereitenden changes in ophiuroid species diversity over time. Thus, a Untersuchungen der Bodenfauna der Deutschen Bucht mit fairly stable composition of brittle stars can be stated for dem Petersen-Bodengreifer. Berichte der Deutschen the inner German Bight over the past 130 years. Wissenschaftlichen Kommission fu«r Meeresforschung,N.F.1,247^ 272. The present study is part of a diploma thesis carried out at the Highsmith, R.C., 1985. Floating and algal rafting as potential Biologische Anstalt Helgoland. Our thanks go to the Foundation dispersal mechnanism in brooding . Marine Alfred Wegener Institute for Polar and Marine Research and in Ecology Progress Series, 25,169^179. particular to the sta¡ of the Marine Station on the Island of Hollertz, K., Sko«ld, M. & Rosenberg, R., 1998. Interactions Helgoland for providing research facilities and personal support between two deposit feeding echinoderms: the spatangoid ^ especially the crew of the MS ‘Utho«rn’. 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