Phylogenomic and morphological relationships among the botryllid ascidians (Subphylum Tunicata, Class Ascidiacea, Family Styelidae)

Abstract

Ascidians (Phylum Chordata, Class Ascidiacea) are a large group of invertebrates which occupy a central role in the ecology of marine benthic communities. Many ascidian species have become successfully introduced around the world via anthropogenic vectors. The botryllid ascidians (Order Stolidobranchia, Family Styelidae) are a group of 53 colonial species, several of which are widespread throughout temperate or tropical and subtropical waters. However, the systematics and biology of this group of ascidians is not well-understood. To provide a systematic framework for this group, we have constructed a well-resolved phylogenomic tree using 200 novel loci and 55 specimens. A Principal Components Analysis of all species described in the literature using 31 taxonomic characteristics revealed that some species occupy a unique morphological space and can be easily identified using characteristics of adult colonies. For other species, additional information such as larval or life history characteristics may be required for taxonomic discrimination. Molecular barcodes are critical for guiding the delineation of morphologically similar species in this group.

Figure 1

Maximum Quartet Support Species Tree of relationships among botryllid species, using the program Accurate Species TRee ALgorithm (ASTRAL). Nodes with less than 50% posterior probability support have been collapsed. The scale bar indicates two coalescent units. Red text = Genus Botryllus, Blue text = Genus Botrylloides. Colored circles refer to geographic regions (Orange = Western Pacific/Hawaii, Green = Eastern Pacific, Purple = Western Atlantic/Caribbean, Pink = English Channel/Mediterranean).

Figure 2

Graphical representation of Principal Components Analysis for morphological characters in botryllid species. The blue circles represent species in the genus Botrylloides, and the red circles species in the genus Botryllus. (a) Graph excluding three outlier species: Botryllus flavus (33), Botryllus magnus (38), and Botryllus renierii (47). (b) Graph including three outlier species: Botryllus flavus (33), Botryllus magnus (38), and Botryllus renierii (47). Species names corresponding to the numbers on the graphs in both (a) and (b) are as follows: (1) Botrylloides anceps (2) Botrylloides aureus (3) Botrylloides chevalense (4) Botrylloides conchyliatus (5) Botrylloides crystallinus (6) Botrylloides diegensis (7) Botrylloides fuscus (8) Botrylloides giganteus (9) Botrylloides israeliense (10) Botrylloides leachii (11) Botrylloides lenis (12) Botrylloides lentus (13) Botrylloides magnicoecus (14) Botrylloides niger (15) Botrylloides perspicuus (16) Botrylloides praelongus (17) Botrylloides saccus (18) Botrylloides simodensis (19) Botrylloides superbus (20) Botrylloides tyreus (21) Botrylloides violaceus (22) Botrylloides Bocas del Toro (23) Botrylloides Rabbit Key (24) Botrylloides Philippines (25) Botryllus arenaceus (26) Botryllus aster (27) Botryllus closionis (28) Botryllus compositus (29) Botryllus delicatus (30) Botryllus eilatensis (31) Botryllus elegans (32) Botryllus firmus (33) Botryllus flavus (34) Botryllus gaiae (35) Botryllus gregalis (36) Botryllus horridus (37) Botryllus japonicus (38) Botryllus magnus (39) Botryllus meandricus (40) Botryllus mortenseni (41) Botryllus ovalis (42) Botryllus planus (43) Botryllus primigenus (44) Botryllus promiscuus (45) Botryllus pumilus (46) Botryllus puniceus (47) Botryllus renierii (48) Botryllus rosaceus (49) Botryllus scalaris (50) Botryllus schlosseri (51) Botryllus separatus (52) Botryllus sexiens (53) Botryllus stewartensis (54) Botryllus stuhlmanni (55) Botryllus tabori (56) Botryllus tuberatus (57) Botryllus Bocas del Drago.