CHAPTER

RED MANGROVE CRAB AS A BIOENVIRONMENTAL INDICATOR

OF THE EFFECTS OF MANGROVE TRIMMING

ALONG WHISKEY CREEK, LEE COUNTY, SOUTHWESTERN FLORIDA

H.J. Mitchell-Tapping Ph.D. Estero Bay Marine Lab Inc. Fort Myers

Andrew Schwartz, Whiskey Creek, Fort Myers.

A mangrove snake A red mangrove crabtop (photos A. Schwartz).

RED MANGROVE CRAB AS A BIOENVIRONMENTAL INDICATOR

OF THE EFFECTS OF MANGROVE TRIMMING

ALONG WHISKEY CREEK, LEE COUNTY, SOUTHWESTERN FLORIDA

INTRODUCTION

As development continues to pressure our estuarine environments, our ability to replace lost mangroves and establish new stands of these valuable trees is becoming increasingly important to maintaining water quality and wildlife habitat. Mangroves are trees that grow along saline, tidally-influenced, tropical and sub-tropical shores. The word "mangrove" is not a taxonomic term but an ecological term. In Florida, there are 3 species of mangrove: black, red, and white. Red mangroves (Rhizophora mangle) usually occur on the seaward margin, while black (Avicennia germinans) and white mangroves (Laguncularia racemosa) are more landward.

The objective of the study was to develop relatively quick and inexpensive way to determine the effects of mangrove trimming or pruning using a bioindicator. This project used the red mangrove crab that lives generally on the red mangrove. The site chosen for the study was Whiskey Creek, located in South Fort Myers on the Caloosahatchee River, southwestern Florida.

The red mangrove crab Artus pisonii

THE RED MANGROVE-TREE CRAB

The crab Artus pisonii is a common mangal browser throughout south Florida and the Caribbean as reported in the literature. Normally, it can be found clinging to bushes, rocks, pilings, wharves and mangroves. In Estero Bay and other areas of Southwest Florida, the creature is a conspicuous resident of the red mangroves, but can be confused with the abundant green crab of the same size which is found in the same environment. Mangrove-tree crabs tend to remain in the same general area all their lives. As reported in the literature, in observations over a period of several months, 12 percent of the tagged individuals were recovered at or near their point of release.

The red mangrove-tree crab found in the mangroves of Estero Bay: left lower view; right is top view: actual size.

Above water, predators of the mangrove-tree crab include birds, raccoons and the spotted mangrove crab. At low tide, the mangrove-tree crab explores the exposed mud bottom to feed on algae and small crustaceans. It also scavenges dead fishes and crustaceans, and consumes insects that it finds on the mud and in the trees. Apparently, however, one of its favorite foods is the leaf of the red mangrove. Laboratory-raised crabs preferred the leaf of the red mangrove over the black mangrove, even when they were originally caught on black mangroves.

The secretive tiny mangrove-tree crab moves slowly with the stealth when traveling over the mangrove branches and roots. The creature easily clings to even the smallest twig as it is equipped with legs that have needle-like tips.

(Left) The claws and a pincer of the red mangrove crab x10 (right) The mouth area of the crab x10

The claws of the red mangrove crab x10; x60

(left) The underside view of the carapace x10 (right) The body hairs of the crab underside x60

The eye of the red mangrove crab. x10; x60; x200

Due to its coloration, the crustacean is difficult to spot as its brown to mottled olive-green carapace color also helps conceal it. Most spend their time in the inner portions of the mangrove forest. During the day it hides by holding itself upside down on roots and branches. If the crab moves, it often gives itself away. When approached too quickly or closely by a human intruder or potential predator, the creature either scampers off or releases its grip and plunges to the waters below. But a watery escape route is not always perfect. Passing fishes and blue crabs sometimes snatch up this tasty handsome crustacean.

Females carrying eggs do, however, migrate to prop roots along the water’s edge to release their young. Mating takes place throughout the year while the female’s shell is hard (many other crabs must wait for the female to shed her shell. mating then takes place while the female’s shell is still soft.). The male body and claws are much larger than the female’s. During mating the male holds the female upside down - belly-to-belly (abdomen) - with his claws and some of his legs. He clings to the mangrove’s roots or branches with his remaining legs. Within a day or two, the female attaches 3,000 to 27,000 (averaging about 11,000) fertilized eggs to her abdomen. As the young (larvae) develop, the eggs pass through four color stagos. The eggs swell and change from khaki to dark brown, light brown and finally grey. The eggs are ready to hatch in about 16 days. Hatching usually takes place at night, during a spring high tide (during a full or new moon). When ready, the female moves down the mangrove roots to the water’s edge. With her body only partially submerged, she moves her abdomen back and forth and dispatches hot larvae into the water.

For about 30 days the larvae drift in the currents; they arc part of a much larger population of ocean drifters known as zooplankton (animals). When first hatched, the crab’s larvae somewhat resemble a mosquito. At that time, the larvae are known as zoeae. As they grow, they shed their exoskeleton (shell) about four times. The fifth and final larva resembles a crab. Called megalopa, the last larvae continue to drift along the coast until they metamorphose into minute crabs. Less than one percent of the larvae survive to become crabs. The highest predation pressure is while in the planktonic larval phase.

Location of Whiskey Creek, Fort Myers, southwestern Florida (USGS aerial photo)

Sampling locations along Whiskey Creek

Site #2 at MacGregor Boulevard

Site #3

Site #5

Site 6

Site #7

Views of the sites along upper Whiskey Creek

OBJECTIVES

This study was designed to evaluate the effect of mangrove trimming on the population of the red mangrove crab along Whiskey Creek. The distribution of red mangrove crabs was studied. Baseline data on the mangrove crab population was recorded. With this data the red mangrove crab may be useful as an indicator species for the health of the mangrove environment. In the future it will be possible to determine what other variables affect the population of the red mangrove crab.

METHODS

An initial search of the creek was carried out to observe the presence of different species of crabs. A seventeen foot canoe was used as transportation for this study. The same two observers were present to conduct each survey. An attempt was made to conduct all surveys at the same times of day. Thirty two sites were chosen along the creek to study. The sites were selected as randomly as possible where access could be achieved. Sites were spaced along the creek evenly to evaluate the population of crabs along the creek accurately. Crabs were surveyed by visual inspection of the mangroves from the canoe at these sites. Each site was visited on multiple occasions . The location of each site was documented using a handheld Garmin GPS unit . The time of day and number of crabs at each site was recorded. Observations on the types of mangroves present at the site as well as tide, temperature, humidity, cloud cover and evidence of mangrove trimming were also noted. A Canon digital camera was used to document the appearance of the site, evidence of mangrove trimming or destruction, and photograph the crabs and predatory species found at the site. The data and observations were recorded in a small spiral bound field notebook and then transferred to data sheets and finally a Microsoft works spreadsheet for analysis and display.

The mangrove crab population of Whiskey Creek was studied. The data proved that the hypothesis was true. The data showed that the number of crabs on trimmed mangroves was considerably less than untrimmed. The graphs and charts also showed that there was a decrease in the amount of crabs the farther south or deeper upstream into the creek. The results also reflect the differences in the mangrove crab habitat at the north and south ends of the creek. They also demonstrate how the red mangrove crab can be used as an indicator species to show the harmful effects of destruction, removal, or trimming of red mangroves.

If a larger sample of data was tested the results would be very similar. Trimming would negatively affect the number of mangrove crabs found. The differences in numbers of crabs on the north and south ends of the creek would not be significant even if more study sites were observed.

The knowledge that was gained in this study can be used to further examine the effect of environmental changes on the red mangrove crab and its habitat. Seasonal variations in rainfall and temperature can be expected to cause changes in this habitat. The population of Red Mangrove crabs might also be effected by chemicals used in insect control and found in runoff from roads.

CONCLUSIONS

This study took many days to complete and the researcher encountered many challenges. At times some of the sites were hard to get into and the visibility at certain times of the day made it difficult to spot the crabs. Keeping accurate and consistent records of the data was also a challenge. During the study changes in the type of data collected were made. In future studies the data to be recorded would be written on preprinted forms from the start. Numbers of crabs seen and their location on the mangroves varied at different times of the day. Interesting species of animals, such as, a large yellow mangrove snake and a tiny fuzzy caterpillar which camouflaged itself under the mangrove leaves, were also observed. In conclusion, the distribution of the red mangrove crab, Aratus pisonii, along Whiskey Creek support the hypothesis that mangrove trimming has a negative effect on its population.

ACKNOWLEDGEMENTS

The author would like to thank the following people for helping with this project : Dr. Hugh Mitchell-Tapping for his help with planning of the project and data analysis, and my parents, Stan and Cindy Schwartz who made sure I carried out the study in a timely manner.

site	date	time	oF	humidity%	cloud%	tide	crab #	GPS N 26	GPS W 81	trimmed
1	28-Sep-02	19:05			30	high	4	34.601	53.856	T
1	6-Oct-02	10:40	87	55	0	mid	1	34.601	53.856	T
1	14-Oct-02	11:15	91	80	0	high	0	34.601	53.856	T
1	20-Oct-02	16:30	81	53	50	high	0	34.601	53.856	T
1	3-Nov-02	16:27	85	42	20	high	0	34.601	53.856	T
2	28-Sep-02	18:56			30	high	2	34.580	53.878
2	6-Oct-02	10:30	87	55	0	mid	1	34.580	53.878
2	14-Oct-02	11:10	87	87	0	high	0	34.580	53.878
2	20-Oct-02	16:35	81	53	50	high	1	34.580	53.878
2	3-Nov-02	16:35	85	45	20	high	0	34.580	53.878
3	28-Sep-02	18:47			30	high	6	34.574	53.852
3	6-Oct-02	10:50	87	55	0	mid	7	34.574	53.852
3	14-Oct-02	11:02	87	80	0	high	11	34.574	53.852
3	20-Oct-02	16:50	81	53	50	high	12	34.574	53.852
3	3-Nov-02	16:44	85	42	20	high	14	34.574	53.852
4	28-Sep-02	18:45			30	high	3	34.574	53.852
4	6-Oct-02	11:00	87	55	0	mid	0	34.574	53.852
4	14-Oct-02	10:50	80	80	0	high	0	34.574	53.852
4	20-Oct-02	19:00	81	53	50	high	0	34.574	53.852
4	3-Nov-02	19:12	83	44	20	high	0	34.574	53.852
5	28-Sep-02	18:35			30	high	0	34.507	53.806	T
5	6-Oct-02	11:10	87	55	0	mid	0	34.507	53.806	T
5	14-Oct-02	11:00	87	80	0	high	0	34.507	53.806	T
5	20-Oct-02	17:05	81	53	50	high	0	34.507	53.806	T
5	3-Nov-02	17:20	83	44	20	high	1	34.507	53.806	T
6	28-Sep-02	18:25			30	high	4	34.500	53.799
6	6-Oct-02	11:15	88	55	0	mid	1	34.500	53.799
6	14-Oct-02	10:47	87	80	0	high	5	34.500	53.799
6	20-Oct-02	17:10	81	53	50	high	3	34.500	53.799
6	3-Nov-02	17:17	83	44	20	high	2	34.500	53.799
7	28-Sep-02	18:13			30	high	5	34.462	53.806
7	6-Oct-02	11:20	87	55	0	mid	3	34.462	53.806
7	14-Oct-02	10:30	80	71	0	high	3	34.462	53.806
7	20-Oct-02	17:15	81	53	50	high	4	34.462	53.806
7	3-Nov-02	17:20	83	44	20	high	2