Blog Posts: Spicules
The group Porifera refers to the 5,000 living species of sponge and is composed of three major groups: the Hexactinellida (glass sponges), the Demospongia (siliceous sponges), and the Calcarea (calcareous sponges). Hecaxtinellid sponges are the most rare, and occur mostly in deep-sea habitats where they are inaccessible for study without the use of ROV technology. Demospongia and Calcarea represent most of the observed diversity in Porifera, and are separated by the elemental composition of their internal structure.
Sponges are considered to be a basal group on the tree of life, representing a primitive multicellular organism lacking in symmetry and without distinct tissue layers. Nevertheless, sponges can achieve an impressive amount of body complexity, and are represented by three body plan types: the ascon, sycon, and luecon. Ascon sponges are the most simplistic in design, and can be described as a simplified basket-shaped organism with holes perforating the exterior. Complexity takes the form of increased folding of the interior to create additional surface area for the absorption of food particles and oxygen. This increase in surface area has allowed some sponges to grow to enormous size, such as the giant barrel sponge (Xestospongia muta), a leuconoid Demosponge which can reach a diameter of almost 2 meters.
Generally, characteristics of sponges include a system of pores (called ostia) and canals set in an external layer called the pinacoderm, through which water passes. Water movement is driven by the beating of flagellated cells known as choanocytes. They are supported in a skeleton composed of the protein collagen and spicules, which are small, hard, calcareous or siliceous bodies. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called the mesohyl. Sponges capture food (detritus particles, plankton, bacteria) that is swept inward by water currents created by the choanocytes. Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells.
Much of the traditional SEM work involving sponges has focused on (1) spicule morphology, (2) elemental composition, (3) associated microbe communites, and (4) spicule presence in sedimentology. Difficulties in Porifera research often stem from an inability to easily identify species without extensive analysis. Spicules are one way to classify organisms, as their presence and distribution throughout the sample are often unique between species. Typically, spicules are designated as being “megascleres” or “microscleres” based on their size, with microscleres accounting for those between 10-60 µm. In this way, scanning electron microscopy makes detailed analyses of these features much easier. The first study detailing spicule formation made use of SEM technology, demonstrating the presence of an axial canal composed of the enzyme silicatein, which synthesizes the spicules (Müller et al. 2005).
Additionally, the use of EDX technology also allows for ease of analysis in determining elemental composition of spicules. As stated previously, Demospongia and Calcarea represent the majority of diversity with the phyla, but it is often not possible to assign a class purely through visual analysis. Interestingly, EDX was used to establish the first evidence of chitin in the skeletal system of a sponge (Ehrlich et al. 2007), suggesting a new alternative hypothesis for the evolution of skeletal formation in this phylum. Beyond basic elemental frameworks, additional studies have detected small-scale differences in heavy metal composition of spicules (Kumar & Shah 2014) and specialized allocation of metabolites in the pinacoderm (Koigoora et al. 2013), both of which have been posited to ease in species identification.
The study sponge-associated symbionts is useful from an ecological standpoint, as sponges are known to inhabit nutrient-poor systems and so must filter massive amounts of water and be very efficient (up to 80%) in their nutrient uptake (Fiore, Jarett, et al. 2013) to subsist. Both sponges and their symbionts can remove dissolved nutrients, particularly nitrogen, from the water column (Fiore et al., Fiore, Baker, et al. 2013). Studies using SEM technology have identified the main prokaryotic drivers behind these symbionts, and also described how their distribution across their host changes to maximize nutrient uptake. One study of the specious genus Aplysina found a high concentration of cyanobacteria, an important nitrogen-fixer, living within the choanocyte chambers of their hosts (Pfannkuchen et al. 2010). Additional studies found more diverse assemblages (Fiore, Jarett, et al. 2013) among different phyla, suggesting different species are cultivating differing microcommunities. Furthermore, Antarctic sponges were found to actively facilitate the uptake of diatoms, which accumulate in the mesohyl underneath the pinacoderm and help to strengthen the sponge cortex, and may in fact be an additional dietary source during oligotrophic periods in the Antarctic environment (Gaino et al. 1994).
Lastly, sponges have served as important bioindicators in marine sediments. Silica is only preserved in sediments which form in areas of high primary productivity, and so the presence of spicules in “siliceious oozes” can be telling of past environmental conditions. Similarly, knowledge regarding the ecology and natural history of calcareous species can be combined with sedimentology to make inferences regarding the geological past. For example, spicules have been used to study the intrusion of salt-water sources into estuarial systems (Volkmer-Ribeirto et al. 2004) over time, and are useful in the analysis of sequence stratigraphy (Neuweiler et al. 2014). The literature regarding this use of vast, and details will be covered more completely in our “Deep Sea Sediments” section of this Atlas.