Spiders are fascinating creatures that inhabit nearly every ecosystem on Earth. With over 47,000 known species, spiders display incredible diversity in their physiology, behavior, and habitat preferences. One question that often comes up regarding spider anatomy and physiology is: do spiders have hearts? The quick answer is yes, spiders do have hearts, but they are quite different from the complex four-chambered hearts of humans and other mammals. In this article, we’ll explore the cardiac system of spiders in detail to understand how their simple circulatory system works to move blood and nutrients throughout their bodies.
Spider Circulation and Heart Structure
Like insects, spiders have an open circulatory system, meaning the blood flows freely within the body cavity and is not restricted to blood vessels. The central organ of the spider’s circulatory system is a tubular heart or “aorta” located along the upper surface of the abdomen.
Small openings called ostia allow hemolymph (the equivalent of blood in spiders) to enter the heart. Contractions of the heart muscles propel the hemolymph through an anterior aorta into the head region. The hemolymph exchanges nutrients and wastes with tissues as it moves through sinuses and lacunae (cavities) in the body. It then reenters the heart through additional ostia and is pumped back through a posterior aorta towards the abdomen and eventually into a pericardial sinus near the heart to complete the circuit.
While more primitive arachnids like scorpions have between four and eight ostia connecting to their tubular hearts, most spiders have four pairs of ostia along the heart to take in and pump out hemolymph. The heart is therefore divided into four chambers corresponding to each of the ostia pairs. However, these chambers are not separated by valves like in mammalian hearts. The pace of heartbeat and strength of contractions are controlled by nerves coming from the central nervous system.
Valve-Like Structures
Some more advanced spiders have valve-like slit structures at the entries to the ostia that can restrict backflow of hemolymph. Species like tarantulas also have a rhythmically contracting sac next to the heart called the ventral sac that helps pressurize the flow of hemolymph. These adaptations allow for more organized circulation and may have contributed to the greater size and metabolic needs of large arachnids compared to smaller spider species.
Hemolymph Composition
In addition to a relatively simple circulatory system, spiders lack many other features of advanced cardiovascular systems like blood vessels, red blood cells, and hemoglobin. The hemolymph that circulates in spiders has a watery consistency similar to insect blood. Just as insects have hemolymph instead of blood, spiders technically do not have true blood. However, the hemolymph of spiders does have various dissolved proteins, salts, nutrients and waste products which aid in circulation and metabolism.
The most abundant protein in spider hemolymph is hemocyanin, which contains copper and is blue in color when oxygenated. Hemocyanin acts similarly to hemoglobin in vertebrate blood, binding to oxygen which can then be transported to tissues. Hemolymph also contains types of white blood cell-like amoebocytes which are involved in immune response and clotting of hemolymph. Fat bodies and nephrocytes function as equivalents to the liver and kidneys, helping remove metabolic waste products from the hemolymph.
Clotting Ability
An important attribute of spider hemolymph is its ability to clot relatively quickly when the exoskeleton is damaged. This limits hemolymph loss and entry of pathogens.Specialized clotting cells called coagulocytes initiate clot formation when triggered by contact with foreign surfaces. Ca2+ ions are important regulators of the coagulation cascade. The clot forms a plug at the wound site from thread-like fibrin proteins while damaged tissues are repaired.
Variable Heart Rates
Spider heart rates are temperature dependent and can vary from around 35 beats per minute in cool conditions to over 200 beats at warmer temperatures. Smaller spiders tend to have faster resting heart rates than larger species. For example, juvenile spiders can have heart rates in the range of 250 to 300 beats per minute.
Some spiders like tarantulas display irregular heart rhythms and rates that can double when they get excited or stressed. During molting, spiders’ heart rates slow down considerably since high activity levels could rupture their soft new exoskeletons. Some spiders even exhibit a temporary pause in the heartbeat during leg movement to maintain blood pressure in the rest of the body.
Circulation Efficiency
The spider heart pumps hemolymph at relatively low pressure compared to vertebrate blood pressures. But due to their small size and hydraulic skeleton (where pressure transmitted through hemolymph plays a role in movement), spiders do not need a high speed circulatory system. Hemolymph can diffuse through tissues readily to deliver oxygen and nutrients. The open layout allows hemolymph access without strict vessels or capillaries.
Still, the single ventricle heart and lack of advanced valves or chambers make spider circulation much less efficient than the double-loop closed circulatory systems of mammals. They rely more on diffusion over shorter distances than bulk transport of hemolymph.
Respiration and Oxygen Delivery
In addition to circulation of hemolymph for delivery of nutrients, the spider cardiovascular system functions to distribute oxygen throughout the body. Like insects, spiders utilize a tracheal system rather than oxygen-binding proteins like hemoglobin for respiration. The tracheal system consists of branching hollow tubes called tracheae that bring oxygen directly into tissues through openings called spiracles in the exoskeleton.
However, spiders also rely on diffusion from hemolymph to supply oxygen to interior tissues and structures that cannot be reached efficiently by the tracheal system. Oxygen binds weakly to hemocyanin proteins and other molecules in the hemolymph for broader circulation.
Book Lungs
A unique respiratory organ found in spiders is the book lung. Book lungs are made up of stacked thin plates or lamellae with air spaces between them. As hemolymph flows over and between the lamellae, oxygen diffuses into the hemolymph while carbon dioxide diffuses out. Most spiders have two pairs of book lungs located ventrally that are connected to the outside by corresponding spiracles. Jumping spiders, however, only have one pair of book lungs.
Tracheae directly supply oxygen to the book lungs, which then absorb oxygen to complement the role of the tracheal system.
Digestion, Nutrient Transport, and Excretion
Like the circulatory system, the spider digestive system also utilizes diffusion and transport via the hemolymph once food is initially digested through secretions and movement through the gut. Digested nutrients from the stomach and gut lining get absorbed into the hemolymph, which circulates these throughout the body. Wastes are filtered out by excretory organs and routed back to the stomach.
The primary excretory organs are coxal glands connected to the base of each walking leg which filter wastes into the hemolymph. Malphigian tubules in the digestive tract work with the coxal glands to remove metabolic wastes from the hemolymph and produce a white guanine-rich substance that is excreted.
Water Regulation
In addition to facilitating digestion and excretion, the spider’s cardiovascular system and in particular the hemolymph play roles in osmoregulation – controlling water balance. Hemolymph contains diluted salts and sugars that help spiders maintain water homeostasis in different environmental conditions. Water regulation is critical since spiders have high surface area relative to their volume and can risk dehydration.
Nervous Control
The spider heart and vascular system are strongly governed by the central nervous system. Neurosecretory cells connect to the heart to regulate heartbeat frequency and strength of contractions. These neurosecretory cells produce peptides and hormones that get delivered through the hemolymph to target organs.
Factors like temperature, activity level, and stress stimulate nerve signals that can increase heart rate to meet elevated metabolic demands. Signals initiating clotting and other hemolymph functions are also mediated by the nervous system. This connection allows spiders to alter circulation in response to both external and internal cues.
Molting Regulation
One critical function governed by the nervous system is coordinating the molting process, including timing of heart rate changes. Molting requires a major coordinated effort using hormones like ecdysteroids to loosen the old exoskeleton. Simultaneously, the lowered heart rate protects the soft fragile body. Once the new exoskeleton hardens, heart rate and hemolymph pressure increase to circulate nutrients and wastes at normal levels again.
Evolution of Spider Hearts
The basic tubular heart and open circulatory system of spiders have evolved over hundreds of millions of years. Primitive arachnids utilized simple diffusion through the body cavity to distribute nutrients rather than a circulatory system. Over time, improved organization through a heart structure and hemolymph-filled cavity allowed larger, more mobile, and more complex spiders to evolve.
However, the open system and lack of advanced pressure-regulation mechanisms constrain how big and metabolically active spiders can grow. Other adaptations like book lungs and tracheae supplemented oxygen delivery as spiders diversified. Overall, the spider cardiovascular system strikes an effective balance between simplicity and meeting basic circulatory needs.
Relation to Other Invertebrates
Many invertebrates like mollusks and annelid worms utilize hearts to pump hemolymph or blood through open circulatory systems. Insects also have tubular hearts along the dorsal side that pump hemolymph without complex vessels. However, key differences are insects have multiple chambers segmented along the heart length whereas most spider hearts have one chamber with ostia openings along the sides.
Unique Features of Spider Circulation
While spiders have some characteristics similar to other invertebrates, a few distinctive features of their cardiovascular system include:
– Single chambered heart with lateral ostia vs. insect’s chambered heart
– Presence of book lungs as supplementary respiratory organs
– Neurosecretory cells to control heart via hormones
– Rapid coagulation of hemolymph induced by coagulocytes
These specialized adaptations contribute to meeting spiders’ requirements like managing hydration, oxygenating tissues, and responding to the environment.
Surviving with a Simple System
Remarkably, a simple tubular heart, low-pressure circulatory fluid, and lack of advanced respiration are sufficient to enable spiders to thrive in many habitats. Diffusion-based transport meets their cellular needs without costly development of complex networks of blood vessels and capillaries. Open flow allows hemolymph access without strict regulation of pressure and flow rates. Though rudimentary compared to mammals, the spider cardiovascular system fulfills the essential priorities of circulation.
Conclusion
In summary, spiders do indeed have hearts that pump hemolymph throughout their bodies open circulatory systems. While less advanced than vertebrate cardiovascular systems, spider hearts efficiently deliver nutrients and oxygen to tissues through the combination of an open central cavity and diffusion-based transport. Adaptations like ostia and book lungs supplement circulation. Neurohormonal control adapts the system to different conditions. So the next time you see a spider crawling around, remember it has a simple yet effective circulatory system powering its movements!
