Humoral immunity has emerged as a vital immune component against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Nevertheless, a subset of recovered Coronavirus Disease-2019 (COVID-19) paucisymptomatic/asymptomatic individuals do not generate an antibody response, constituting a paradox. We assumed that immunodiagnostic assays may operate under a competitive format within the context of antigenemia, potentially explaining this phenomenon. We present a case where persistent antigenemia/viremia was documented for at least 73 days post-symptom onset using ‘in-house’ methodology, and as it progressively declined, seroconversion took place late, around day 55, supporting our hypothesis. Thus, prolonged SARS-CoV-2 antigenemia/viremia could mask humoral responses, rendering, in certain cases, the phenomenon of ‘non-responders’ a misnomer.

Reproduction in barnacles relies on chemical cues that guide their gregarious settlement. These cues have been pinned down to several sources of settlement pheromones, one of which is a protein termed Settlement-Inducing Protein Complex (SIPC), a large glycoprotein acting as a pheromone to induce larval settlement and as an adhesive in the surface exploration by the cyprids. Settlement assays in laboratory conditions with Amphibalanus (=Balanus) amphitrite cyprids in the presence of SIPC showed that cyprids exhibit settlement preference behaviour at lower concentrations (EC₅₀=3.73 nM) and settlement avoidance behaviour at higher concentrations of SIPC (EC₅₀=101 nM). By using truncated fragments of SIPC in settlement assays, we identify that domains at the N-terminal of SIPC transduce settlement preference cues that mask the settlement avoidance cues transduced by domains at its C-terminal. Removing the N-terminal 600 amino acids from SIPC resulted in truncated fragments that transduced only settlement avoidance cues to the cyprids. From the sexual reproduction point of view, this bimodal response of barnacles to SIPC suggests that barnacles will settle gregariously when conspecific cues are sparse but will not settle if conspecific cues inform of overcrowding that will increase reproductive competition and diminish their reproductive chances.