Ch. 10 Flashcards
(32 cards)
Key features of Gnathifera
- Pharyngeal Jaws: All animals discussed have pharyngeal jaws with a complex ultrastructure, a defining characteristic.
- Taxonomic History: These animals were once grouped with nematodes and other phyla in Aschelminthes, but it is now recognized that their similarities are due to convergence, not common ancestry.
- Gnathifera: The non-molting Aschelminthes are now placed in the taxon Gnathifera, which is considered to have evolved from a common
ancestor. - Pseudocoelom: These animals have a pseudocoel (fluid-filled body cavity) instead of a true coelom, which is not lined by mesodermal epithelia.
- Eutely: Gnathifera exhibit eutely, where growth occurs by increased cell size rather than cell number, and specific tissues have a fixed number of cells, making regeneration of lost body parts impossible.
- Gnathifera Groups: The two main groups are the free-living Rotifera and parasitic Acanthocephala, which are closely related based on both morphological and molecular evidence.
- Syndermata Hypothesis: Some researchers suggest combining Rotifera and Acanthocephala into a single phylum, Syndermata, due to similarities, while others propose retaining them as separate phyla.
- Current Classification: For this edition, the animals are kept as separate phyla (Rotifera and Acanthocephala), though there is no consensus on their exact relationship.
Key features of Phylum Rotifera:
- Defining Characteristics:
• Muscular Pharynx: Contains jaws (trophi) for grasping, crushing, or grinding prey, or attaching to a host.
• Toes with Adhesive Glands: Helps in attachment to surfaces or hosts. - Ciliated Lobes (Corona): Rotifers have two ciliated anterior lobes called the corona, which beat metachronally (in a wave-like pattern), creating a rotating appearance. The degree and pattern of ciliation vary by species.
- Habitat: Approximately 1,850 species, mostly freshwater (about 95%), found in lakes, ponds, moist surfaces of mosses, and some interstitial environments. A small percentage
of species are marine or estuarine. - Pseudocoelomate and Eutely: Rotifers have a pseudocoel, not a true coelom, and are generally eutelic (growth by cell size increase, not cell number). Specific organs and tissues have a fixed number of nuclei.
- Syncytial Epidermis: The epidermis is syncytial (incomplete cell membranes between nuclei) and produces a non-chitinous intracellular “cuticle” that isn’t molted. This is supported by protein filaments.
- Musculature: Rotifers have smooth and striated muscle fibers, with striated muscles used for rapid movement of spines and appendages.
- Lack of Specialized Systems: They do not have specialized respiratory or circulatory systems.
Rotifer life span, parasitism, size and ecological importance
- Life Span: Most species are free-living and short-lived, typically living 1-2 weeks, with some surviving up to 5 weeks. Some species remain permanently attached to substrates, while others can move.
- Parasitism: Some rotifers are parasitic, mainly affecting invertebrates like arthropods and annelids, but they do not significantly impact humans.
- Size: Rotifers are generally small, ranging from 100-500 micrometers, with the largest species reaching up to 3 mm.
- Ecological Importance: Rotifers play a role in aquatic community structure, energy flow in freshwater ecosystems, and are used in research on aging (senescence) and pollution monitoring. Some species are important food sources for commercially reared fish and crustaceans.
Rotifer feeding, unique feeding structures, food collection and locomotion
- Feeding: Free-living rotifers are omnivores, consuming algae, zooplankton, detritus, ano small animals. Some species are carnivores,
preying on smaller organisms, and others feed on intracellular juices of algae. - Unique Feeding Structures:
• Mastax: A muscular modification of the pharynx used for food processing.
• Trophi: Rigid structures in the mastax used for grinding, sucking, or grabbing food. The structure of the trophi varies by species and is important for identification. - Food Collection: In planktonic and sedentary species, coronal cilia form parallel bands that generate water currents for respiration and food collection, directing captured particles into the mouth via a food groove.
- Locomotion:
• Swimming: Free-living rotifers swim using the coronal cilia, with some species also using spines for leaping movement.
• Attachment: Many species can attach to substrates using cement glands on their feet. The feet may have up to four toes.
Key features of Rotifer Locomotion
- Locomotion on Solid Substrates:
• Attachment: Free-living rotifers can attach temporarily to substrates using their foot, forming a secure hold.
• Body Elongation: The contraction of circular muscles allows elongation of the body, functioning like a hydrostatic skeleton. This allows for the antagonism between circular and longitudinal muscles, generating temporary increases in hydrostatic pressure.
• Movement: As the body elongates, the anterior end becomes a suction-
generating proboscis, used to form a new attachment. The posterior attachment is then released, and the body shortens via contraction of longitudinal muscles.
• Looping Movement: This alternating contraction and elongation allow the animal to “loop” forward, moving progressively on the substrate. - Shape Changes in Locomotion:
• Some species exhibit extraordinary shape changes during movement, with the foot and trunk divided into sections that can telescope, allowing the rotifer to elongate and contract like a collapsible drinking cup. - Sessile Species:
• Some rotifers are sessile as adults and cannot move. They attach permanently to a substrate via pedal gland secretions.
• Protective Tubes: Sessile rotifers may secrete protective tubes, often incorporating debris, sand, or feces into the walls.
• Free-Swimming Young: Even sedentary species have free-swimming young, which allow movement between adult generations.
Key features of Rotifer Reproduction:
• Asexual Reproduction: Asexual reproduction occurs through parthenogenesis, where unfertilized eggs develop into new individuals.
• No Fission or Fragmentation: Rotifers do not reproduce by fission or fragmentation due to eutely (fixed cell number).
• Sexual and Asexual Reproduction:
Specific details of reproduction vary by class within the phylum Rotifera, with three distinct classes identified in the
phylum.
Key features of Class Seisonidea:
- Parasitic Lifestyle:
Seisonidea species are exclusively ectoparasites of marine crustaceans. - Reduced Corona:
• The corona (ciliated feeding structure) is often greatly reduced in size, reflecting the parasitic nature of these organisms. - Reproduction:
• Reproduction is exclusively sexual.
• Members of this class are gonochoristic (dioecious), meaning individuals are of separate sexes.
• Fertilization is internal:
• It can occur via true copulation or hypodermic impregnation, where the male injects sperm into the female’s pseudocoel. From there, the
sperm move to the ovary for fertilization.
Key features of Class Belloidea:
- Free-living and Mobile:
• All species in this class are free-living and mobile.
• No tube-dwelling, sessile, or parasitic species. - Feeding:
• Most members are omnivorous suspension feeders.
• The corona is well-developed and bilobed, aiding in feeding.
• One predatory species has been identified. - Reproduction:
• Parthenogenesis (asexual reproduction) is the only form of reproduction, as no males have been discovered.
• All known species are female.
• Molecular evidence suggests they have existed for millions of years without genetic exchange between individuals, challenging the idea that asexual reproduction leads to extinction. - Survival in Stressful Environments:
• Belloid rotifers are found in environments subject to extreme conditions like freezing, dehydration, or high temperatures.
• They are capable of entering cryptobiosis, a state of extremely low metabolism that allows them to survive environmental extremes. - Adaptation to Stress:
• Some species secrete a gelatinous covering that hardens into a cyst during desiccation.
• Cryptobiosis enables them to survive desiccation for extended periods, with some species rehydrating after over 20
years.
• They can also tolerate exposure to ionizing radiation. - Species Count:
• About 370 species have been described in this class.
Key features of Class Monogononta:
- Diverse Habitat and Locomotion:
• Monogonont rotifers can be either free-swimming or sessile.
• They include the only free-living sessile rotifers, which can attach to plants, filamentous algae, or tubes of other rotifers. - Feeding:
• Sessile species typically use the corona for feeding, but some have modifications such as long spines around the anterior end that capture prey.
• Many are carnivores, using spines to entrap small metazoans and then ingesting them.
• Ciliation around the mouth aids in food ingestion. - Protective Structures:
• Many sessile species live in protective tubes, which can be made of gelatinous secretions or by cementing particles and feces together.
• Both free-swimming and sessile species have a cuticle that forms a lorica (protective shell), unique to this class. The lorica’s shape can be influenced by environmental factors. - Reproduction:
• Parthenogenesis (asexual reproduction) is common in monogononts.
• Females produce diploid eggs by mitosis that develop into female rotifers (amictic, or asexual females).
• These females reproduce rapidly, with populations doubling in as little as 15 hours.
• Mictic reproduction occurs under specific conditions:
• Mictic females produce haploid eggs through meiosis. Unfertilized eggs develop into haploid males.
• Males are small, nonfeeding, ano short-lived but crucial for fertilization.
• Resting eggs (diapause or winter eggs) are produced through fertilization, which are highly resistant to environmental stress and can remain dormant for years.
• After favorable conditions return, resting eggs hatch into amictic females, restarting the asexual reproductive cvcle. - Unique Reproductive Behavior:
• A small percentage of amphoteric females (less than 0.5%) produce both haploid eggs (developing into males if unfertilized) and diploid eggs (developing into females).
• Most free-living monogononts encountered in nature are females.
Key features of the digestive system of rotifers:
- Tubular Digestive System:
• Most rotifers have a tubular digestive system with a mouth at the anterior end and an anus at the posterior end.
• Cilia lining the inner surface of the gut help move food through the system. - Digestion:
• Extracellular digestion primarily occurs in the stomach, where gastric glands or gastric caeca release digestive enzymes.
• Undigested waste moves through a short intestine and is excreted through the cloaca (a common chamber that also receives the excretory and reproductive ducts). - Anus Location:
• The anus is located dorsally, near the junction between the trunk and the foot. - Belloid Rotifers:
• Some belloid rotifers do not have a distinct stomach cavity or anus. Instead, they have a continuous syncytial mass that serves as a digestive space through which food is circulated.
• In these species, digestion is primarily intracellular. - Male Rotifers:
• In male rotifers, except for Seisonidea species, the digestive system is typically nonfunctional or absent.
Key Features of the nervous and sensory systems in rotifers:
- Brain Structure:
• The rotifer brain consists of a bilobed mass of ganglia, located dorsally to the mastax (the muscular pharynx).
• Nerves extend throughout the body, connecting the brain to the musculature, organ systems, and various sensory receptors. - Sensory Receptors:
• Sensory bristles are present on the rotifer’s body, aiding in sensory detection.
• Antennae: Most rotifers have three antennae — two lateral and one median-dorsal - which function as
chemoreceptors (detecting chemicals) and mechanoreceptors (detecting physical stimuli like touch or vibration). - Photoreception:
• A pigmented, cuplike photoreceptor is often located directly on the brain, helping the rotifer sense light.
• Additional photoreceptors may be found on the corona (the ciliated structure near the head) to assist in light detection.
Key features of Excretion and Water Balance in rotifers:
- Excretion Mechanisms:
• Excretion is partially carried out by diffusion across the general body surface.
• Rotifers also possess protonephridia, similar to those in flatworms, which help in fluid regulation. - Protonephridia Function:
• Flagella within each protonephridium create negative pressure, drawing fluid from the pseudocoel.
• Up to 50 flame bulbs may be associated with each protonephridium, assisting in waste filtration.
• The collecting tubules from each protonephridium lead to a common bladder. Bladder contractions expel fluid into the cloaca, where waste is released alongside digestive and reproductive products. - Water Balance:
• Freshwater rotifers have a higher concentration of dissolved materials in their tissues and body fluids compared to the surrounding medium, causing water to constantly diffuse into the body through the permeable surface.
• The protonephridia play a crucial role in maintaining water balance and body volume by regulating the influx of water.
• The osmotic gradient between the rotifer’s body and the external environment aids in maintaining body
turgor, which is important for structural integrity and function. - Role of Protonephridia:
• While excretion is a secondary function, the primary role of the protonephridia in freshwater rotifers is the regulation of water balance, ensuring the animal does not absorb excessive water due to the osmotic gradient.
Key Features of Phylum Acanthocephala:
Defining Characteristics
- Defining Characteristics:
• Acanthocephalans have 1-2 large, acellular, collagenous sacs (ligament sacs) in the pseudocoel that support the
gonads.
• Proboscis containing intracellular hooks, which is a hallmark feature of the phylum.
The proboscis is retracted into a specialized pouch in most species, and the hooks are used for attachment to the host’s gut. - Parasites of Vertebrates:
• Acanthocephalans are gut parasites found primarily in vertebrates, with a preference for the small intestines of fishes, particularly freshwater species, but also affecting birds, mammals, reptiles, and amphibians.
• A single host can harbor hundreds of acanthocephalans.
Key Features of Phylum Acanthocephala:
Reproduction
Reproductive System:
• Acanthocephalans are gonochoristic (dioecious, meaning there are separate sexes.
• Male acanthocephalans have cement glands that secure the female’s vagina after copulation.
• The female acanthocephalan lacks a well-defined ovary, instead having fragmented ovaries called ovarian balls floating in the ligament sac fluid. The female can release several hundred
thousand fertilized eggs daily.
Life Cycle Development:
• Eggs (acanthors) develop into the advanced stage within the female’s pseudocoel.
• After being released with the host’s feces, acanthors need to be ingested by an appropriate intermediate host (usually an arthropod).
• The acanthor emerges from its shell, burrows through the gut tissue into the hemocoel (blood space) of the intermediate host, and continues to develop.
• Only when the intermediate host is eaten by a definitive vertebrate host can the parasite reach adulthood.
Key Features of Phylum Acanthocephala:
Digestive System
Absence of Digestive System:
• Acanthocephalans do not have a digestive tract at any life stage. Instead, they absorb solubilized nutrients from the host. This lack of a digestive system is typical for parasites.
No Digestive Tract:
• Throughout its life, the acanthocephalan lacks a digestive system. It absorbs nutrients directly from the host’s tissues, an adaptation to its parasitic lifestyle.
Key Features of Phylum Acanthocephala:
Life cycle
Life Cycle:
• Arthropods often serve as intermediate hosts, crucial for the completion of the life cycle.
• The life cycle involves the release of acanthors, which are fertilized eggs that exit the host and develop further only in certain invertebrate hosts (typically arthropods). These hosts must be consumed by a vertebrate (definitive host) for the acanthocephalan to reach adulthood.
Key Features of Phylum Acanthocephala:
Body structure
Body Structure:
• The acanthocephalan body is divided into three parts:
1. Proboscis (retractable and hooked)
2. Neck
3. Trunk
• The trunk contains mainly the gonads and associated glands, with the
pseudocoel serving as a large space for reproductive organs.
• The proboscis’s hooks and spines are used for attachment to the gut wall,
ensuring the parasite stays in place in the host’s intestines.
Key Features of Acanthocephalans and Their Evolutionary History:
Adaptations for Endoparasitism:
• Acanthocephalans exhibit pronounced adaptations for an endoparasitic lifestyle, making it challenging to trace their evolutionary history. Their specialized morphology serves their parasitic function, with minimal external features that indicate their ancestral traits.
Key Features of Acanthocephalans and Their Evolutionary History:
Fossil Evidence:
• Fossil evidence suggests that acanthocephalans may have evolved from free-living marine worms, such as priapulids (a small group of marine worms discussed in Chapter 17). Some
fossilized priapulids found in the Burgess Shale (about 540 million years ago) show characteristics that resemble the acanthocephalans, indicating a shared ancestry.
• This evolutionary history implies that acanthocephalans may have initially been free-living mud dwellers, adapting to an endoparasitic life in response to intense predation by arthropods, potentially as a survival strategy (“Parasitize or perish”).
Key Features of Acanthocephalans and Their Evolutionary History:
Possible Relation to Rotifers:
• There is growing molecular evidence suggesting that acanthocephalans are more closely related to rotifers than to other groups. Some researchers propose that acanthocephalans and rotifers may be classified together in a single phylum, Rotifera or Syndermata.
• Rotifers and acanthocephalans share several features, including:
• Protonephridia: Some acanthocephalans possess
protonephridia, which is also found in rotifers and flatworms, suggesting a similar excretory system.
• Syncytial Epidermis: Acanthocephalan epidermis is syncytial (a multinucleated structure), with an intracellular network of protein fibers, similar to that found in rotifers, further indicating a close evolutionary link.
Key Features of Acanthocephalans and Their Evolutionary History:
Evolutionary Considerations:
• The current theory suggests that acanthocephalans and rotifers may share a common evolutionary origin, with acanthocephalans being drastically modified for a parasitic lifestyle.
• Molecular studies, including the analysis of more species and genetic sequences, may help clarify this connection and resolve whether acanthocephalans and rotifers should be classified in the same
phylum, providing insights into their shared evolutionary history.
Key Features of Phylum Gnathostomulida:
General Characteristics:
• Gnathostomulids are small, soft-bodied worms, typically less than 1 mm long, and they inhabit interstitial spaces between sand grains. Some species can form dense populations of up to 6,000 individuals per kilogram of sand, especially in deep sediment layers rich in hydrogen sulfide.
• They were initially described as turbellarian flatworms in 1956, but later associations have included gastrotrichs,
annelids, and even the planula larvae of cnidarians. Recently, molecular and ultrastructural evidence have confirmed
their closer relationship to rotifers.
Key Features of Phylum Gnathostomulida:
Body Plan and Morphology:
• Acoelomate: Like flatworms, gnathostomulids do not possess a body cavity (coelom).
• Ciliated epidermis: They have externally ciliated cells, but unlike flatworms, each epidermal cell has only one cilium rather than the multiple cilia per cell found in flatworms and other spirally-cleaving animals.
• Triploblastic: The body is made up of three germ layers: ectoderm, mesoderm, and endoderm.
• No functional anus: They have a mouth but lack a functional anus, similar to other simple invertebrates.
• Excretory system: They possess simple protonephridia for excretion, as in many other related species.
Key Features of Phylum Gnathostomulida:
Feeding and Mouthparts:
• The most complex structure in gnathostomulids is their mouthparts, which consist of hardened jaws and an associated basal plate. These jaws are
used to scrape bacteria and fungi from sand grains and other substrates, suggesting a close relationship with the feeding structures (trophi) of rotifers.
