Hemichordates

Hemichordates, meaning ‘half-chordates,’ are a group of worm-like marine animals that share features of both chordates (animals with a backbone) and invertebrates (those lacking a backbone). They have gill slits, similar to all chordates, but unlike chordates, they lack a notochord.

These invertebrates are characterized by a tripartite (threefold) division of the body: an anterior proboscis (prosome), an intermediate collar (mesosome), and a posterior trunk (metasome).

They belong to the phylum Hemichordata, a sister group to echinoderms. Currently, around 130 described species are divided into two major classes: Enteropneusta (Acorn worms) and Pterobranchia (Pterobranchs). The third monotypic class, Planctosphaeroidea, contains a single species, Planctosphaera pelagic.

Description

Size

While some species, like Saccoglossus pygmaeus, measure less than an inch, others, such as Balanoglossus gigas, can reach lengths of up to 7 ft (2.1 m).

Body Plan

Hemichordates are bilaterally symmetrical and triploblastic, with three primary germ layers: the ectoderm, mesoderm, and endoderm.

Acorn Worms

They have a vermiform or worm-like appearance. Their muscular proboscis is ciliated, trapping food particles easily and directing them to the mouth between the proboscis and the collar. 

The trunk, the longest part of the body, houses several vital organs, including the pharynx, which is perforated with numerous gill slits (around 200 in some species) that facilitate gas exchange and filter feeding. Additionally, the trunk contains the esophagus, a long intestine, the terminal anus, and the gonads. In juvenile members of the acorn worm family Harrimaniidae, a post-anal tail is also present.

Pterobranchs

The proboscis of pterobranchs is modified into a ciliated, muscular cephalic shield that helps in their locomotion. The shield also produces the coenecium, an elaborate, sclerotized structure (made of collagen) that houses their zooid colonies.

The metasome, or trunk, extends into a contractile stalk that connects individuals to other colony members formed through asexual budding. It houses a looped digestive tract and gonads.

Organ System

Circulatory

These invertebrates have an open circulatory system with a contractile heartlike vesicle located dorsally within the proboscis. This vesicle constantly expands and contracts against the dorsal blood vessel, indirectly helping blood circulate throughout the body.  

Respiratory

The gill slits facilitate to exchange gases with their surroundings during respiration.

Digestive

They possess a well-developed digestive tract that begins with the mouth and extends all along the trunk.

Excretory

Hemichordates expel their body wastes through the proboscis in the anterior end of the body.

Nervous

Although hemichordates lack a brain, they possess a nerve net of interconnected neurons under their epidermis. The nerve net is directly linked to the two primary nerve tracts: the dorsal and the ventral median tracts.

On the dorsal side of the collar lies a neurochord (an inpocketing of the epidermis) comprising large nerve cells that probably facilitate prompt reflex actions.

Taxonomy

The name of this phylum, Hemichordata, is derived from the Greek words hemi, meaning ‘half,’ and chorde, meaning ‘string.’ Such etymology stemmed from the discovery of the buccal diverticulum (a tubular outgrowth from the mouth cavity), which resembles a rudimentary notochord.

Around 120 living species of hemichordates are broadly divided into three classes. Planctosphaeroidea is monotypic, with a single known species, Planctosphaera pelagic, whereas within acorn worms and pterobranchs, Saccoglossus kowalevskii and Ptychodera flava are the most widely studied.

Some DNA-based studies suggest that hemichordates are close relatives of echinoderms and are hence placed under the clade Ambulacraria.

• Class: Enteropneusta (Acorn worms)
• Class: Pterobranchia (Pterobranchs)
  • Subclass: Graptolithina (Graptolites)
• Class: Planctosphaeroidea

Distribution and Habitat

• Acorn worms are found in both the hemispheres, ranging between the White Sea and Greenland in the north to New Zealand and the Cape of Good Hope in the south. They mostly live along the seashore in intertidal zones at depths of over 10,500 ft.
• Pterobranchs are mainly restricted to the Southern Hemisphere but have also been spotted in northern waters.

Diet

Hemichordates consume floating microscopic algae, diatoms, and bacteria.

Reproduction and Life Cycle

Hemichordates are dioecious, meaning they have distinct male and female individuals. They reproduce sexually by releasing their gametes into the environment, where fertilization takes place.

Development in hemichordates can be either direct or indirect. For example, Saccoglossus kowalevskii develops directly, while Ptychodera flava undergoes indirect development. The latter includes an extended tornaria larval stage, during which the larvae feed on plankton.

Saccoglossus kowalevskii 

After fertilization, this species lays large spherical eggs (approximately 300 μm) that undergo a series of cleavages before developing into an adult. Here is the sequence of cleavage stages:

1. First Cleavage: This occurs almost equally along the animal to the vegetal pole. Sometimes, the division may be unequal, forming the two-cell stage.
2. Second Cleavage: Similar to the first cleavage, this results in the four-cell stage, with each cell called a blastomere.
3. Third Cleavage: A latitudinal division that produces the eight-cell stage.
4. Fourth Cleavage: The animal pole cells divide into eight radially asymmetric blastomeres called mesomeres. Simultaneously, the four blastomeres of the vegetal pole divide into four large macromeres and four tiny micromeres, resulting in the sixteen-cell stage.
5. Fifth Cleavage: Both the animal and vegetal pole cells divide again, forming the thirty-two-cell stage.
6. Sixth Cleavage: This follows the pattern of the fifth cleavage, leading to the sixty-four-cell stage.
7. Seventh Cleavage: This results in a blastula with 128 blastomeres, marking the end of the cleavage stage.

Through gastrulation, the blastula develops gill slits and forms a short-lived, non-feeding larva, which further grows into an adult. 

Ptychodera flava 

After fertilization, the relatively small eggs (approximately 120 μm) of this species follow a cleavage pattern similar to that of Saccoglossus kowalevskii. Here is the detailed sequence of cleavage stages:

1. First and Second Cleavages: These cleavages are equal, orthogonal to each other (at 90 degrees), and involve both the animal and vegetal poles, resulting in the four-cell stage with equal cells.
2. Third Cleavage: This cleavage is equal and equatorial, producing four blastomeres in both the vegetal and animal poles, leading to the eight-cell stage.
3. Fourth Cleavage: The cells of the animal pole divide equally and transversally to form eight blastomeres called mesomeres. In contrast, the vegetal pole cells divide equatorially but unequally, resulting in four large macromeres and four tiny micromeres, forming the sixteen-cell stage. The mesomeres usually give rise to the ectoderm of the future larva.

Subsequent cleavages continue in a manner similar to Saccoglossus kowalevskii, leading to the formation of a blastula. The blastula then undergoes gastrulation to form a planktotrophic tornaria larva. This larva features a ciliated tuft called the telotroch, oral and aboral feeding bands, and a complete gut. Over time, the larva gradually grows into its adult form.

Adaptations

Some hemichordate species, like Saccoglossus bromophenolosus and Protoglossus graveolens, produce and accumulate toxic compounds, such as halogenated phenols and pyrroles. These compounds are used for self-defense and help deter predators. The production of these chemicals is an important adaptation that enhances their survival in the marine environment by making them less palatable or even toxic to potential predators. 

Ecological Importance

• Acorn worms are active burrowers that engage in bioturbation (mixing of soil sediments) for aeration, thereby increasing microbial activity. This process plays a crucial role in maintaining the health and fertility of marine ecosystems.
• They are predators of small invertebrates and are integral to the food chain. Their predation helps regulate prey populations, maintaining ecological balance.
• Pterobranchs, living in colonies, provide attachment surfaces for small crustaceans, polychaete worms, and other tiny invertebrates, thereby serving as microhabitats. These colonies create a supportive environment for various marine organisms, contributing to the biodiversity of marine ecosystems.
• Hemichordates that undergo a larval stage during development disperse to new habitats, helping to establish new populations and facilitate gene flow.