Habitat Characteristics of the Horse Mussel Modiolus modulaides (Röding 1798) in Iloilo, Philippines

Despite the ecological importance of horse mussels, they have not received enough attention because they are considered of less economic value than other fisheries resources and not as charismatic as other marine resources. As a result, research efforts are often limited and information on biology and ecology is scant, affecting resource management. Recognizing this, the present study investigated the habitat characteristics of a local bioengineering species in Iloilo, Philippines – the horse mussel Modiolus modulaides. Analyses of water properties, sediments, phytoplankton composition in the water column, and food items pre-ingested by M. modulaides during the wet and dry seasons in Dumangas, Iloilo, Philippines were conducted. The water temperature (27.33–27.76 °C), dissolved oxygen (4.22–5.21 mg/L), salinity (30.97– 32.63‰), pH (7.55–8.01), total dissolved solids (31736.70–33079.48 mg/L), and conductivity (50121.56–53971.26 µS/cm) favor the growth and maintenance of M. modulaides. Sediments exhibited increasing deposition of fine material and high organic matter content as a result of the deposition of feces and pseudofeces. Diatoms comprised the majority of phytoplankton in the water column and among pre-ingested food items of M. modulaides. Based on pre-ingested food items, M. modulaides appears to practice selective feeding with preferences on Thalasionema spp., Diploneis spp., Pleurosigma spp., Detonula spp., and Rhizosolenia spp. Copepods and tintinnids may also be important additional food sources. Horse mussel density drastically decreased from 105.33 ± 30.15 individuals/m2 during the wet season to 24.47 ± 3.78 individuals/ m2 during the dry season due to intense exploitation in the area. Aside from providing baseline data, these results are essential in the development of a management framework and an initial step towards the development of culture technology for M. modulaides.