Urochordates

Larvaceans (Appendicularians)

Larvacean in its house

Larvaceans (Appendicularians) are urochordates (as a subdivision of phylum Chordata). The urochordates consists of three groups, the Ascidians (Sea Squirts), Thaliaceans (Salps, Doliolids and Pyrosomids) and the Larvaceans or Appendicularians. The name 'larvacean' derives from the fact that these organisms resemble the tadpole larvae of the ascidians, and they presumably evolved from something similar by a process of neoteny: retention of juvenile characteristics by the adult. Adult larvaceans retain the tail of the tadpole-like ascidian larva, but reproduce sexually. Sexual reproduction is the only mode of reproduction in larvaceans and the free-swimming larva metamorphoses directly into the adult. The alternative name 'appendicularian' refers to the appendage or tail they possess.

Larvacean in its house

Larvaceans construct an elaborate mucous house for themselves, which serves in food capture and locomotion and in protection. These houses are typically the size of a walnut, but in giant species may reach one to to meters in diameter. They are very fragile and translucent and often obscured by embedded particles, and so have proven difficult to study. By beating its tail the larvacean can draw water currents through its house. water enters through one or two apertures covered with fine fibrous meshes or grids that filter out particles too large to be of interest. These are inhalent prefilters and the water passing into the house then passes through an intricate set of finer mucus filters which sieve out very fine particles, with a mesh-size as small as 0.2 by 1 micrometer (μm) they can filter out microscopic algae, bacteria (nanoplankton) and tiny organic debris. They are said to be more efficient at filtering such fine materials than any other animal (though I expect sponges may be at least as effective, but confined to benthic habitats whereas larvaceans are planktonic).

External link: a nice larvacean animated graphic.

Larvacean with its filters

The skin of the larvacean consists of a single layer of epithelium, there is generally no tunic as in ascidians, though Fritillaria pellucida has no epithelium over most of its body, but an acellular layer instead, a tunic of sorts, except near the front of the trunk where epithelium does occur. the epithelium near the front end of larvaceans is also called the oikoplast and secretes the new mucous house in a compacted form, called the house rudiment. When the old house has been vacated and a new house is needed, this house rudiment is expanded with water by action of the tail.

Larvacean in its house

The larvacean house is a temporary structure. If touched, even lightly, then the larvacean will flee its house through the emergency exit and swim freely in the water column and eventually produce a new house. Similarly, when the filters become blocked the house is discarded. As many as 5 to 10 (4 to 16) new houses may be produced in one day and up to 46 have been counted in an individual's lifetime (which is typically only about 7 days). Generally, a single house is kept for no more than 4 hours in Oikopleura. These discarded houses are laden with organic debris as they rain down on the seabed they provide important food and may be fed upon by crustaceans as they descend and other animals like sea cucumbers when they sediment.

Larvacean in its house

The body of a larvacean is flask-like or ovoid and certain of its organs and systems are rather 'embryonic' and undeveloped.

In many species the house is bioluminescent, both when occupied and when vacated. A simple touch is sufficient to induce the luminescence. This is probably defensive, with a house discarded in an emergency continuing to provide a useful distraction.Some of this luminescence may be due to organisms on or entangled in the house, but empirical evidence suggests that primarily it arises from granules deposited in the house rudiment from the paired oral gland (see below).

Larvacean in its house

Classification

There are three recognised families of larvacean: Oikopleuridae, Fritillariidae and kowalevskiidae, but larvaceans are abundant in the oceans and poorly studies due to the difficulties associated with collection (fragility of the houses) and study and many more species probably await discovery. More species of giant larvacean have recently been discovered.

External link: the giant larvacean Bathochordaeus (Youtube).

Nervous System

The brain or cerebral ganglion is located above the oesophagus (supra-oesophageal ganglion) and puts out one anterior and a posterior nerve. The anterior nerve connects to an anterior circum-esophageal nerve (refs). The brain contains one vesicle or sac (the fluid-filled statocyst) that contains a spherical otolith: a sensor of equilibrium (i.e. it senses one or more of: balance (orientation in space), acceleration of the animal, gravity etc.). The posterior nerve (dorsal nerve cord) passes left of the oesophagus then between the lobes of the stomach then arrives at the caudal ganglion in the base of the tail. A caudal nerve from the caudal ganglion courses along the tail, left of the notochord. (Originally the left side of the tail was dorsal to the notochord, but the tail twists during development so that dorsal becomes left and ventral right).

The brain also sends nerves to the lips, which contain sensory cells, and to the stigmata. The stigmata are a left-right pair of pharyngeal openings (pharyngeal slits) to the outside. These nerves coordinate the action of the ciliary rings - there is one ciliary ring just inside each stigmata.

The caudal ganglion also puts out a pair of recurrent nerves that travel back into the trunk to innervate the paired Langerhans receptors. Each Langerhans receptor consists of an epithelium cell with a sensory cilium.

In addition to a few nerves, the epithelium itself conducts all-or-none action potentials. these propagate through the epithelium itself, from cell to cell via gap junctions, in a non-decremental way (meaning the signal does not lose strength over distance). The epithelium lining the posterior trunk and tail is conductive in this way and will conduct signals when stimulated. These signals spread throughout the epithelium in all directions. This epithelium conduction system is important for conveying sensory stimuli to the tail and posterior trunk, but sensory inputs from the lips are conveyed to the stigmata by a nervous pathway. epithelium impulses can also spread to and from the gonadal syncytia but do not involve the secretory oikoplastic epithelium.

Notochord - musculoskeletal systems and locomotion

As chordates the larvaceans possess a notochord which is retained in the adult, forming an elastic cord along the middle of the tail. Notochords are generally spongy, stiff and elastic rods. Its function in the larvacean is to assist movement of the tail by acting as a flexible skeleton for the paired muscle cells (myocytes) that run down the tail axis (see below). It stores elastic energy from contraction of these tail muscles which is used to spring the tail back in the opposite direction. During swimming and ventilating movements the tail undulates sinusoidally.

Larvacean in its house

The caudal nerve puts out motor neurons to synapse with the muscle cells on either side. Water drawn into the house, by rhythmic undulation of the tail can be circulated within the house or expelled through the exhalent aperture. To achieve the latter, the tail fits into a mucous funnel or tail sheath that connects to the exhalent aperture in at least some species. This also propels the house along. The tail can be withdrawn from the tail sheath to circulate water in other ways within the house. The circulation of water within the house is poorly understood, but hopefully studies using laser beams to probe larvaceans in life will shed light on this (Katija et al., 2017).

In Oikopleura, the tail pumps at 2 to 3 beats per second during pumping and at 5 to 6 bps during free swimming. during free swimming, outside the house, the larvacean swims upwards in bursts then sinks vertically head first and then repeats this pattern a few times before expanding a new house.

Larvacean animation

Circulatory System

A 'heart' or contractile dorsal vessel is situated between the lobes of the stomach. The main body cavity is the blastocoel or fluid-filled haemocoel and is not a true coelom since it is lined by basement membrane rather than epithelium. The heart consists of a coelomic sac with a muscular wall and has an epithelial lining (heart and pericardium). This heart pumps haemolymph into the base of the tail - extensions of the haemocoel extend into each wing or lateral fin of the tail. Apart from the pericardium, the coelom persists as the gonadal cavities.

Reproductive System

The precise arrangement of gonads depends on species, but the gonads are generally situated at the rear of the trunk. The testis is greenish when young but becomes a deep orange-red in adults. In Oikopleura gracilis there is a hermaphroditic gonad with two subcompartments: a testis and an ovary. Both the testis and ovary are covered by syncytium and correspond to coelomic cavities. The testis is connected to the outside via a duct and gonopore, supporting its coelomic origin. House building stops at spawning when larvaceans spawn and then die.

Feeding

Food trapped and sieved from the water currents by the filters is passed to the mouth, via a mucous tube which acts also as a food reservoir. Occasional cessation of pumping may allow the animal to clear a back-log of food particles in this reservoir. Food is drawn into the mouth with water currents generated by the action of the ciliary rings of the stigmata which propel water in through the mouth, through the pharynx and to the outside through the stigmata (pharyngeal slits). Food particles are gathered by the mucus-film 'conveyor-belt' produced by the endostyle, a rod-shaped structure in the floor of the pharynx, and directed into the oesophagus to the stomach. A typical-sized larvacean filters about 300 ml of water each day.

There is a well-defined cardiac valves where the oseophagus meets the stomach to regulate food entry to the stomach. The gut lining of Fritillaria consists of three types of cell: 1) ciliated microvillar cells which propel the gut contents by ciliary action and also absorb micronutrients through their microvilli; 2) gastric band cells which possibly secrete digestive enzymes and 3) globular cells which undergo endocytosis and intracellular digestion (phagocytosis) of small particles. Food passes from the stomach into the intestine (it is not clear whether or not there is a pyloric valve at the stomach/intestine junction). The final portion of the intestine, the rectum, prepares the fecal pellet for ejection through the anus. Fecal pellets carry important nutrients to life further down in the water column as a component of the organic snow. In Oikopleura dioica it takes only about 8 minutes for materials to pass the gut from mouth to anus. However, in Appendicularia sicula (family: Fritillaridae) there is no anus! The rectum simply stores an ever more compacted fecal pellet throughout the short life of the animal.

Larvacean animation

References

New technology reveals the role of giant larvaceans in oceanic carbon cycling. Katija, K., Sherlock, R.E., Sherman, A.D. and Robison, B.H. 2017.  Sci. Adv. 3(5): e1602374. DOI:10.1126/sciadv.1602374.