ANNALES • Ser. hist. nat. 10 2000 ■ 2 (21)
original scientific paper	UDC 594.1(2623}
received: 5. 9. 2000
DISTRIBUTION OF CORBULA GIBBA{BIVALV1A, CORBULIDAE) IN THE NORTHERN ADRIATIC SEA
Moriana ALEFFI & Nicola BETTOSO Laboratorio di Biologie Marina, IT-34010 Trieste, Via A. Piccard 54
ABSTRACT
Corbula gibba is widespread on soft bottom of the northern Adriatic Sea it is considered a species with a wide ecologies! distribution, particularly abundant in zones characterized by environmental instability- Its distribution along the western side of the northern Adriatic Sea is discussed, furthermore data about its shell length, biomass and dominance in this area are analyzed.
Key words: Corbula gibba. distribution, northern Adriatic Sea
DISTRIBUZIONF; Di CORBULA GIBBA {BIVALVIA, CORBULIDAE)
NEL NORD ADRIATICO
S INTE SI
Corbula gïbba è un bivalve a suai diffuso sui fonds molli del Nord Adriático. E' una specie ad ampia ripartizione ecobgica, particolarmente abbondante in zone caratterizzate da instabilité ambiéntale. Puô resfstere, mfattt, a condizioni di ipossia delte acque di fondo e si comporta da specie pioniera nella ricohnizzazione dei fondait a seguito di crisi anossiche, inoltre risulta dominante in condizioni di arricchimento orgánico.
Nel presente studio C. gibba è risultata presente in sedimenti pelitici c politico sabbiosi, abbondante in prossimità dei più important i fiumi nord adriatici (TagHamento, Pi ave e Ad.ige) ed in paiticotare iungo la fascia cositera a sud del fíume Po, zona caralierizzata da una forte sedimenlazione di materia I i fini e interessata da periodici fenomeni di ipossia e anossia. E' s ta to evidenziato un decremento délia densità di C gibba dalla costa verso il largo, legato alle caratteristiche de! sedimento, alla profondità ed aile condizioni di stress ambiéntate. La dominanza di C. gibba in aree soggette ad instabilité ambiéntale conferiría il comporta/vento di specie, pioniera di questo bivalve, al contrario m zone non soggette a forti stress ambientali essa tende a seguire ¡'andamento dell'intera comunica.
Parole chiave: Corbula gibba, distribuzione, Nord Adriático
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Roriana ALFFF' & Nicola BETTO'-O: DISTRIBUTION OF COKH(,'IA CIUBA (BIVALVIA, CORUUIIDAI) IN FML NORTHERN ADRIATIC SCA, 173-! 80
INTRODUCTION
Corbula gibba Oiivi (1792} is a bivalve mollusc belonging to the Corbulidae family; it is distributed into the Atlantic area, from Northern Europe to Angola and in the Mediterranean Sea, Adriatic Sea included (Tebble, 1966).
C. gibba lives from the intertidai zone to considerable depth (Parerizan, 1976) and prefers muddy sand bottom with larger pieces of gravel and pebbles utilized for byssal-thread attachment, in order to maintain its siphons flush with the surface (Yonge, 1946).
Yonge (1946) also supposes that the asymmetry of the shell valves permits an efficient elimination of pseudo-faeces, so the obstruction originated by fine sediment is prevented and an efficient valves lock is assured.
The growth of the juveniles is very rapid (Jensen, 1988, 1990) and the specimens are retained by a 1 mm sieve in a few weeks after settlement. The length at metamorphosis is 0.25-0.33 mm (Muus, 1973) and the adults can exceed 13 mm in length (Hrs-Brenko, 1981).
It is considered to be a species with a wide ecological distribution (Bellan et al., 1975), particularly abundant in zones characterized by environmental instability. C. gibba is resistant to severe hypoxia (Diaz & Rosenberg, 1995; Brizzi ef al., 1994) and it is a pioneer species in recolonization of bottoms after anoxic crises; it is predominant in polluted or enriched areas (Crema et a/., 1991; Hrs-Brenko ef a/., 1994; Theodorou, 1994). Moreover, its shell is particularly resistant to mechanical stress, thanks to conchiolin layers that increase shell strength and toughness by acting as crack stoppers (Kardon, 1998). For this reason C. gibba is not damaged by trawl-fishing (Rumohr & Krost, 1991).
The first studies on C. gibba population in the Adriatic Sea were made by Hrs-Brenko (1979, 1981) along the western part of tstria, where this species is particularly abundant during hypoxic and after anoxic crises (Hrs-Brenko el at., 1994).
In the Gulf of Trieste, the first studies devoted to growth rates were made by Aleffi et at. (1993) at two stations in the middle of the Gulf, one located in an area affected by recurring hypoxia (Orel et al., 1993). More recently in the inner part of the Trieste harbour, biornet-ric analyses were made (Goriup ef a/., 1997) in relation to pollution and particularly to heavy metals concentration in the sediment (Adami ef al., 1997).
The aim of the present study was to analyze the distribution, shell length, biomass and dominance of C. gibba along the western part of the northern Adriatic Sea in relation to the environmental features of the area.
MATERIALS AND METHODS
Sampling was carried out at 40 stations located in the northern Adriatic Sea, from 3 to 12 May 1995, during the PRISMA research project (founded by the Italian Ministry of University, Scientific Research and Technology).
The sites were located on soft bottom affected by sedimentation of riverine fine material, and along transects perpendicular to the shore characterized by different bottom textures, front pelite to coarse sand (Brambati et al., 1983), at. depths ranging from 12 to 70
A 0.1 m-! Van Veen grab was used and a total of 5 samples randomly positioned were removed from the substratum at each sampling site. Grab samples were processed through sieve of 1 mm mesh size and the re tained material was preserved in a solution of 4% buffered formaldehyde.
In the present study only the stations with at least 15 specimens of C gibba sampled were considered to analyze its shell length, biomass and dominance. The shell length was measured with a calliper and the specimens were separated in 1 mm length classes. For each sample, about 70% of total specimens was measured, whereas for the stations with high abundance (st. 18, 26, 31), only 200 specimens were chosen randomly.
The Kolmogorov-Smirnov two-sample test was applied to the size-frequency distribution of C. gibba in the sampling stations, in order to verify the null hypothesis that two samples belong to the same distribution {Sokal & Rohlf, 1997).
Finally the dry weight of C. gibba was determined for each station, drying the soft parts at 105° for 24 hours (Stirn, 1981) and the dominance within the communities of the sampling sites was calculated as percentage.
RESULTS
Figure 1 shows the position of sampling stations and the abundances of C. gibba. The specimens were more abundant at the stations near the shore and particularly south to the Po river mouth, where C gibba reached abundance of 2043 ind. 0.5 m-> at st. 31. In table 1, the depth, sediment type (Brambati et al., 1983) and number of individuals of each sampling station are given. All the specimens were divided in four depth classes and five sediment types: the major part of specimens (84%) was found in the pelitic sediment (Fig. 2) and 66% of the total at depths between 12 arid 15 rri (Fig. 3). Only few individuals were associated with sandy sediments and were found at depths > 25 m.
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Floriana Al £pfi A Nicola BFTTOSO DISTRIBUTION OF CO/tPULA CMS A ¡BIVAIVIA. COKBUKOAt) IN THfc NORTHFRN ADRIATIC SEA. 173 180
<L> TJ
r>
<D
13° Longitude
Fig. 1: Location ofsampting stations and abundance of C. gibba (ind. 0.5 m 2). SI. 1: Lokacija vzorčišč in relativna gostota školjke C gibba (ind. 0,5 nr-).
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Florinna ALfffl & Niçois BETl'OSO: DISTRIBUTION OF COR RU LA GtBBA 0IVAIVIA. COR81K.IDAI) IN THE NORTHERN ADRIATIC SEA, 173-180
The mean-length for every site was calculated (Tab. 2): it. varied from 6.57 (± 1.07) at st. 29, to 10.42 (± 1.30) at st. 2 (Gulf of Trieste); the biggest specimen, sampled in st. 2, measured 13.5 mm.
The Kolmogofov-Smirnov test applied to size-frequency distribution of the populations revealed significant differences between sampling stations, compared two by two (Sokal & Kohlf, 1997). The results are given in table 3, where "1" indicates stations with different distribution of the populations, whereas "0" indicates the same distribution at the two sites. Altogether, the 59.6% of comparisons gave significant differences between stations; in particular, st. 2 and st. 31 frequency distribution of shell length differed from those at all other stations.
The Kruskal-Wallis test applied on dominance expressed as percentage of the two groups (Tab. 4), consisting of stations north {st. 2-14) and south (st. 1740) to the Po river mouth, revealed significant differences {Tab. 5); on the contrary the same analysis applied on biomass values (Tab. 4) did not show any significant differences (Tab. 5)
DISCUSSION AND CONCLUSJON
C gibbû is widespread on soft bottoms of the northern Adriatic Sea. Vatova (1949) includes it in "Chione gallina" and "Schizaster chiajei" zooœriosis, but the abundances were lower than at present time.
Orel era/. (1987), studying benthic populations between the Gulf of Trieste and the Po river mouth, found it at about 90 sites, characterised by the biocenoses of "Vase Terrigene Cotière" and "Fonds Détritique Cotiers" (Pérès & Picard, 1964).
SP
11 06% :...
Tab. I: Depth and sediment type of all sampling stations in relation to the individual number of C. gibba. S: sand, PS: pelitic sand, VSP: very sandy pelite, SP: sandy pelite, P: pelite
Tab. 1: Globina in tip sedimenta na vseh vzorčiščih glede na posamezno število školjke C. gibba. S: pesek, PS: peiitski pesek, VSP: zelo peščen pelit, SP: peščen pelii. P: pelit
Fig. 2: Percentage of specimens in relation to the sediment types.
S: sand, PS: pelitic sand, VSP: very sandy pelite, SP: sandy pelite, P: pelite
SI. 2: Odstotek primerkov glede na tipe sedimenta. S: pesek, PS: peiitski pesek, VSP: '/elo peščen pelit, SP: peščen pelit, P: pelit
	ind. 0.5 m 2	depth (m)	sediment
st. 1	14	13	P
st. 2	51	23	SP
st. 3	2	14	S
st. 4	205	16	SP
st. 5	274	12	p
st. 6	131	16	SP
st. 7	87	15	SP
st. 8	0	21	s
st. 9	0	31	VSP
st. 10	0	31	PS
st. 11	55	26	PS
st, 12	165	22	SP
st. 13	55	21	p
st, 14	27	28	PS
st. 15	0	32	SP
st. 16	5	30	SP
st. 17	69	18	p
st. 18	409	21	p
st. 19	49	27	p
st. 20	8	30	p
st. 21	0	38	VSP
st. 22	0	40	PS
st. 23	0	41	PS
st. 24	11	37	VSP
st. 25	5	31	p
st. 26	3 BQ	14	p
st. 27	24.5	12	p
st. 28	162	23	p
st. 29	18	34	p
st. 30	1	40	VSP
st. 31	2043	12	p
st. 32	9	26	p
st. 33	8	23	p
st. 34	0	27	p
st. 35	1	60	PS
st. 36	5	62	PS
sf. 37	0	70	VSP
st. 38	0	69	VSP
sf. 39	2	45	p
st. 40	143	23	P
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Tab. 2: Size-frequency distribution and mean length of the measured samples. Tab. 2: Velikostna porazdelitev in povprečna dolžina izmerjenih primerkov.
	2.5	3.5	4.5	5.5	G.5	length classes (mm) 7.5 8.5 9.5			10.5	11.5	12.5	13.5	measured samples	mean length	S.D.
st. 2	0	0	0	0	0	1	2	16	17	to	3	1	50	10.42	1.3
st. 4	1	0	2	1	18	28	26	33	31	8	2	0	ISO	8.81	2.84
st. 5	0	0	1	10	17	28	47	72	23	2	0	0	200	8.64	1.91
St. 6	1	0	1	1	2	20	19	28	36	7	3	?	119	9.35	2.64
St. 7	0	0	1	2	5	10	5	11	25	3	0	0	62	9.15	2.89
St. 11	0	0	0	0	3	S	10	3	8	6	1	0	36	9.33	2.77
sf. 12	0	1	3	15	27	35	9	12	14	12	5	0	133	8.09	4.43
st. 13	0	0	3	7	17	8	9	4	2	1	0	0	51	7.25	2.55
st. 14	0	0	0	0	0	6	9	6	0	2	0	0	23	8.76	1.29
St. 17	0	0	0	0	7	23	24	9	2	0	0	0	65	8.13	0.92
st. 18	0	0	0	0	12	72	69	38	7	2	0	0	200	8.31	0.99
st. 19	0	0	0	0	0	6	7	5	12	7	2	0	39	9.83	2.23
st. 26	0	0	1	1	3	34	56	63	39	2	1	0	200	9.02	1.36
st. 27	0	0	0	2	14	42	72	43	18	8	1	0	200	8.66	1.58
st. 28	0	0	0	3	7	34	33	35	22	8	1	0	143	8.85	1.99
st. 29	0	0	0	5	6	2	2	0	0	0	0	0	15	6.57	1.07
St. 31	0	0	0	5	56	SO	23	16	16	3	1	0	200	7.77	1.84
St. 40	0	0	0	1	4	35	39	42	12	0	0	0	133	8.65	1.11
Tab. 3: Kolmogorov-Smirnov test: "1" indicates the stations with different distribution of the length. Tab. 3: Kolmogorov-Smirnov test: "1" označuje vzorčišča s školjkami različnih dolžin.
	5t. 2	si. 4	st. 5	st. 6	st. 7	St. 11	st. 12	st. 13	st. 14	st. 17	st. 18	st. 19	st. 26	st. 27	st. 28	st. 31	it. 40
St. 2	0	1;1S	1	1	1	i	1	1	1	1	1	0	1		1	1	1
st. 4		0	0	0	0	0	1	1	0	1	1	0	0	0	0	1	0
st. 5			0	iifl	ill	C	1	1	0	1	1	1	0	0	0	1	0
st. 6				0	0	0	1	1	0	m\i	1	0	0	1	0	1	1
st. 7					0	0	0	1	0	i	1	0	1	iij I'lijlK	0	1	■i-tl:-';
St. 11						0	•it:-	1	0	i	1	0	0	0	0		1
st. 12							0	0	1	0	0	1		1	-1	0	1
st. 13								0	1	1	1	1	1	1	1	0	1
st. 14									0	0	0	1	0	0	0	1	0
St. 17										0	0	1	1	0	1	1	0
st. 18											0	1	1	0	' 1	1	0
st. 19												0	1			1	0
st. 26													0	w,:	0	1	0
st. 27														0	0	1	0
st. 28															0	1	0
st. 31																0	1
st. 40																	0
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ANNALES Ser. hist. nat. 10 2000 2 (21)
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Tab. 4: Biomass values and percentage dominance of the stations with at least 15 specimens ofC. gibba. Tab. 4: Biomasa in odstotkovna dominanca na vzor-čiščih z najmanj 15 primerki školjke C. gibba.
		dry weight	
	ind. 0.5 m*2	% 0.5 m-2	% dominance
St. 2	51	0,344	16
st. 4	205	1,311	37
st. 5	274	1,631	26
st. 6	131	0,989	32
st. 7	87	0,638	10
St. 11	55	0,328	9
st. 12	165	0,744	34
St, 13	55	0,226	10
st. 14	27	0,051	4
st. 17	69	0,335	14
st. 18	409	2,211	73
st. 19	49	0,370	15
st. 26	380	3,261	65
St. 27	.245	2,195	43
st. 28	162	0,989	64
st. 29	18	0,033	8
St. 31	2043	11,493	66
st. 40	143	1,170	44
Attribute	% dominance	Biomass
Data transform	None	None
		
H	3.947*	1.996
P	0.047	0.158
df	1	1
During the present study, C. gibba was found on pelitic and sandy-pelitic sediments, being abundant in proximity to the main northern Adriatic rivers (Taglia-mento, Piave and Adige) and especially along the shoreline south to the Po river, characterized by high ¡ates of riverine fine material sedimentation and affected by periodic hypoxia and/or anoxia (Rinaldi et at., 1992). There is an evident decreasing gradient from coast to the offshore z one (Fig. 1, Tab. 1) that appear related to the sediment type, depth and stress conditions.
Viceversa the analysis applied on the shell length does not reveal any trend similar to that regarding its abundance. Only st. 2 and st. 31 were separated from all others, probably because the former has the biggest specimens, as already found by Ateffi et a!. (1993) and the latter shows the highest individuáis number, about 45% of total sampled specimens.
In the area north to the Po river, where the faunistic composition of the macrobenthic communities are richer than in the coastal southern area (Aleffi, 1997), C gibba resulted abundant but not dominant. The variance analysis applied on percentage dominance values showed significant differences between stations north and south to the Po river mouth (Tab, .5). The dominance of C. gibba south to the river Po has been observed in previous studies and linked to the features of the area: high organic-matter input, high sedimentation rates, periodic bottom hypoxia and anoxia (Crema et a!.,
1991; Tahey ei a!., 1996; Tomassetti et a!., 1997; Ai-bertelli etai., 1998; Moodiey et a!., 1998).
Tab. 5: Kruskal-Wallis test for percentage dominance and biomass (g dry wt) compared between the groups of stations north and south to the Po river mouth. Tab. 5: Kruskalov-Wallisov test odstotkovne dominance in biomase (g suhe teže) ter primerjava med skupinami vzorčišč severno in južno od ustja reke Pad.
Its dominance in the areas affected by environmental instability, as south to the Po river mouth, confirms the pioneer species behaviour. In these ateas, in fact, C. gibba has no competitors and its abundance increases considerably; probably favoured by its particular morphology (Yonge, 1946) that permits it to survive in areas with high sedimentation rates. On the contrary, in the areas not subjected to environmental stress, C. gibba follows the trend of the entire benthic community, as already noted in the Gulf of Trieste (Aleffi ei al., 1993).
Fig. 3: Percentage of specimens in relation to the depth.
SI. 3: Odstotek primerkov glede na globino morja.
ACKNOWLEDGEMENT
The authors are grateful to Donatella Dei Piero and Serena fonda Urriani for the review of the manuscript, and to Francesca Goriup and Barbara Martincic for their contribution in the field and laboratory analyses.
178
15-25 m
25-40 m >40 m 4.05% 0.17%
ANNALES • Ser. hist. nat. ■ 10 • 2000 • 2 (21)
florlaiw ai r.FFi a Nrcolo BÍTTOSO: DISTRH3UTION' OF C.ORSUl A C!$ÜA OtVAlVtA. CORBL'UDAE) <N THE NOS ti líRN ADRÍATfC SEA, 1 7J-180
RAZŠIRJENOST ŠKOLJKE CORBULA C1BBA (B1VALV1A, CORBULIDAE)
V SEVERNEM JADRANU
Fiaría na ALEFFI & Nicote BETTOSO Morski biološki laboratorij, IT-34010 Trst, Via A. Piccard 54
POVZETEK
Školjka Corbula gibba je splošno razširjena na mehkem dnu severnega Jadrana. Velja za široko ekološko razširjeno vrsto, še posebno številna je v območjih, značilnih po spremenljivosti okolja. C. gibba je pravzaprav odporna proli hudi hipoksiji in je pionirska vrsta v vnovičnem podeljevanju morskega dna po krizah, ki jih povzroča anoksija. Poleg tega je prevladujoča v onesnaženih območjih.
C. gibba je bila med pričujočim preučevanjem najdena na pelitskih ah peščeno-pelitskih sed imen tih. Močno je razširjena v bližini glavnih severnojadranskih rek (Tilment, Piava m Adiža) in Še posebno vzdolž obrežja južno od reke Pad z značilno močno sedimentscijo drobnega rečenega materiala ter občasno Ivpoksijo in apoksijo. Od obrežja se proti odprtemu morju spušča greben, kar vpliva na tip sedimenta, globino in stresne razmere v tem območju. Prevlada školjke v območjih, izpostavljenih okoljski nestabilnosti, kol na primer južno od ustja reke Pad, potrjuje vedenje pionirske vrste Po drugi strani pa v območjih, ki niso izpostavljeni okoljskim stresom, ta Školjka sledi trendu popolnoma bentoške združbe.
Ključne besede: Corbula gibba, razširjenost, severni Jadran
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