Spearman rank correlations between the shell factors evaluated (Table 1), and the total taphonomic grade (TTG) (see also S1 Table).

<p>Spearman rank correlations between the shell factors evaluated (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184745#pone.0184745.t001" target="_blank">Table 1</a>), and the total taphonomic grade (TTG) (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184745#pone.0184745.s005" target="_blank">S1 Table</a>).</p

Sclerobionts coverage on mollusks.

<p>(A) Gastropoda genera: Ade.: <i>Adelomelon</i>, Buc.: <i>Buccinanops</i>, Cre.: <i>Crepidula</i>, Epi.: <i>Epitonium</i>, Oli.: <i>Olivancillaria</i>, Psa.: <i>Psania</i>, Sin.: <i>Sinum</i>. (B) Bivalvia genera: Ama.: <i>Amalarillodesma</i>, Ami.: <i>Amiantis</i>, Ana.: <i>Anadara</i>, Bra.: <i>Brachidontes</i>, Chls: <i>Chlamys</i>, Cra.: <i>Crassostrea</i>, Don.: <i>Donax</i>, Gly.: <i>Glycymeris</i>, Lae.: <i>Laevicardium</i>, Mac.: <i>Mactra</i>, Ost.: <i>Ostrea</i>, Per.: <i>Perna</i>, Pho.: <i>Pholas</i>, Pit.: <i>Pitar</i>. Und.: Unidentifiable.</p

Some examples of sclerobionts that colonized molluscan shells gathered from Concheiros Beach, on the Southern coast of Rio Grande do Sul, Brazil.

<p>(A) The inside of a <i>Buccinanops</i> gastropod that contained sclerobionts, such as serpulid polychaete, bryozoans, and an oyster. On the external side of the shell, there is evidence of bioerosion by Spionidea polychaeta (arrow). (B) A gastropod shell with a sand structure made by the polychaete <i>Phagmatopoma caudata</i>. (C) A gastropod covered by bryozoans. (D) A fragment of a gastropod that was fouled by eggs of the bivalve <i>Stramonita haemastoma</i> (arrow). (E) An external view of a <i>Pholas</i> bivalve shell with encrusting Ostreidae and bryozoans and a sediment-tube made by a polychaete (arrow). (F) Internal view of another <i>Pholas</i> shell with several oysters (<i>Ostrea equestris</i>) and bryozoans. (G) External view of an <i>Anadara brasiliana</i> bivalve shell with a boring sponge. (H) Internal view of a <i>Mactra</i> bivalve shell encrusted by Ostreidae. The hole was made by a spionidea polychaete. Scale bars: 10 mm.</p

The occurrence of sclerobionts exposed to distinct ornamentation and mineralogy of the host substrates.

<p>(A) Gastropod external ornamentation. (B) Bivalvia external ornamentation. (C) Bivalvia internal ornamentation. (D) Bivalvia mineralogy. Und.: Unidentifiable. The vertical lines denote the 95% confidence intervals (standard error*1.96), and the lowercase letters indicate similarities (the same letters) or significant differences (different letters) between the factors (Tukey test).</p

The occurrence of sclerobionts exposed to distinct life modes, sizes, and colors of the host substrates.

<p>(A) Gastropod life modes. (B) Bivalvia life modes. (C) Gastropod sizes (D) Bivalvia sizes. (E) Gastropod color. (F) Bivalvia color. Und.: Unidentifiable. The vertical lines denote the 95% confidence intervals (standard error*1.96), and the lowercase letters indicate similarities (the same letters) or significant differences (different letters) between the factors evaluated (Tukey test).</p

Categorical classification of external ornamentation, mineralogy (1 = calcite; 2 = aragonite; 3 = bimineralic) and frequency of occurrence (FO) data.

<p>Categorical classification of external ornamentation, mineralogy (1 = calcite; 2 = aragonite; 3 = bimineralic) and frequency of occurrence (FO) data.</p

Sclerobiont occurrence on different shell size.

<p>(A) Encrustation occurrence on Gastropoda. (B) Bioerosion occurrence on Gastropoda. (C) Encrustation occurrence on Bivalvia. (D) Bioerosion occurrence on Bivalvia.</p

Biofilm community on shells.

<p>(A) Bacterial biofilm density (bact cm^-2) on different shells. (B) The relative size (FSC-A) and complexity (SSC-A) of the bacterial cells measured by a flow cytometer. Each point represents a bacterial cell. The lighter colors (central part) are related to higher density cells with a determined feature (size × complexity) being characterized as one population. (C) Microorganism communities stained with acridine orange under epifluorescence microscopy (1000X). The vertical lines denote the 95% confidence intervals (standard error*1.96), and the lowercase letters indicate similarities (the same letters) or significant differences (different letters) between the shells (Tukey test).</p

Zooplankton colonization on shells.

<p>(A) The colonization density on the internal and external surfaces of different shells. (B) The richness of colonizers on internal and external surfaces. (C) Settled zooplankton composition (%) on different shells sides. The vertical lines denote the 95% confidence intervals (standard error*1.96), and the lowercase letters indicate similarities (the same letters) or significant differences (different letters) between the shells (Tukey test).</p

What determines sclerobiont colonization on marine mollusk shells?

<div><p>Empty mollusk shells may act as colonization surfaces for sclerobionts depending on the physical, chemical, and biological attributes of the shells. However, the main factors that can affect the establishment of an organism on hard substrates and the colonization patterns on modern and time-averaged shells remain unclear. Using experimental and field approaches, we compared sclerobiont (i.e., bacteria and invertebrate) colonization patterns on the exposed shells (internal and external sides) of three bivalve species (<i>Anadara brasiliana</i>, <i>Mactra isabelleana</i>, and <i>Amarilladesma mactroides</i>) with different external shell textures. In addition, we evaluated the influence of the host characteristics (mode of life, body size, color alteration, external and internal ornamentation and mineralogy) of sclerobionts on dead mollusk shells (bivalve and gastropod) collected from the Southern Brazilian coast. Finally, we compared field observations with experiments to evaluate how the biological signs of the present-day invertebrate settlements are preserved in molluscan death assemblages (incipient fossil record) in a subtropical shallow coastal setting. The results enhance our understanding of sclerobiont colonization over modern and paleoecology perspectives. The data suggest that sclerobiont settlement is enhanced by (<i>i</i>) high(er) biofilm bacteria density, which is more attracted to surfaces with high ornamentation; (<i>ii</i>) heterogeneous internal and external shell surface; (<i>iii</i>) shallow infaunal or attached epifaunal life modes; (<i>iv</i>) colorful or post-mortem oxidized shell surfaces; (<i>v</i>) shell size (<50 mm² or >1,351 mm²); and (<i>vi</i>) calcitic mineralogy. Although the biofilm bacteria density, shell size, and texture are considered the most important factors, the effects of other covarying attributes should also be considered. We observed a similar pattern of sclerobiont colonization frequency over modern and paleoecology perspectives, with an increase of invertebrates occurring on textured bivalve shells. This study demonstrates how bacterial biofilms may influence sclerobiont colonization on biological hosts (mollusks), and shows how ecological relationships in marine organisms may be relevant for interpreting the fossil record of sclerobionts.</p></div