A superficial meta-analysis of calcium/vitamin-D pathways in crustaceans and their associated ultraviolet needs.

Ian Green
Land Hermit Crab Owner Society
7/19/2017

I. Introduction

There seems to be some controversy and confusion regarding the need for UV-B lighting with hermit crabs. Due to the esoteric nature of our chosen pets, few studies are available to draw from which explicitly study hermit crabs. There is however a considerable volume of research regarding crustaceans, and more specifically decapod biology available to study.

The issue at had falls firmly between the worlds of cellular biology and physics. Integration of the two fields requires a degree of supposition and intuition, hence the presumptive nature of this analysis. Aside from the biological facets of this issue we must also take into consideration variations in bulb performance and placement. Ultraviolet energy is a form of electromagnetic radiation which follows an “inverse square law” function1 which may further confound real world application of UV-B equipment.

II. Presence of 7-DHC and absence of UV-B pathways

Crustaceans have been found to contain 7-dehydrocholesterol as well as other compounds typically associated with vertebrate calcium sequestration pathways3. It does not however appear to be involved in the metabolization or synthesis of calcium in invertebrates as is seen in vertebrates4. There does not seem to be any indication that UV-B radiation is utilized in the conversion of 7-DHC or other sterols for crustaceans. This is not unexpected due to the relatively high levels of free calcium in marine environments.

Furthermore, based on current understanding of evolutionary biology, the utilization of D3 and UV-B radiation in the sequestration and utilization of calcium appears to have evolved after the development of decapods2. While many of the direct precursors involved in vertebrate calcium absorption such as 7-DHC are present in crustaceans, they likely have other biological rolls.

“A full vitamin D (refers to vitamin D2 and D3) endocrine system, characterized by a specific VDR (vitamin D receptor, member of the nuclear receptor family), specific vitamin D metabolizing CYP450 enzymes regulated by calciotropic hormones and a dedicated plasma transport-protein is only found in vertebrates. “2

“ Serum calcium homeostasis in marine animals was tightly regulated as in higher animals but mainly required potent hypocalcemic hormones such as stanniocalcin with CT as second fiddle. Hypercalcemic factors, such as PTHrP and the hypophysial prolactin/growth hormone, had a minor role for calcium homeostasis. Terrestrial animals needed primarily hypercalcemic hormones to allow better nutritional absorption of calcium and the vitamin D hormone was and is thereby essential from amphibian onward. A large internal reservoir of calcium was hardly important for animals living in a calcium-rich ocean but gained a strategic survival benefit in tetrapods. The use of an internal calcium reservoir for maintaining serum calcium is not totally novel in tetrapods, as calcium reservoirs in scales and jaw bones were already used in fish without calcified postcranial bones. Similarly, the exoskeleton of crustaceans and snails can be resorbed for reuse during molting. In these cases, resorption of calcium (carbonate) reservoirs used mechanisms different from the well-regulated bone turnover in higher vertebrates. “2

III. Crustacean physiology of calcium pathways

Summary of this section is left absent since it is convoluted and requires a high degree of familiarity with cellular biology. Suffice to say, crustacean calcium sequestration and resorption have been well studied and does not appear to be a function of UV-B radiation. The specifics of how they get it, and how it is handled during molt cycles is explained very well in reference #4.

IV. Studied involvement of UV and crustaceans

Little is known regarding whether or not hermit crabs can see UV light. Deep sea crabs have been studied and appear to use it as a form of color vision to distinguish predators and food5 however their environment is considerably different from that of hermit crabs. Other non-decapod crustaceans are known for being able to sense UV wavelengths.

V. Types of UV-B bulbs and their installation

Linear fluorescent fixtures greatly outperform coil type fluorescent bulbs. Only linear bulbs are worth using due to something called the “photogradient”. The UV-B radiation produced by coil type lamps weakens with distance very quickly due to bulb/hood geometry. It may be such that the distances where useful UV-b is produced are close enough to the bulb that captive animals may suffer damage from other wavelengths of light8.

Within the range of linear UV-B products sold there are various intensities and mixes of UV-A/UV-B. T5 HO bulbs outperform T8 bulbs considerably and waste less energy. It is important that you use high-output(HO) bulbs and an HO hood/fixture/ballast.

Reflectors are required otherwise approximately 50% of the UV-B output can be lost9. Mesh coverings also negate a large amount of the radiation produced10. Glasses other than quartz block all UV-B transmission11. Special consideration should be taken to determine if you can safely use an unshielded fixture due to hermit crab’s propensity for climbing.

Manufacturer claims and specifications for their reptile/pet bulbs are often found to be crafted specifically to sell their product and don’t always reflect the actually use of their products. Photogradients and output levels should always be self tested or researched via independent sources prior to purchase.

The producers are callous and aware of this discontinuity. Zoo Med representatives(to name just one producer) have dismissed valid claims that most of their “Starter Kits” will kill pets within a year. Their opinion is that if a new pet owner buys their kits and fails to get other equipment, its completely the owner’s fault regardless of claims made on the packaging. Other producers such as Exo Terra have also been found to make overly generous claims for their products. It cannot be stressed enough that packaging claims cannot be trusted when it comes to UV-B. Even if the specific output levels are valid, they don’t include information such as what type of hood and what distances were used in their testing methodology.

VI. Metered radiation levels

Correct use of UV meters is vital for your tests to be meaningful. Proper meter use and meter variation can greatly impact the readings taken6.

VII. Conclusion

This analysis is superficial and may contain errors or generalizations. Explicit information on this subject is not readily available. Steps have been taken to contact qualified marine biologists at the University of Melbourne(Australia) to try to get a more exact answer.

While it may seem intuitive that all animals need UV-B radiation for healthy metabolization of calcium, there is no direct evidence to support that notion in decapod biology. Conversely, there is some degree of evidence to suggest that it is not involved. The addition of UV-B lighting to artificial hermit crab habitats may aid some in providing the most nature-like environment that we can, however the fact that we have no solid information on the subject should not be overlooked.

It can safely be assumed that high levels of UV-A are harmful and should be avoided for the ocular health of hermit crabs. Care should be taken in bulb selection to avoid high UV-A output. UV-B producing products vary vastly in their output intensity and wavelength clustering which means they cannot all be considered based on their labeling. Certain brands and types may largely outperform similarly advertised products, therefore in-situ testing and independent review is necessary when deciding which product to use. The lights stop producing useful UV radiation rather quickly and need to be viewed as a disposable product.

UV-B lighting should only be added to a habitat once all other needs are met or exceeded and it should not be considered a basic biological need. Due to the confusing nature of this subject and the products available, the addition of UV-B lighting should not be emphasized to new hermit crab owners or those of limited means but instead should be handled as an advanced/extra aspect of care.

References:
1. Inverse Square Law, General
Georgia State University.
Accessed: 7/19/2017
http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html

2. Vitamin D: calcium and bone homeostasis during evolution
National Center for Biotechnology Information
Accessed: 7/19/2017
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3899559/

3. Sterol composition of shellfish species commonly consumed in the United States
National Center for Biotechnology Information
Accessed: 7/19/2017
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3484358/

4. Calcium Homeostasis in Crustacea
The Evolving Role of Branchial, Renal, Digestive and Hypodermal Epithelia
University of Massachusetts Amherst
Accessed: 7/19/2017
http://www.bio.umass.edu/…/P…/Wheatley_Mineraliz-JEZ1999.pdf

5. Who needs eyes? Crabs in the Bahamas use UV light to tell the difference between food and predators
Daily Mail
Duke University/Nova Southeastern University
Accessed: 7/19/2017
http://www.dailymail.co.uk/…/Who-needs-eyes-Bahama-crabs-us…

6. Using the Solarmeter 6.5 UVI and 6.2 UVB Meters and the ZooMed Digital Radiometers
UV Guide UK
Accessed: 7/19/2017
http://www.uvguide.co.uk/usinguvmeter.htm

7. UV Light in Nature: Solar Ultraviolet Light
UV Guide UK
Accessed: 7/19/2017
http://www.uvguide.co.uk/uvinnature.htm

8. Frances Baines and UV reptile lighting
Accessed: 7/19/2017
http://youtu.be/AUu24MNO2Ho

9. Reflectors and Double Fixtures with UVB Fluorescent Tubes
UV Guide UK
Accessed: 7/19/2017
http://www.uvguide.co.uk/fluorescenttubereflectortests.htm

10. The Effect of Mesh and Screen upon the output of UVB Fluorescent Tubes
UV Guide UK
Accessed: 7/19/2017
http://www.uvguide.co.uk/fluorescenttubemeshtests.htm

11. The role of glass as a barrier against the transmission of ultraviolet radiation: an experimental study.
National Center for Biotechnology Information
Accessed: 7/19/2017
https://www.ncbi.nlm.nih.gov/pubmed/19614895