Oikopleura dioica

2. Larvaceans are an important class of zooplanktons
Larvaceans are transparent planktonic animals, commonly less than 1cm in body size. Most species live in the pelagic zone within the upper 100m of the water column , though oikopleurids and fritillarids are found at depths extending to 3,500m [7–9]. Owing to their fast growth, voracious appetites and contribution to the vertical flux of organic matter, they have substantial impact on marine nutrient cycles [10,11]. 

2.1 Larvacean lifestyles in cellulose houses
Tunicates, the subphylum to which larvaceans belong, are the closest living relatives of vertebrates [12,13]. Uniquely among tunicates, larvaceans retain a tadpole-like planktonic morphology throughout their lives (Figure 1). Each animal is enclosed in a complex cellulose-based structure, or “house”, for filter-feeding on a concentrated suspension of bacteria and algae extracted from seawater [1,14]. Clogged houses are replaced several times a day and abandoned houses gradually sink to the ocean floor with food remnants, thus making up a significant portion of marine snow - the constant fall of organic matter from upper water layers to the depths. The size of animals and their accompanying houses vary according to species, from 4mm in diameter for Oikopleura dioica up to 1m among Bathochordaeus giant larvaceans [15].  

Figure 1. Adult (a) male and (b) female Oikopleura dioica (photos by Aki Masunaga, OIST); (c) O. dioica individual in its house (image credit Sars International Centre for Marine Molecular Biology). 

2.2 Larvaceans share morphological features with vertebrates
Larvaceans share several morphological features with vertebrates, including a central notochord and dorsal nerve chord, as well as cells analogous to the neural crest typical of derived chordate morphologies [13,16]. Molecular phylogenetic techniques have revealed that tunicates and vertebrates most likely shared a common ancestor more recently than cephalochordates, despite possessing fewer morphological similarities [12]. This implies that tunicates lost some of the morphological features present in the last chordate common ancestor. The genomes of several tunicate species have been sequenced, revealing remarkable differences between them in the presence and absence of conserved genes, rates of evolution and genome size [17–20]. 
 
Figure 2. Phylogenetic relationships among deuterostomes (adapted by Aleksandra Bliznina, OIST,  from Delsuc et al. 2006, 2018).

2.3 Some larvacean genomes evolve very quickly 
Oikopleura dioica is perhaps the most dramatic example of diversification among larvaceans [3,21]. Its genome has reduced to a mere 70Mbp (less than half the 170Mbp genome of the model tunicate Ciona robusta/intestinalis), and exhibits unique genomic characteristics, such as non-canonical splicing and the scattering of Hox genes [18,22]. It is thought that the accelerated life cycle (<5 days) and the loss of DNA repair pathways contribute to highly elevated rates of evolution [23]. However, although the first Oikopleura dioica genome was reported in 2010 [3], the extent of the species’ global genetic diversity remains unknown. Considering the organism’s rapid evolution (the transcriptomes of the Norwegian and Japanese isolates vary ~10% at the nucleotide level [24]) and its broad cosmopolitan distribution, the large-scale genetic diversity of Oikopleura might be expected to be high, but this is completely unknown.

2.4 Larvaceans are key vectors of microplastics 
Like other zooplankton, larvaceans are highly sensitive to changes in ecosystem and they respond quickly to seasonal variations in water temperature, nutrient balance, and ocean currents. The effects of environmental perturbations are reflected through rapid changes in species compositions; but there is little knowledge of how or to where populations suddenly appear and disappear [25]. Longitudinal studies have tracked larvacean composition in several coastal locations [26–28], but little is known about possible population genetic changes or evolutionary adaptations to long-term changes. Finally, larvaceans are potential key links in the biomagnification of environmental pollutants: a recent study revealed how giant larvaceans could serve as efficient vectors of microplastics using their filter-feeding systems [29](Figure 3).  Little is yet empirically known about the impact of such pollutants on larvacean biology, and so on marine food webs and nutrient cycles.