Fig 1.
A view of the studied coralligenous of Bogliasco (Ligurian Sea) mixed with patches of the seagrass Posidonia oceanica.
The drawing shows a core sample conduced on a build-up and reaching the basal rock.
Table 1.
Calibrated radiocarbon ages of the different layers of each core sample.
Uncertainty refers to one standard deviation confidence level. Present assumed as 1950AD.
Fig 2.
Sponge species number recorded in the Bogliasco coralligenous assemblage during the samplings of 1973 and 2014.
The grey bar indicates the species number shared by the two surveys.
Fig 3.
Sponge species number recorded in the Bogliasco coralligenous assemblage during the samplings of 1973 (white bars) and 2014 (black bars), according to the sponge different growth patterns.
The main reduction was recorded within the massive/erect sponges.
Table 2.
Percent abundance and growth pattern of the sponge species recorded during the two surveys.
The species in bold were shared by the two surveys.
Fig 4.
Examples of spicules embedded in the sediment inside the coralligenous crevices.
A, achantostyle of Agelas; B, rhabdostyle of Rhabderemia; C, spiraster of Cliona; D, selenaster of Placospongia; E, diplaster of Diplastrella; F, cladotylote of Acarnus; G, forceps of Forcepia; H, anisochela of Mycale; I, dichotriene of Dercitus; J, sterraster of Erylus; K, sterraster of Geodia; L, microstrongyle of Pachastrella; M, dichomesotriene of Triptolemma; N, tubercolate oxea of Alectona; O, dilophose calthrop of Plakina.
Table 3.
List of the genera identified on the basis of spicular remains recorded in the layers belonging to the considered spans of time.
Genera in bold were also recorded in the recent surveys.
Fig 5.
Alveospongia sp. A, B two different examples of sinuous acanthomicrostrongyles typical of the genus; C, detail of the microspiny surface of acanthomicrostrongyles; D, magnification of the acanthomicrostrongyle tip.
Fig 6.
Number of sponge genera recorded in the considered periods.
Note the sharp decreases corresponding to the temperature collapse at the end of the Bronze Age (3000–2500 YBP). After this period, the sponge diversity progressively increased during the Little Climatic Optimum (2500–1500 YBP) to decrease again during the Dark Age Cold Period (1500–1000 YBP).
Fig 7.
Average total spicule amount per sediment g (±SE) in each considered period.
Fig 8.
Percent of sponge genera recorded in all the considered periods according to the different growth patterns.
Fig 9.
Hypothetical evolutionary scenario of the coralligenous accretions of Bogliasco.
A) In a first phase the algal growth resulted in pillar-like bioherm. B) Periods of heavy floods could have increased the bottom sediments, partially or totally burying the pillars and killing the algal coverage. C) During the burying or after the removal of the sediments, a part of the structure could be prone to erosive processes, giving rise to mushroom-like structures. D) In following phases, the coralline algae could grow again in sciaphilous microhabitats, determining the irregular temporal layering of the structure (the number from 1 to 4 indicated different sheets of deposition from the oldest to the youngest). In this situation, in a core sample (dotted rectangle), younger sheets can be overlapped by older ones.
Fig 10.
Trend of sponge diversity evaluated as number of genera present in each period (grey bars) compared with the trend of sponge abundance evaluated as average amount of spicules per sediment g present in the same periods.